In the late 1800s and early 1900s, classical theories of physics began to crumble in light of new evidence accumulated by experiments on the quantum scale. The new explanations proposed at the time irked some of the biggest names in physics, mathematics, and philosophy.
For example, Poincare's philosophy of science was based on what philosophers call instrumentalism, which dismisses unobservable entities. (see for example the paper by Milena Ivanova) This viewpoint is dismissive of the reality of atoms because they are not directly observed. Even when the evidence mounted, Milena Ivanova argues that, "...Poincaré’s paper motivates a non-fundamentalist view about the world, and that this is compatible with his structuralism. ... Poincaré advanced structural realism, which commits one to the structural claims of scientific theories and not the claims regarding unobservable entities." In other words, the theory describes what is observed in terms of a mathematical structure, but the observation does not imply, for example, that atoms - an assumption underpinning the theory - exist.
The early 1900s must have been an exciting time of discovery. Scientists had to shed their preconceived notion about certainty and absolute space. I wonder if I would have been a staunch supporter of the status quo or if I would have accepted the new way of thinking. We'll never know, though I'm sure many of us would fancy ourselves falling on the right side of history.
Quantum mechanics became accepted as the theory of atoms and molecules because of its success in predicting atomic spectra and differential scattering cross-sections. As such, the underlying structure of atoms and molecules as suggested by the theory have become accepted as reality. Our classical view and intuition about the world falls apart at the quantum scale. Electrons are not tiny point-like particles whose position and momenta can be simultaneously determined to arbitrary precision.
The rates of thermodynamic processes are typically accelerated when the
temperature is increased, but we find a dramatic decrease in the rates
of self healing when we turn up the heat. Many such strange things
characterize the underlying process.
In the modern era, physicists picture a single electron in an atom as a fuzzy cloud rather than the old planetary picture of one tiny particle orbiting a nucleus. An electron can pass simultaneously through two slits in a wall and two particles can be entangled so that measuring the spin of one particle instantaneously determines the spin of another one on the other side of the universe. Observations like these are so commonplace that we accept them without question.
We have warmed to the reality of something very bizarre because all measurements support this view. Is it really that way? I would respond, yes, because reality is probed by observation, even if by indirect means. Because of the theory's great success, it is assumed to work for highly complex systems even when it is impossible to test the theory because of our computational limitations and inaccurate experiments.
In my field of nonlinear optics, we measure the nonlinear susceptibilities of molecules and compare them with theory. However, the measurements have large experimental uncertainties and the calculations use approximation techniques that render the calculations imprecise. It would be wonderful if experiments and calculations could reliably reach 10% uncertainties. In many cases, it's more like 25-50% uncertainty. Contrast this with the test of quantum electrodynamics in which theory and experiments agree to 12 decimal places. That is an amazing theory!
One of our projects seeks to understand the self-healing process and our experiments suggest that strange things are at work. When a material is burned with light, it does not recover just like ashes don't recombine into a log from which they came. We have been observing self-healing of molecules after being burned with highly intense light. Though this may seem weird to most physicists, it is commonplace in our lab, where we have been observing the phenomena for over a decade. The rates of thermodynamic processes are typically accelerated when the temperature is increased, but we find a dramatic decrease in the rates of self healing when we turn up the heat. Many such strange things characterize the underlying process.
As described in a previous post, we postulated that molecules in the company of others heal more quickly. We call these groupings domains. At elevated temperatures, thermal jiggling breaks up the domains and therefore self healing is suppressed. Do domains really exist? We haven't seen a domain but every measurement is consistent with the predictions of the theory. In other words, the mathematical structure corresponds to the reality of what we are measuring. At what point can we say that domains actually exist in our samples?
It takes lots of evidence for a theory to be accepted as the true description of a phenomena, and the picture that it suggests starts to become accepted slowly as the theory predicts other phenomena that were not intentionally added to the theory when it was originally formulated. For example, the famous Dirac Equation accurately predicts all of the relativistic corrections to the hydrogen atom, and naturally includes spin, which in the Schrodinger theory needs to be separately added -- an inelegant solution.
Dirac's theory of the electron worked flawlessly, but also had a major defect; it had negative energy solutions that were not observed. Dirac perseverated over this flaw and tried many approaches to sweep the problem under the proverbial rug. The negative energy solutions were later shown to be those of the positron, the anti-particle to the electron. The Dirac equation in effect predicted the existence of antimatter. Dirac quipped that his equation was smarter than him.
While to the best of our knowledge, the domain theory of self healing has not been making any new predictions, we have been testing it in new ways. One of my students (Ben) wrote a dissertation that focused on measurements of self healing under the influence of an electric field. An electric field induces an electric dipole moment in a molecule. The molecules in a domain will then interact with each other through the electric fields generated by the induced dipole moments. It is straightforward to calculate the energy of interaction, and thus determine if a domain grows or gets torn apart by the electric field.
Ben's calculations of the effect of the electric field on the distribution of domains in a sample, and thus it's healing properties, agrees well with his experiments. As the evidence accumulates, the domain model is not only holding up well, but predicts with reasonable precision what we observe.
The problem is that we have not actually "seen" a domain. It may be possible to do scattering experiments in which particles such as neutrons probe the microscopic structure of a domain. We have also been trying to come up with an explanation of the nature of a domain and how it is held together. My preferred picture is that a domain is made of molecules that are attached to a polymer chain (the domain model suggests wispy string-like domains and not clumps) rather than a string of molecules connected to each other. Our model allows us to determine the binding energy of a molecule to a domain, and experiments show it to be in the ballpark of hydrogen binding energies between molecule and polymer.
There is even a more intriguing possibility. This system may be exhibiting a phenomena that arises from the complexity of the system, and cannot be reduced to a description in terms of simpler units. Perhaps the picture of bonds and molecules starts falling apart when many large molecules interact with each other and with long polymer chains. The bond, which chemists hold sacred, is not as sacred as the Schrodinger equation, but rather a convenient form of book keeping that helps chemists understand what molecules are formed in reactions between smaller units.
Chances are that the explanation of the phenomena is more mundane than we propose. At worst case, we may be deluding ourselves into seeing something that is not there. We won't know until we do lots more work and other researchers test our models with new experiments.
too much research these days is focused on narrow topics, so new and interesting work might be missed simply because it falls outside the mainstream.
Our group enjoys the luxury of having a huge lead in this area of research. We may be sitting on a very interesting discovery. Though my talented grad students have done lots of excellent work to eliminate hypotheses, we are still puzzled by what we are observing. We continue to whittle away the false hypotheses in our quest to uncover the truth.
This lead is a two-edged sword. Our paper on the affect of the electric field on self-healing, which we submitted to Physical Review E and which appears on the Physics Archives, has been sent to half a dozen reviewers, all of whom have turned down the request to review. The editors subsequently notified us that they were going through a second wave of review requests. Because we are so far ahead, other researchers may not understand our work. Another unfortunate consequence of our sequential series of papers is that we reference a large number of our own papers. This is unavoidable since we have done all of the foundational work on the topic. But, it raises eyebrows in the community, which makes us cringe. However, I am happy to report that our paper eventually got two reviewers who made some good suggestions that we implemented, and the paper is in print.
I believe that too much research these days is focused on narrow topics, so new and interesting work might be missed simply because it falls outside the mainstream. Perhaps the huge success of science has lead to such a high volume of activity that researchers can only understand work in their own narrow area of specialization. We are too busy studying the scales to see the huge serpent wriggling in our midst. This is not to say that one research paradigm should be pursued at the exclusion of all others.
A balance needs to be struck between detailed narrow work, which can miss the big stuff, and broader investigations, which may temporarily lead us astray but eventually lead to something big. Though the process is sloppy, science has a way of eventually sorting through the trash and finding the real gems. Hopefully, my lifetime commitment to the process will eventually uncover something of value. Even if I fail, I take comfort in the fact that we are all part of a highly interconnected human network, where each part contributes to the success of the whole.
We must take pleasure in the process of discovery, give it our best shot, and see where it takes us. I am fortunate to be part of the most incredible journey, keeping me excited throughout my life. I still suffer fitful nights, being kept awake by ideas running around my head and starting the morning with impatience for all the administrative obligations that keep me away from my true passion. The professorial life still gives a fair amount of time and more importantly encouragement to pursue the big ideas, giving me the resolve to pursue my ideas even if in a quixotic manner. For this I am eternally grateful.
Showing posts with label self healing. Show all posts
Showing posts with label self healing. Show all posts
Thursday, April 3, 2014
Tuesday, March 26, 2013
A refreshing review of our new paper on a model of self healing
There are times when one of my less stellar papers, in my opinion, gets accepted for publication without trouble; and, at other times, what I think are very significant papers have lots of trouble. Perhaps this is a matter of taste, or perhaps papers that are outside the norm are misunderstood and therefore rejected.
As a case in point, I was invited to submit a paper to a special issue of a journal dedicated to self-healing polymers. The focus of this special issue is on polymers that are made to self heal after mechanical cracking by incorporating tiny reserves of monomer that runs into cracks as they form, thus filling them and repairing the material.
Our work is very different, so I thought that our invitation was an effort by the editors to broaden the scope of the journal. However, we were shocked to find that our paper was rejected without review based on the assessment that the work was "incremental." We responded to the editors, reminding them that we were invited to write the paper. The next email informed us that the editors had made an error,and that the paper would go out for review.
Two reviewers responded and one of them recommended that our paper be rejected on the grounds that the work was incremental. However, the editor gave us the opportunity to respond.
In the meantime, we had found data at the extremes that were inconsistent with our model. We fixed the model with one simple change in the underlying assumptions, and the new model fit all of our data. (This in itself is a very interesting story which I will report on later.) We revised the manuscript to include the new data and resubmitted it.
The editors sought an opinion from a forth reviewer. An excerpt from his/her comments, follow,"One of the reviewers evidently commented that this work is incremental. However, I don’t agree. The authors are clearly refining their model, and this is an entirely new set of data and observations. The authors search for a better physical understanding will naturally require a significant amount of investigation, and it is helpful to the community to see the work as it unfolds, not wait 25 years for a definitive explain‐all paper that may never appear."
We, of course, agree with this reviewer, and are glad that this paper came to a happy ending, especially in light of the fact that I believe that our new results provide important insights that are taking us a step closer to understanding a new phenomena.
The new version was accepted with minor revisions suggested, which we made. The paper was then accepted and the page proofs arrived a couple weeks ago. We fixed minor typos and now the paper is in the queue for publication in the early summer. I ust learned today that the electronic version is already available online. It's ironic that we were even invited to provide an artistic rendition of a figure that might be used as a cover photo. What a difference a revision makes! From incremental to cover story material with the change of one variable!
For the interested reader, below is the introductory paragraph, which describes our work and how it differs from the norm, "Structural damage and degradation of a polymer is usually associated with cracking. Mitigating damage or developing methods to promote healing in polymeric materials after cracking is an active area of research motivated by its practical utility. White and coworkers reported on a structural polymeric material with the ability to autonomically self-repair cracks. Such polymers incorporate a microencapsulated healing agent that is released in the cracking process with polymerization being triggered by contact of a catalyst with the healing agent, thus bonding the crack faces. White observed as much as 75% recovery in toughness.
"Our work presented here is different in two regards. First, the sample is a dye-doped polymer rather than a neat polymer and the degradation process is through optically induced burning, so chemical changes are induced rather than solely mechanical/structural damage – though cracking can accompany burning. The dopant molecules thus mediate the phenomena. The degree of damage is observed using optical techniques, the simplest of which is the detection of a color change. Secondly, the healing process is a microscopic one, originating at a molecular level that we believe involves a cooperative process of aggregates of molecules. The polymers of interest to our work have applications as optical materials where photodegradation is a common cause of optical and optoelectronic device failures, either as catastrophic failure or a slow deterioration of performance."
The final paragraph in the conclusion succinctly states what we believe is cool about our work, as follows, "The concept that a material would exhibit such complex behavior without intentional design by the experimenter is an interesting one. Though self-healing is a process with great practical utility, it is intriguing that nature has been kind enough to provide an inherently smart material system that appears to behave in a way contrary to most others; it mediates recovery in a world in which irreversible damage is the norm. Further advances in understanding the physics underlying this phenomena will surely enable new applications that require materials to withstand high light intensities; and, may lead to new physics.
Now the next battle...
As a case in point, I was invited to submit a paper to a special issue of a journal dedicated to self-healing polymers. The focus of this special issue is on polymers that are made to self heal after mechanical cracking by incorporating tiny reserves of monomer that runs into cracks as they form, thus filling them and repairing the material.
Our work is very different, so I thought that our invitation was an effort by the editors to broaden the scope of the journal. However, we were shocked to find that our paper was rejected without review based on the assessment that the work was "incremental." We responded to the editors, reminding them that we were invited to write the paper. The next email informed us that the editors had made an error,and that the paper would go out for review.
Two reviewers responded and one of them recommended that our paper be rejected on the grounds that the work was incremental. However, the editor gave us the opportunity to respond.
In the meantime, we had found data at the extremes that were inconsistent with our model. We fixed the model with one simple change in the underlying assumptions, and the new model fit all of our data. (This in itself is a very interesting story which I will report on later.) We revised the manuscript to include the new data and resubmitted it.
The editors sought an opinion from a forth reviewer. An excerpt from his/her comments, follow,"One of the reviewers evidently commented that this work is incremental. However, I don’t agree. The authors are clearly refining their model, and this is an entirely new set of data and observations. The authors search for a better physical understanding will naturally require a significant amount of investigation, and it is helpful to the community to see the work as it unfolds, not wait 25 years for a definitive explain‐all paper that may never appear."
We, of course, agree with this reviewer, and are glad that this paper came to a happy ending, especially in light of the fact that I believe that our new results provide important insights that are taking us a step closer to understanding a new phenomena.
The new version was accepted with minor revisions suggested, which we made. The paper was then accepted and the page proofs arrived a couple weeks ago. We fixed minor typos and now the paper is in the queue for publication in the early summer. I ust learned today that the electronic version is already available online. It's ironic that we were even invited to provide an artistic rendition of a figure that might be used as a cover photo. What a difference a revision makes! From incremental to cover story material with the change of one variable!
For the interested reader, below is the introductory paragraph, which describes our work and how it differs from the norm, "Structural damage and degradation of a polymer is usually associated with cracking. Mitigating damage or developing methods to promote healing in polymeric materials after cracking is an active area of research motivated by its practical utility. White and coworkers reported on a structural polymeric material with the ability to autonomically self-repair cracks. Such polymers incorporate a microencapsulated healing agent that is released in the cracking process with polymerization being triggered by contact of a catalyst with the healing agent, thus bonding the crack faces. White observed as much as 75% recovery in toughness.
"Our work presented here is different in two regards. First, the sample is a dye-doped polymer rather than a neat polymer and the degradation process is through optically induced burning, so chemical changes are induced rather than solely mechanical/structural damage – though cracking can accompany burning. The dopant molecules thus mediate the phenomena. The degree of damage is observed using optical techniques, the simplest of which is the detection of a color change. Secondly, the healing process is a microscopic one, originating at a molecular level that we believe involves a cooperative process of aggregates of molecules. The polymers of interest to our work have applications as optical materials where photodegradation is a common cause of optical and optoelectronic device failures, either as catastrophic failure or a slow deterioration of performance."The final paragraph in the conclusion succinctly states what we believe is cool about our work, as follows, "The concept that a material would exhibit such complex behavior without intentional design by the experimenter is an interesting one. Though self-healing is a process with great practical utility, it is intriguing that nature has been kind enough to provide an inherently smart material system that appears to behave in a way contrary to most others; it mediates recovery in a world in which irreversible damage is the norm. Further advances in understanding the physics underlying this phenomena will surely enable new applications that require materials to withstand high light intensities; and, may lead to new physics.
Now the next battle...
Sunday, August 12, 2012
Perhaps this time it may be right - taking a big chance
When preparing my talk for SPIE a couple days ago, I drew the PMMA polymer chain with a molecule drawing program and added a few DO11 tautomer molecules to see where they would fit. Miraculously, as a plopped the DO11 molecules on the page, I immediately saw that the NH from the DO11 tautomer cozies up to one oxygen in the PMMA polymer chain while the OH group naturally attaches itself to another oxygen in the chain, as shown above. And he energy? You got it; the sum of the two hydrogen bound energies is 0.30eV, a match. The table below shows the energies of four types of hydrogen bonds.
There are always other possibilities that we have not yet considered, but this smells right. Perhaps we are onto something. Future experimentalists will allow us to test this hypothesis and zero in on what is going on when a molecule self heals.
This project has been one huge puzzle, were each new experiment presents to us a new piece. It reminds me of how the discovers of the structure of DNA (Crick, Watson, and Wilson ) pieced together cardboard cutouts of molecules to guess its molecular structure, and confirmed their results using x-ray scattering data from Rosalind Franklin. Incidentally, the story behind Franklin's contributions to the discovery of DNA and not being recognized at the time makes for interesting reading. I also recommend readers to check out Schrodinger's guess as the structure of DNA using simple physics principles. The title of his very thin but fascinating book is "What Is Life?: with 'Mind and Matter' and 'Autobiographical Sketches'"
I can imagine the thrill of discovery experienced by Crick, Wason, Wilson, and Farklin. From little cardboard pieces and an "X" on a piece of film from an x-ray scattering experiment (shown above), they revolutionized our understanding of the workings of DNA. Ironically, the forces that hold together the double helix reside in the hydrogen bond, the very forces that seem to be at work in our molecule/polymer system.
I am preparing my talks this morning, and plan to go on a limb proposing stating that the interaction between a DO11 molecule and a polymer chain through hydrogen bonding underpins the phenomena of self healing. I am not a chemist and have a naive view of the intricacies of how molecules interact. But, I hope that my bold proposal will result in good feedback form my audience that will help us fine tune our models of the mechanisms of self healing.
I have been very excited in recent months by all of the discoveries that we are making. Even if they end up being wrong, the process of the search for the truth is exhilarating. Gotta run. Too much to do. And again, sorry for the typos!
Wednesday, July 25, 2012
A pendent necklace and a new insight about self-healing molecules
In a recent post on our research on self healing, I discussed our new theory, which is posted in the Physics Archives (see it here). The paper has been accepted for publication in the Journal of Chemical Physics and will appear soon.
We used lots of data as input to construct the model, which took years to complete. Data that seemed to support one model initially would later be contradicted by additional data. Over time, the model evolved into a coherent picture as more hypotheses were eliminated by experiments. Finally, we had a model that fit the data AND had as its cornerstone the formation of domains of molecules that together, would help a damaged molecule heal.
There is no direct evidence for domain formation, though the behavior of all the experiments to date are consistent with this model, and only this model. Remove the domains and the predicitve power of the theory is lost. The burning question pertains to the nature of the domains. What are they? Are they clumps of molecules or molecules that are somehow stuck to the same polymer chain? What is the nature of the force that keeps the domains together, and how is it that a domain of healthy molecules acts to promote healing in a damaged one?
We may be closer to an answer.
The lab is in a wonderful buzz of activity with lots of new measurements -- always an exciting time. There are bold new hypotheses based on initial data that generalize our model, followed by letdowns after new data or a more detailed calculation proves us wrong. The process is highly stimulating. I can just smell it; something new and wonderful is brewing.
In the midst of all this activity, I found myself sitting at my computer writing my conference paper for SPIE, where I will give a couple of papers in August. I completed writing the introduction and then explained our new model. What next? I needed something new that did not detract from the presentations of my students. So, I drew the molecular structures of the polymer and the molecules, and started to play with them, rotating this one this way and that one here, etc.
In less than a few minutes, I realized again that a molecule could stick to a polymer through what is called hydrogen bond -- an attractive force between a hydrogen molecule and in this case, an oxygen, very much like the forces found between water molecules. This thought had crossed my mind in the past, and is indeed a motivation for a subset of projects. However, having all this jumbled data running around my head made me realize that Shiva, my coauthor on the theory paper, had already determined the three parameters of our model, one of which is the force that binds the molecule to a domain. If the molecules are sticking to the polymer chain through a hydrogen bond, the hydrogen bond energy should have the same value as the corresponding parameter in the model.
This is an excellent example of a model that we built to explain the data is now guiding us in figuring out what is going on.
I got on the internet and searched for hydrogen boding and found a table of numbers. The energy between a hydrogen and oxygen was one of the first values listed, at 0.3 eV. Then I nervously clicked through the directory tree on my computer to find its measured value. As I scrolled to the table with the results, my eyes focused on the value of the lambda parameter -- 0.29 eV with an uncertainty of 0.01. The two matched!
It is not often that things work out this easily, so I considered the next question, and that was how self-healing is mediated by molecules attached to a chain. A polymer with molecules connected by hydrogen bonding looks a lot like a necklace (polymer) with pendents (molecules) thrown on the night dresser as shown in the figure below.
The hypothesis that I proposed is as follows. (a) When a molecule absorbs a photon, (b) it breaks into two fragments that are charged. There is evidence from earlier work that charged species are involved. One of the fragments is fixed in place by the polymer and (c-e) the other hops from molecule to molecule along the chain (f) until it finds its mate and recombines.
An alternative explanation is that the attached fragment attracts a small fragment from a neighboring molecule. The neighboring molecule then attracts a fragment from its neighbor, and so on, which propogates down chain like a wave of fans at a stadium until the original damaged piece combines with an adjacent fragment. The more molecules in the domain (i.e number of molecules attached to a polymer chain), the bigger the chance that there is a contiguous path for the fragment to find a mate.
This is indeed an exciting time. In addition to this work, there are other very exciting developments that I will post in the near future. Breakthroughs can be addictive. I can't wait for the next one!
We used lots of data as input to construct the model, which took years to complete. Data that seemed to support one model initially would later be contradicted by additional data. Over time, the model evolved into a coherent picture as more hypotheses were eliminated by experiments. Finally, we had a model that fit the data AND had as its cornerstone the formation of domains of molecules that together, would help a damaged molecule heal.
There is no direct evidence for domain formation, though the behavior of all the experiments to date are consistent with this model, and only this model. Remove the domains and the predicitve power of the theory is lost. The burning question pertains to the nature of the domains. What are they? Are they clumps of molecules or molecules that are somehow stuck to the same polymer chain? What is the nature of the force that keeps the domains together, and how is it that a domain of healthy molecules acts to promote healing in a damaged one?
We may be closer to an answer.
The lab is in a wonderful buzz of activity with lots of new measurements -- always an exciting time. There are bold new hypotheses based on initial data that generalize our model, followed by letdowns after new data or a more detailed calculation proves us wrong. The process is highly stimulating. I can just smell it; something new and wonderful is brewing.
In the midst of all this activity, I found myself sitting at my computer writing my conference paper for SPIE, where I will give a couple of papers in August. I completed writing the introduction and then explained our new model. What next? I needed something new that did not detract from the presentations of my students. So, I drew the molecular structures of the polymer and the molecules, and started to play with them, rotating this one this way and that one here, etc.
In less than a few minutes, I realized again that a molecule could stick to a polymer through what is called hydrogen bond -- an attractive force between a hydrogen molecule and in this case, an oxygen, very much like the forces found between water molecules. This thought had crossed my mind in the past, and is indeed a motivation for a subset of projects. However, having all this jumbled data running around my head made me realize that Shiva, my coauthor on the theory paper, had already determined the three parameters of our model, one of which is the force that binds the molecule to a domain. If the molecules are sticking to the polymer chain through a hydrogen bond, the hydrogen bond energy should have the same value as the corresponding parameter in the model.
This is an excellent example of a model that we built to explain the data is now guiding us in figuring out what is going on.
I got on the internet and searched for hydrogen boding and found a table of numbers. The energy between a hydrogen and oxygen was one of the first values listed, at 0.3 eV. Then I nervously clicked through the directory tree on my computer to find its measured value. As I scrolled to the table with the results, my eyes focused on the value of the lambda parameter -- 0.29 eV with an uncertainty of 0.01. The two matched!
It is not often that things work out this easily, so I considered the next question, and that was how self-healing is mediated by molecules attached to a chain. A polymer with molecules connected by hydrogen bonding looks a lot like a necklace (polymer) with pendents (molecules) thrown on the night dresser as shown in the figure below.
The hypothesis that I proposed is as follows. (a) When a molecule absorbs a photon, (b) it breaks into two fragments that are charged. There is evidence from earlier work that charged species are involved. One of the fragments is fixed in place by the polymer and (c-e) the other hops from molecule to molecule along the chain (f) until it finds its mate and recombines.An alternative explanation is that the attached fragment attracts a small fragment from a neighboring molecule. The neighboring molecule then attracts a fragment from its neighbor, and so on, which propogates down chain like a wave of fans at a stadium until the original damaged piece combines with an adjacent fragment. The more molecules in the domain (i.e number of molecules attached to a polymer chain), the bigger the chance that there is a contiguous path for the fragment to find a mate.
This is indeed an exciting time. In addition to this work, there are other very exciting developments that I will post in the near future. Breakthroughs can be addictive. I can't wait for the next one!
Thursday, July 5, 2012
Zeroing in on the cause of self healing
As I have mentioned in the past, one of our biggest projects seeks to develop an understanding of the mysterious self healing process following damage to a molecule by a zap of light. Recently, a former graduate and I developed a model of the healing process that hinges on the formation of domains of molecules. Members of these domains are highly cooperative: they accelerate the healing of a damaged molecule in proportion to the size of the group and they prevent their comrades from being damaged. This behavior is as strange from the sociological perspective as it is from the underlying physics. Why do the molecules aggregate and how does their community enhance healing and prevent physical damage?
We have gone out on a limb and made what I believe is a bold assertion; that there are forces between the molecules that cause them to aggregate, and that these same forces are responsible for healing. Such an assertion would be just a wild guess if it were not for lots of data t
hat we find to be consistent with our model. With only three parameters, our data fits the model as a function of temperature, concentration, time, and intensity. The model also makes predictions beyond our present experimental capabilities, so it will gain acceptance only if it holds up to future scrutiny.
When submitting something this interesting (at least to us) that may go past the present paradigms (Shiva got some lifted eyebrows and jaw dropping during an interview talk, which turned to nods of approval after he presented supporting evidence), one always worries that the work will not be understood. There are many examples of Nobel-prizewinning work being rejected by a journal. In our case, the first journal did not even send the paper out to review, claiming that our work was not appropriate. How can a physics paper not be appropriate to a physics journal?
Of course, I have no illusions that this is a Nobel-prizewinning paper, but if the underlying mechanism is found to be new, it could very well end up being a significant achievement for whoever makes this discovery.
Rather than fight the editor, back in mid May, we sent the paper to a second journal of equal quality. Then we waited. I was still concerned that the reviewers may not see the importance of the work. But alas, they accepted it on the first pass, suggesting only minor revisions. And it was also incredibly fast given the nature of our paper. The first reviewer summarizes the paper as follows,
"This interesting manuscript continues the authors' work aimed at discovering the mechanism behind the observation of self-healing of photoluminescence in chromophore doped polymers. The authors have proposed a phenomenological model for their observations that is able to predict aspects of the time, temperature, concentration and intensity dependence. The model focuses on the formation of dye domains in the polymer and studies the dynamics of these..."
Then (s)he goes on,
"While these are interesting results, the manuscript could be more satisfying if the authors did more to understand the physical mechanisms behind the model. Some well-considered speculation on the materials physics in the conclusions would suffice. "
We tried to hold back on speculation, but this review gives us an opportunity to present what we think is happening. Incidentally, the reviewer is right that we need to work more on the mechanisms, which is exactly what we are doing now. We are already getting data that is pointing at the mechanism, but its still too premature to mention.
The second reviewer made no suggestions for revisions and believes that the paper is in good shape in its present form. (S)he writes,
"In this paper authors present a model on photodegradation/self-healing kinetics of dye molecules doped in a polymer matrix. This investigation is an extension of their previous work. Using phenomenological arguments the authors generalize their model. They allow (implicitly) for association of dye molecules which form correlated domains interacting with the polymer matrix. A healing rate is assumed to be proportional to the number of undamaged molecules in a correlated region and a decay rate is proportional to the intensity normalized to the correlation volume. The model proposed by the authors predicts decay and recovery of the population of doped molecules. The results of the theory are successfully tested with experimental data.
"The paper is generally well written and contains several interesting results. I recommend it to be published as it stands..."
The next step will be to determine the physical significance of these parameters. I am excited by the prospects that we may be looking at some very new physics because this process is like no other that I have ever seen. As I sit at my computer bogged down with lots of administrative tasks, new physics is in the air. I hope to be able to get back with pencil and paper to work on the next set of ideas. But first I need to work on some proposals so that we have the resources to do lots of wonderful work in the future. And as penance for writing proposals, I also have some that I need to review. Similarly, I have a pileup of papers to review.
Hopefully in my next post I will report on even more interesting physics. On another project, something very exciting is brewing. Again, new physics! Until then, ...
We have gone out on a limb and made what I believe is a bold assertion; that there are forces between the molecules that cause them to aggregate, and that these same forces are responsible for healing. Such an assertion would be just a wild guess if it were not for lots of data t
hat we find to be consistent with our model. With only three parameters, our data fits the model as a function of temperature, concentration, time, and intensity. The model also makes predictions beyond our present experimental capabilities, so it will gain acceptance only if it holds up to future scrutiny.When submitting something this interesting (at least to us) that may go past the present paradigms (Shiva got some lifted eyebrows and jaw dropping during an interview talk, which turned to nods of approval after he presented supporting evidence), one always worries that the work will not be understood. There are many examples of Nobel-prizewinning work being rejected by a journal. In our case, the first journal did not even send the paper out to review, claiming that our work was not appropriate. How can a physics paper not be appropriate to a physics journal?
Of course, I have no illusions that this is a Nobel-prizewinning paper, but if the underlying mechanism is found to be new, it could very well end up being a significant achievement for whoever makes this discovery.
Rather than fight the editor, back in mid May, we sent the paper to a second journal of equal quality. Then we waited. I was still concerned that the reviewers may not see the importance of the work. But alas, they accepted it on the first pass, suggesting only minor revisions. And it was also incredibly fast given the nature of our paper. The first reviewer summarizes the paper as follows,
"This interesting manuscript continues the authors' work aimed at discovering the mechanism behind the observation of self-healing of photoluminescence in chromophore doped polymers. The authors have proposed a phenomenological model for their observations that is able to predict aspects of the time, temperature, concentration and intensity dependence. The model focuses on the formation of dye domains in the polymer and studies the dynamics of these..."
Then (s)he goes on,
"While these are interesting results, the manuscript could be more satisfying if the authors did more to understand the physical mechanisms behind the model. Some well-considered speculation on the materials physics in the conclusions would suffice. "
We tried to hold back on speculation, but this review gives us an opportunity to present what we think is happening. Incidentally, the reviewer is right that we need to work more on the mechanisms, which is exactly what we are doing now. We are already getting data that is pointing at the mechanism, but its still too premature to mention.
The second reviewer made no suggestions for revisions and believes that the paper is in good shape in its present form. (S)he writes,
"In this paper authors present a model on photodegradation/self-healing kinetics of dye molecules doped in a polymer matrix. This investigation is an extension of their previous work. Using phenomenological arguments the authors generalize their model. They allow (implicitly) for association of dye molecules which form correlated domains interacting with the polymer matrix. A healing rate is assumed to be proportional to the number of undamaged molecules in a correlated region and a decay rate is proportional to the intensity normalized to the correlation volume. The model proposed by the authors predicts decay and recovery of the population of doped molecules. The results of the theory are successfully tested with experimental data.
"The paper is generally well written and contains several interesting results. I recommend it to be published as it stands..."
The next step will be to determine the physical significance of these parameters. I am excited by the prospects that we may be looking at some very new physics because this process is like no other that I have ever seen. As I sit at my computer bogged down with lots of administrative tasks, new physics is in the air. I hope to be able to get back with pencil and paper to work on the next set of ideas. But first I need to work on some proposals so that we have the resources to do lots of wonderful work in the future. And as penance for writing proposals, I also have some that I need to review. Similarly, I have a pileup of papers to review.
Hopefully in my next post I will report on even more interesting physics. On another project, something very exciting is brewing. Again, new physics! Until then, ...
Saturday, April 28, 2012
Expertise is stagnation
A PhD degree in Physics represents a new contribution to the body of scientific knowledge; but, it is more than that. The power of new physics is not in the generation of new information, though that is one important side effect. It is the new or deeper understanding of the nature of how how things work that are its treasures. It is not a process in which a student goes through steps 1, 2, 3, 4, 5, and then is done, but a journey of exploration that often can come up empty handed. Putting in lots of time and effort is not enough. There needs to be a tangible result that adds to the corpus of Physics.
The dissertation documents the contribution a PhD student has made to science. The process of writing up ones work and results inevitably uncovers errors, weakness in logic, and oversights that need to be addressed. As such, the period of writing prior to submitting the dissertation to the examining committee is filled with stress. One never knows if the errors that are uncovered will be fatal to the thesis. Many students are unaware of the magnitude of the demands. In the end, it has to be right (internally self consistent and in accord with the rest of Physics) and the dissertation committee needs to be convinced that the work is significant enough to be worthy of a PhD.
I write this as Shiva is finishing up his dissertation and getting ready to defend. The work is excellent and I believe that it will be a major contribution to the body of Physics. We have proposed a novel model based on experimental observations that pass the test of simplicity - with only three parameters all of the data is explained over a huge range of conditions; and, it suggests new physics, namely, that a polymer mediates the interaction between molecules in a way that coerces them into healing after they are damaged by powerful laser pulses. This phenomena is new and its explanation is bound to be controversial; and it may end up being wrong...
In the process of writing his dissertation, Shiva had to make major changes to the analysis of the data, needed to take additional data, and had to take into account complications that had slipped by our attention. Each time he thought he was done, there always seemed to be one more thing to check, one more experiment to do or one more calculation to correct. I can imagine the ups and downs associated with the relief of being done followed by the anxiety over a potential error that could mean the downfall of the dissertation.
As I write this post, I believe that his dissertation is finally done, and I am comfortable with the scrutiny that is to come from the committee. There are certainly loose ends that will be addressed, but those can be completed prior to the oral or as minor revisions after the defense.
I have advised dozens of graduate students over the years. Many have vocalized the childhood question asked of parents during a long road trip, "are we there yet?" Others think about their progress quietly while some may assume that they will get a degree as a result of making an effort. In the end, only students who persevere after what appears to be endless failure and hardships will make it through to the end. The process includes hard work, independence, cleverness, deep thinking, and extreme grit. As a result, the future employer of a PhD physicist is not getting just an expert. If that is what they think, the employers miss the best part. They are getting an individual who is fearless in the face of new challenges that require a nonexistent expertise.
Expertise is stagnation. Dealing with the unknown is wrought with fear, insecurity, and doubt; but, there is an air of exhilaration from the possibility of successes. The PhD student builds problem-solving skills and the ability to think beyond his or her knowledge base and to thrive in a world of uncertainty. It is character not just expertise that the PhD represents.
The beauty of nature is that she is consistent and filled with rich and wondrous phenomena. On the downside, she holds the highest standards and is intolerant of contradiction. I am glad to have a job that allows me to be continually confused, insecure and humbled. Every new piece of knowledge or expertise gained drives me into a new unknown realm. I am simultaneously frustrated and ecstatic, perhaps a required blend of opposites that lead to fulfillment and happiness.
Having acquired a taste for this life, I wish the same for my students. As Shiva is finishing up, I look to the next generation of students coming up through the ranks in the hopes that they too will succeed.
The dissertation documents the contribution a PhD student has made to science. The process of writing up ones work and results inevitably uncovers errors, weakness in logic, and oversights that need to be addressed. As such, the period of writing prior to submitting the dissertation to the examining committee is filled with stress. One never knows if the errors that are uncovered will be fatal to the thesis. Many students are unaware of the magnitude of the demands. In the end, it has to be right (internally self consistent and in accord with the rest of Physics) and the dissertation committee needs to be convinced that the work is significant enough to be worthy of a PhD.
I write this as Shiva is finishing up his dissertation and getting ready to defend. The work is excellent and I believe that it will be a major contribution to the body of Physics. We have proposed a novel model based on experimental observations that pass the test of simplicity - with only three parameters all of the data is explained over a huge range of conditions; and, it suggests new physics, namely, that a polymer mediates the interaction between molecules in a way that coerces them into healing after they are damaged by powerful laser pulses. This phenomena is new and its explanation is bound to be controversial; and it may end up being wrong...
In the process of writing his dissertation, Shiva had to make major changes to the analysis of the data, needed to take additional data, and had to take into account complications that had slipped by our attention. Each time he thought he was done, there always seemed to be one more thing to check, one more experiment to do or one more calculation to correct. I can imagine the ups and downs associated with the relief of being done followed by the anxiety over a potential error that could mean the downfall of the dissertation.
As I write this post, I believe that his dissertation is finally done, and I am comfortable with the scrutiny that is to come from the committee. There are certainly loose ends that will be addressed, but those can be completed prior to the oral or as minor revisions after the defense.
I have advised dozens of graduate students over the years. Many have vocalized the childhood question asked of parents during a long road trip, "are we there yet?" Others think about their progress quietly while some may assume that they will get a degree as a result of making an effort. In the end, only students who persevere after what appears to be endless failure and hardships will make it through to the end. The process includes hard work, independence, cleverness, deep thinking, and extreme grit. As a result, the future employer of a PhD physicist is not getting just an expert. If that is what they think, the employers miss the best part. They are getting an individual who is fearless in the face of new challenges that require a nonexistent expertise.
Expertise is stagnation. Dealing with the unknown is wrought with fear, insecurity, and doubt; but, there is an air of exhilaration from the possibility of successes. The PhD student builds problem-solving skills and the ability to think beyond his or her knowledge base and to thrive in a world of uncertainty. It is character not just expertise that the PhD represents.
The beauty of nature is that she is consistent and filled with rich and wondrous phenomena. On the downside, she holds the highest standards and is intolerant of contradiction. I am glad to have a job that allows me to be continually confused, insecure and humbled. Every new piece of knowledge or expertise gained drives me into a new unknown realm. I am simultaneously frustrated and ecstatic, perhaps a required blend of opposites that lead to fulfillment and happiness.
Having acquired a taste for this life, I wish the same for my students. As Shiva is finishing up, I look to the next generation of students coming up through the ranks in the hopes that they too will succeed.
Thursday, August 11, 2011
My voice from the past
A while ago, David Bradly, a reporter from ScienceBase had contacted me about a paper from my group on self healing in a molecule called AF455. He wrote a short news piece on our work. After the piece was posted, he contacted me with additional questions. In response, I shot him an email, which he posted in its entirety. This was back in April of 2007, more than 4 years into my past.
Just 5 minutes ago, I was searching for articles related to our research and ran across my email. I tend to write emails from the top of my head, without much editing, so it was eerie to see myself in an unguarded moment. In effect, it was my own voice form the past, real and uncensored. When writing for the public, as I do in papers and proposals - and even in this blog, I choose my wording carefully, though often not with good results. While you may not notice the tone, reading this email rekindles in me the excitement of discovery that I was feeling at that time. It is better than any diary entry.
I am glad that David Bradley posted this email, which is truly a window into my past. It is reproduced below. As you may have guessed, he asked me about applications of our work.
Dear David,
The molecule AF455 is indeed complex, and that is what makes its irreversibility so puzzling. The DO11 dye, which we previously studied for reversibility is a relatively small molecule; and, the mechanisms for the recovery is the breaking up of dimers that form in the degradation process. This requires the molecules to be able to move around a bit. AF455 clearly can not move around easily, so another mechanism must be responsible.
Any device that operates at high intensity, such as lasers, displays, and all-optical switches and logic, suffer from photodegradation. Solid state lasers, for example, live longer than ion lasers and dye lasers; but, dye lasers have much more flexibility is the range of colors that are available. Polymer displays, on the other hand can be mechanically flexible and can be used to host all sorts of organic molecules. The general theme is that organic molecules have a much broader pallet of what they can do, but, they are not as stable.
So in our work, we are not so much interested in targeting specific applications. Rather, we want to understand the mechanisms for recovery since most materials degrade irreversibly. And here we have two very different molecules that behave the same way. There is one similarity. We discovered this property by accident!
If a material absorbs light strongly, it will damage when the absorbed optical power reaches the material's damage threshold. In applications where the material is transparent, light can be absorbed through a two-photon absorption process. Not as much light is absorbed in the process, but, over long-enough periods of time, cumulative effects cause the material to degrade.
Bright light can cause all sorts of things to happen in a material. If it induces a chemical reaction that causes a molecule to break apart into pieces, that process is irreversible. On the other hand, if the light causes the molecules to change shape into a form that no longer absorbs light or perhaps causes some charge to jump from one side of the molecule to the other, this change is reversible. The trick is to find materials that are not killed by the zap of laser, but that prefer to take a nap.
Another intriguing observation is that when such molecules wear out, rest, then recover many times, they seem to degrade more slowly and recover to a higher level of efficiency upon further cycling. It's like a weight lifter that gets stronger after each workout. So, it may be possible to make our molecules more buff by giving them a good workout. We observed this kind of response in the DO11 dye, but have not seen it in the AF455 dye.
So, while we see two-photon absorption (TPA) as a universal nuisance that destroys materials, and that's the motivation for our studies, there are many important applications. Two-photon absorption is strongest where the light intensity is the highest, and is ideal in applications where a chemical reaction in a material operates above a certain threshold power. The important consideration is that for absorption to occur, two photons must participate.
Cancer therapies are one such application. The patient drinks a cocktail of molecules that like to stick to a particular type of tumor cell. Also, these molecules are tailored to be strong two-photon absorbers to a color of light to which cells and flesh are transparent. Then, just aim a laser beam at the tumor right through the skin. In this way, only the tumor cells are zapped. Since the skin is not perfectly transparent, it will also absorb some of this light, causing a bit of damage. Ideally, you want to make the strength of two-photon absorption as high as possible so that the amount of damage to the tumor is as big as possible relative to the damage to healthy cells. You want the special molecules to live as long as possible so that they can be repeatedly zapped without the patient having to ingest more of the cocktail, which could have side effects.
Since TPA is a process where two photons are simultaneously absorbed, it can be used to drive chemical reactions at the intersection point of two beams of light. As an example, a liquid can be made to turn solid (i.e. polymerize) at the crossing points. In this way, a three-dimensional object can be made piece by piece inside the liquid, such as gears, shafts, and other nano-scale parts. So, it's like having the ultimate nanolab.
So, TPA is something that is simultaneously very useful in important applications; but, can be a nuisance in all applications that require the use of light. We are thinking more about ways to make a molecule snooze to help it recover rather than find more ways to put it to work. Happy dreams!
Mark
Just 5 minutes ago, I was searching for articles related to our research and ran across my email. I tend to write emails from the top of my head, without much editing, so it was eerie to see myself in an unguarded moment. In effect, it was my own voice form the past, real and uncensored. When writing for the public, as I do in papers and proposals - and even in this blog, I choose my wording carefully, though often not with good results. While you may not notice the tone, reading this email rekindles in me the excitement of discovery that I was feeling at that time. It is better than any diary entry.
I am glad that David Bradley posted this email, which is truly a window into my past. It is reproduced below. As you may have guessed, he asked me about applications of our work.
Dear David,
The molecule AF455 is indeed complex, and that is what makes its irreversibility so puzzling. The DO11 dye, which we previously studied for reversibility is a relatively small molecule; and, the mechanisms for the recovery is the breaking up of dimers that form in the degradation process. This requires the molecules to be able to move around a bit. AF455 clearly can not move around easily, so another mechanism must be responsible.
Any device that operates at high intensity, such as lasers, displays, and all-optical switches and logic, suffer from photodegradation. Solid state lasers, for example, live longer than ion lasers and dye lasers; but, dye lasers have much more flexibility is the range of colors that are available. Polymer displays, on the other hand can be mechanically flexible and can be used to host all sorts of organic molecules. The general theme is that organic molecules have a much broader pallet of what they can do, but, they are not as stable.
So in our work, we are not so much interested in targeting specific applications. Rather, we want to understand the mechanisms for recovery since most materials degrade irreversibly. And here we have two very different molecules that behave the same way. There is one similarity. We discovered this property by accident!
If a material absorbs light strongly, it will damage when the absorbed optical power reaches the material's damage threshold. In applications where the material is transparent, light can be absorbed through a two-photon absorption process. Not as much light is absorbed in the process, but, over long-enough periods of time, cumulative effects cause the material to degrade.
Bright light can cause all sorts of things to happen in a material. If it induces a chemical reaction that causes a molecule to break apart into pieces, that process is irreversible. On the other hand, if the light causes the molecules to change shape into a form that no longer absorbs light or perhaps causes some charge to jump from one side of the molecule to the other, this change is reversible. The trick is to find materials that are not killed by the zap of laser, but that prefer to take a nap.
Another intriguing observation is that when such molecules wear out, rest, then recover many times, they seem to degrade more slowly and recover to a higher level of efficiency upon further cycling. It's like a weight lifter that gets stronger after each workout. So, it may be possible to make our molecules more buff by giving them a good workout. We observed this kind of response in the DO11 dye, but have not seen it in the AF455 dye.
So, while we see two-photon absorption (TPA) as a universal nuisance that destroys materials, and that's the motivation for our studies, there are many important applications. Two-photon absorption is strongest where the light intensity is the highest, and is ideal in applications where a chemical reaction in a material operates above a certain threshold power. The important consideration is that for absorption to occur, two photons must participate.
Cancer therapies are one such application. The patient drinks a cocktail of molecules that like to stick to a particular type of tumor cell. Also, these molecules are tailored to be strong two-photon absorbers to a color of light to which cells and flesh are transparent. Then, just aim a laser beam at the tumor right through the skin. In this way, only the tumor cells are zapped. Since the skin is not perfectly transparent, it will also absorb some of this light, causing a bit of damage. Ideally, you want to make the strength of two-photon absorption as high as possible so that the amount of damage to the tumor is as big as possible relative to the damage to healthy cells. You want the special molecules to live as long as possible so that they can be repeatedly zapped without the patient having to ingest more of the cocktail, which could have side effects.
Since TPA is a process where two photons are simultaneously absorbed, it can be used to drive chemical reactions at the intersection point of two beams of light. As an example, a liquid can be made to turn solid (i.e. polymerize) at the crossing points. In this way, a three-dimensional object can be made piece by piece inside the liquid, such as gears, shafts, and other nano-scale parts. So, it's like having the ultimate nanolab.
So, TPA is something that is simultaneously very useful in important applications; but, can be a nuisance in all applications that require the use of light. We are thinking more about ways to make a molecule snooze to help it recover rather than find more ways to put it to work. Happy dreams!
Mark
Another paper accepted, but in record time
In a recent post, I mentioned a paper that took about 4 years to get published. In contrast, we did some work that took about 3 months to complete, and today, the manuscript was accepted for publication in JOSA B.
The topic:
Testing the diffusion hypothesis as a mechanism of self-healing in Disperse orange 11 doped in PMMA
This is an important piece of work that get's us one step closer to understanding self healing in dye-doped polymers, a phenomena that was discovered in our lab a decade ago. The first comment I always get when introducing our work at scientific meetings is that the laser is heating the material, causing the dye molecules to diffuse away from the beam. When the laser is turned off, the dye molecules diffuse back. So, rather than the molecules breaking apart and then reassembling themselves, they are just moving out then into the beam - a much less sexy phenomena.
The report of one of the reviewers summarizes our results best:
Congratulations to Shiva and Nathan for a job well done!
The topic:
Testing the diffusion hypothesis as a mechanism of self-healing in Disperse orange 11 doped in PMMA
This is an important piece of work that get's us one step closer to understanding self healing in dye-doped polymers, a phenomena that was discovered in our lab a decade ago. The first comment I always get when introducing our work at scientific meetings is that the laser is heating the material, causing the dye molecules to diffuse away from the beam. When the laser is turned off, the dye molecules diffuse back. So, rather than the molecules breaking apart and then reassembling themselves, they are just moving out then into the beam - a much less sexy phenomena.
The report of one of the reviewers summarizes our results best:
This manuscript describes a combined experimental and theoretical study on the recovery of absorption in dye-doped polymer samples exposed to high light intensities. There are several possible mechanisms for such a self-healing effect and it is of significant interest to understand which one(s) contribute. The paper presents a set of experimental data on the dynamics and spatial profile of the optical properties of the damage region. It then presents a thorough and detailed model of what is expected if diffusion of undamaged dye molecules into the damaged region is responsible for all or part of the recovery. The difference between the broadened profile of the concentration predicted by the diffusion model and the constant profile shape observed experimentally is persuasive in demonstrating that diffusion is not a significant contributor to the self-healing. The manuscript is well-presented, thorough, and sound. It warrants publication in JOSA B.
Congratulations to Shiva and Nathan for a job well done!
Friday, February 25, 2011
Confirmation Bias and the Scientific Method
Marc Hauser, a primate psychologist at Harvard and expert on the evolution of morality, was recently found guilty of eight counts of scientific misconduct. At issue was his interpretation of video tapes of rhesus monkeys performing tasks that test their ability to learn sound patterns. Hauser "saw" the behavior that supported his hypothesis and was convinced that he was right even when other members of his research group could not. Being an eminent scientist, his group members deferred to his authority, and his interpretation prevailed in the publications that followed.
Physicist Robert Park wrote in a web column that Hauser fudged the data, implying that it was premeditated and deliberate. Scott Lilienfeld, a psychologist, feels that this might be a simple case of confirmation bias, a psychological response of the brain to reinterpret the world by distorting the data to favor the believer's expectations - a phenomena that is commonly at play in strengthening religious faith.
I just returned from a trip to Wright Patterson Air Force Base, where I gave a seminar about our work on self-healing materials and fundamental limits. My visits to the materials lab are always gratifying because the interdisciplinary team of researchers there understand our work from a broad range of angles. There are chemists who understand structural subtleties of chromophores and how they aggregate as well as the role of the host polymer on the properties of the embedded chromphores. The physicists and quantum chemists, on the other hand appreciate the beauty and utility of our models of light-matter interactions. Each individual brings a unique perspective that enriches my understanding of materials and potential mechanisms of a variety of interesting phenomena that we can apply to interpreting our data.
The initial response of people who have just learned about our observations of self healing conclude that diffusion is responsible. Some of the air force scientists shared this concern. The idea is simple; the laser heats the molecules in the polymer, and the added kinetic energy causes them to move away from the laser. When the laser is turned off, the random walk associated with thermal jiggling causes the molecules to return. Thus, rather than the molecule burning (i.e. breaking into pieces) and then recovering (i.e. resembling), they simply move away and return.
When we first observed this phenomena, diffusion was the first hypothesis that we tested using optical absorption spectroscopy. All molecules absorb light at a set of discrete characteristic wavelengths. The DO11 molecule, our model system, has a big absorption peak centered in the middle of the visible part of the electromagnetic spectrum. If molecules move away from the beam, then the height of the peak will drop, but the shape will remain unchanged. If the molecular structure changes, as should happen in the photo-decomposition process, then a new peak forms that is characteristic of the "burnt" molecule. One can show that as one set of molecules is being converted into another set, then the spectrum evolves in a way where all the spectra cross at one point. This point is called an isobestic point.
When I am asked if self healing might be due to diffusion, I can confidently respond that we see an isobestic point in the linear spectrum. Everyone in the audience then usually thoughtfully shake their heads and acknowledge that this is strong evidence against the diffusion hypothesis.
However, upon reading the article about the Hauser case, I began wondering if I am not being deluded by confirmation bias. While we do see an isobestic point, the process of aligning the probe light (used to measure the spectrum) with the pump is difficult, so the measured absorption changes are not always clean. So, I started wondering if we were not being fooled by subconsciously dismissing data that does not meet our expectations. Upon my return, I met with my students and suggested that they try all sorts of other experiments that differentiate between the two mechanisms. I regret this extra burden that I place on my graduate students, but I would hate to be wrong.
On the computational front, we are also seeing weird results that may render some of our ideas invalid. In particular, we normally observe that when a quantum system is at the fundamental limit, only two excited states contribute to the nonlinear response. We call this the three-level ansatz. In recent Monte Carlo calculations, we are seeing rare outliers where more states contribute. Even more disturbing is that in these rare cases, the limit appears to be broken by a tad.
There are two interpretations to these results. First, there may be a flaw in the fundamental limit calculations. In our work using variational approaches of potential energy functions and vector potentials, we never see a system with a nonlinear response larger than 0.709 times the fundamental limit. In these cases, the three-level ansatz seems to always hold. That gives me some degree of confidence that the calculated limits are correct. The resolution to the problem might lie in the fact that the Monte Carlo work uses truncated sum rules. However, such truncated sum rules are also used to calculate the limits. Are we dismissing the effects of truncation when it suits our purposes?
It is likely that there are many subtle issues that we will need to consider to resolve our new observations. I fear that my brain may be driven by confirmation bias into believing in the fundamental limits and to blame counterexamples on the problem of truncation. We must consider the possibility that the limit calculation may be flawed, in which case it needs to be fixed. In the end, getting to the truth should be our top priority.
I learned about the quirky Monte Carlo results just before my trip to Dayton, Ohio. In addition to my preoccupation with my possible affliction with confirmation bias, I am overwhelmed with all sorts of other work, such as doing the annual reviews of our faculty, refereeing papers, reading dissertations, writing letters of recommendation, and writing new papers, as well as struggling to keep up with my lectures and homework solutions - all while fighting a nasty cold.
I look forward to weekends as a time to catch up with my work. Given my workload, I cannot realistically climb out of this hole; but, I take pleasure in my expectations that in the process of preparing for class and pondering our problems, I will learn new physics. Perhaps these activities will lead to the next new breakthrough on our understanding of the universe. I'll keep you posted.
Physicist Robert Park wrote in a web column that Hauser fudged the data, implying that it was premeditated and deliberate. Scott Lilienfeld, a psychologist, feels that this might be a simple case of confirmation bias, a psychological response of the brain to reinterpret the world by distorting the data to favor the believer's expectations - a phenomena that is commonly at play in strengthening religious faith.
I just returned from a trip to Wright Patterson Air Force Base, where I gave a seminar about our work on self-healing materials and fundamental limits. My visits to the materials lab are always gratifying because the interdisciplinary team of researchers there understand our work from a broad range of angles. There are chemists who understand structural subtleties of chromophores and how they aggregate as well as the role of the host polymer on the properties of the embedded chromphores. The physicists and quantum chemists, on the other hand appreciate the beauty and utility of our models of light-matter interactions. Each individual brings a unique perspective that enriches my understanding of materials and potential mechanisms of a variety of interesting phenomena that we can apply to interpreting our data.
The initial response of people who have just learned about our observations of self healing conclude that diffusion is responsible. Some of the air force scientists shared this concern. The idea is simple; the laser heats the molecules in the polymer, and the added kinetic energy causes them to move away from the laser. When the laser is turned off, the random walk associated with thermal jiggling causes the molecules to return. Thus, rather than the molecule burning (i.e. breaking into pieces) and then recovering (i.e. resembling), they simply move away and return.
When we first observed this phenomena, diffusion was the first hypothesis that we tested using optical absorption spectroscopy. All molecules absorb light at a set of discrete characteristic wavelengths. The DO11 molecule, our model system, has a big absorption peak centered in the middle of the visible part of the electromagnetic spectrum. If molecules move away from the beam, then the height of the peak will drop, but the shape will remain unchanged. If the molecular structure changes, as should happen in the photo-decomposition process, then a new peak forms that is characteristic of the "burnt" molecule. One can show that as one set of molecules is being converted into another set, then the spectrum evolves in a way where all the spectra cross at one point. This point is called an isobestic point.
When I am asked if self healing might be due to diffusion, I can confidently respond that we see an isobestic point in the linear spectrum. Everyone in the audience then usually thoughtfully shake their heads and acknowledge that this is strong evidence against the diffusion hypothesis.
However, upon reading the article about the Hauser case, I began wondering if I am not being deluded by confirmation bias. While we do see an isobestic point, the process of aligning the probe light (used to measure the spectrum) with the pump is difficult, so the measured absorption changes are not always clean. So, I started wondering if we were not being fooled by subconsciously dismissing data that does not meet our expectations. Upon my return, I met with my students and suggested that they try all sorts of other experiments that differentiate between the two mechanisms. I regret this extra burden that I place on my graduate students, but I would hate to be wrong.
On the computational front, we are also seeing weird results that may render some of our ideas invalid. In particular, we normally observe that when a quantum system is at the fundamental limit, only two excited states contribute to the nonlinear response. We call this the three-level ansatz. In recent Monte Carlo calculations, we are seeing rare outliers where more states contribute. Even more disturbing is that in these rare cases, the limit appears to be broken by a tad.
There are two interpretations to these results. First, there may be a flaw in the fundamental limit calculations. In our work using variational approaches of potential energy functions and vector potentials, we never see a system with a nonlinear response larger than 0.709 times the fundamental limit. In these cases, the three-level ansatz seems to always hold. That gives me some degree of confidence that the calculated limits are correct. The resolution to the problem might lie in the fact that the Monte Carlo work uses truncated sum rules. However, such truncated sum rules are also used to calculate the limits. Are we dismissing the effects of truncation when it suits our purposes?
It is likely that there are many subtle issues that we will need to consider to resolve our new observations. I fear that my brain may be driven by confirmation bias into believing in the fundamental limits and to blame counterexamples on the problem of truncation. We must consider the possibility that the limit calculation may be flawed, in which case it needs to be fixed. In the end, getting to the truth should be our top priority.
I learned about the quirky Monte Carlo results just before my trip to Dayton, Ohio. In addition to my preoccupation with my possible affliction with confirmation bias, I am overwhelmed with all sorts of other work, such as doing the annual reviews of our faculty, refereeing papers, reading dissertations, writing letters of recommendation, and writing new papers, as well as struggling to keep up with my lectures and homework solutions - all while fighting a nasty cold.
I look forward to weekends as a time to catch up with my work. Given my workload, I cannot realistically climb out of this hole; but, I take pleasure in my expectations that in the process of preparing for class and pondering our problems, I will learn new physics. Perhaps these activities will lead to the next new breakthrough on our understanding of the universe. I'll keep you posted.
Tuesday, February 8, 2011
Good news comes in threes or fours (or fives!)
In the fall, I wrote of my despair when hearing about three papers that had been rejected, all in the same week. Finally, all of these papers are in various stages of the publication process.
First, our paper on photo-mechanical optical devices (which we call PODs), just appeared in Optics Communications this month, which you can view by clicking here. I have talked about this work at several conferences, and I am getting very positive feedback. I am hoping to get this work funded in the near future. I believe that breakthroughs in this area could lead to amazing new technologies that will excite even the most jaded techies.
Secondly, the paper that got the nasty review, which I described in my post The Good, the Bad, and the Nasty, just got accepted. The letter from the editor is appended to this post.
A recent paper that we submitted to Advanced Materials, a high-impact journal, just appeared in print. This is a comment that my former student and colleague Javier Perez-Morena and I wrote on the work of some of our colleagues from Australia. We showed that their results were even more important than they originally thought - a happy outcome for all!
I learned about all three good news items this morning.
Finally, our paper on imaging studies of self-healing will appear shortly in the Journal of the Optical Society of America B. In addition, we have several papers that are in various stages of preparation and under review. New trials and tribulations surely await us!
***Update*** After all of this good news, I was pleased to learn of a fifth item of good news. In a previous post, I had talked about our newest Monte Carlo work, which we submitted to JOSA B. I just heard from Shoresh that this paper was accepted for publication with optional minor revisions. For once, the reviewers and I agree!
February 8, 2011
Title: The effect of electron interactions on the universal properties of systems with optimized off-resonant intrinsic hyperpolarizability
Author(s): David Watkins and Mark Kuzyk
Professor Mark G. Kuzyk
Washington State University
Department of Physics and Astronomy
Post Office Box 642814
Pullman, WA 99164-2814
Dear Professor Kuzyk,
The above manuscript has been accepted for publication in the Journal of Chemical Physics. You may receive requests from our office to ensure that all manuscript files are complete and suitable for typesetting. Once the manuscript files are in an acceptable format, they will be forwarded to the American Institute of Physics publication office.
This e-mail is the only notification you will receive of the acceptance of your paper. If you have questions about the production of your manuscript, you may find contact information for AIP production staff at...
No revisions of the manuscript can be made before the galley proof stage.
Sincerely yours,
Dear Dr. Perez-Moreno,
We are pleased to inform you that your Comment
"A Correspondence on "Organometallic Complexes for Nonlinear Optics. 45.
Dispersion of the Third-Order Nonlinear Optical Properties of Triphenylamine-Cored Alkynylruthenium Dendrimers". Increasing the Nonlinear
Optical Response by Two Orders of Magnitude." by Javier Perez-Moreno, Javier Perez-Moreno
Mark G. Kuzyk
has now been published online.
Your article is available from http://dx.doi.org/10.1002/adma.201003421
The citation data and abstract (if applicable) are available free of charge from the same link; access to the full text may require a subscription.
Please use the above-mentioned URL to link to the article from your institutional homepage, e.g., on publication lists.
A reference to your article is also available from your personal homepage by selecting "Author" and then "My Published Articles".
Best wishes,
Advanced Materials
(Editorial Office)
First, our paper on photo-mechanical optical devices (which we call PODs), just appeared in Optics Communications this month, which you can view by clicking here. I have talked about this work at several conferences, and I am getting very positive feedback. I am hoping to get this work funded in the near future. I believe that breakthroughs in this area could lead to amazing new technologies that will excite even the most jaded techies.
Secondly, the paper that got the nasty review, which I described in my post The Good, the Bad, and the Nasty, just got accepted. The letter from the editor is appended to this post.
A recent paper that we submitted to Advanced Materials, a high-impact journal, just appeared in print. This is a comment that my former student and colleague Javier Perez-Morena and I wrote on the work of some of our colleagues from Australia. We showed that their results were even more important than they originally thought - a happy outcome for all!
I learned about all three good news items this morning.
Finally, our paper on imaging studies of self-healing will appear shortly in the Journal of the Optical Society of America B. In addition, we have several papers that are in various stages of preparation and under review. New trials and tribulations surely await us!
***Update*** After all of this good news, I was pleased to learn of a fifth item of good news. In a previous post, I had talked about our newest Monte Carlo work, which we submitted to JOSA B. I just heard from Shoresh that this paper was accepted for publication with optional minor revisions. For once, the reviewers and I agree!
Letter from Journal of Chemical Physics
February 8, 2011
Title: The effect of electron interactions on the universal properties of systems with optimized off-resonant intrinsic hyperpolarizability
Author(s): David Watkins and Mark Kuzyk
Professor Mark G. Kuzyk
Washington State University
Department of Physics and Astronomy
Post Office Box 642814
Pullman, WA 99164-2814
Dear Professor Kuzyk,
The above manuscript has been accepted for publication in the Journal of Chemical Physics. You may receive requests from our office to ensure that all manuscript files are complete and suitable for typesetting. Once the manuscript files are in an acceptable format, they will be forwarded to the American Institute of Physics publication office.
This e-mail is the only notification you will receive of the acceptance of your paper. If you have questions about the production of your manuscript, you may find contact information for AIP production staff at...
No revisions of the manuscript can be made before the galley proof stage.
Sincerely yours,
Letter from Advanced Materials
Dear Dr. Perez-Moreno,
We are pleased to inform you that your Comment
"A Correspondence on "Organometallic Complexes for Nonlinear Optics. 45.
Dispersion of the Third-Order Nonlinear Optical Properties of Triphenylamine-Cored Alkynylruthenium Dendrimers". Increasing the Nonlinear
Optical Response by Two Orders of Magnitude." by Javier Perez-Moreno, Javier Perez-Moreno
Mark G. Kuzyk
has now been published online.
Your article is available from http://dx.doi.org/10.1002/adma.201003421
The citation data and abstract (if applicable) are available free of charge from the same link; access to the full text may require a subscription.
Please use the above-mentioned URL to link to the article from your institutional homepage, e.g., on publication lists.
A reference to your article is also available from your personal homepage by selecting "Author" and then "My Published Articles".
Best wishes,
Advanced Materials
(Editorial Office)
Thursday, August 12, 2010
Kicking Around New Ideas
For a couple months now, we have been struggling with calculations of the nonlinear-optical response of quantum wires. Our idea is to build up complex structures by connecting together pieces of straight wire segments. The problem is that the sum rules appear to have pathologies. But in reality, the problems lie in the way that we idealize the wire.
As I discussed in a previous post, the case of the quantum rotor is a specific example that had been treated rigorously by Stavros Fallieros. I had an idea of how to apply a similar argument to a straight section of wire. The upshot is that along a wire, the sum rules hold. The problem with an idealized one-dimensional wire is that the wave function is by definition confined to the wire, and therefore vanishes outside. By the Heisenberg uncertainty principle, a particle that is confined in that way must have an infinite transverse momentum, implying an infinite energy state.
If these infinite energy states are included, the sum rules are obeyed. I came up with a simple textbook approach that models transverse confinement with a Dirac delta function potential in the limit when the strength of the delta function is infinite. While I had not solved the full problem, I wrote up the concept in a file LaTeX where I wrote out the form of the solutions. Then, I passed the document along to my students for them to do the hard part: evaluating infinite sums of complicated expressions in the limit when various parameters are large and small. Since there are no loopholes in the way the sum rules are derived, I am confident that this approach will work.
The other day, after I emailed this file to the students, we had a spirited debate. They disagreed with my approach and gave all sorts of counterarguments to prove me wring. They constructed special cases that seemed airtight arguments against my approach. But slowly, they became convinced; not because I am the expert, but because my argument is sound. This is one of the most satisfying aspects of the community of science. In the end, reason wins. This time, I may have been vindicated, but I have made enough mistakes in the past to not be overly dejected when I am proven wrong. Being scientists requires us to admit error.
As an update to one of our papers that was initially rejected, in the process of responding to a substantive comment made by one of the reviewers, David Watkins found an intrinsic hyperpolarizability that exceeds unity - an impossibility, according to my theory. Though my theory has been tested over an over again using different computational techniques under a broad range of conditions, I panic when it appears that I might have missed something. David and I sent many emails back and forth on the topic, trying to understand if somehow the sum rules were being violated by the new case under study. To my delight, David found and fixed a couple of bugs in his code, which solved the problem. The results for this new case is now consistent with all our other calculations.
On another front, three students from my nonlinear optics class and I had finished a nice paper on cascading at the beginning of the summer. As I had reported in a previous post, just prior to submitting the paper, we had found a case where the fundamental limits were exceeded. Since then, we have tried all sorts of approaches to reconcile the problem, but to no avail. Nathan, the lead student on the project, believes that cascading is a way to beat the limits. However, based on general principles, I know the limits must hold. And it's not that I want my theory to be true, but, based on general arguments, the cascading results - which are a special case - must agree with the more general theory. If a specific case appears to violate the more general one, it is incumbent upon us to track down the source of the inconsistency. In other words, we have to specifically show how this case falls outside the realm of the theory. At this point, cascading seems to be formulated in a way that makes it a simple subset of the more general theory.
I have been writing much about theory, but our experimental work has been going well. Shiva has built a beautiful temperature-controlled chamber that will allow him to do experiments from temperatures well bellow ambient to over 100 C. Since the temperature-dependence of a measurement provides a window into the energetics of a process, we hope the new experiments will provide us with a clue as to the metastable species involved that usher self heal self healing of a molecule upon photodegradation. In parallel, I am trying to work out a general theory of self-healing based on our past observations. The real test of this theory will be its power to predict the behavior of new observations as better experiments push the envelope of our knowledge. As a sneak preview, the theory includes a recovery process that is akin to stimulated emission, but in the case of dye recovery, has to do with coupling between the guest molecules and phonons in the host polymer.
Prabodh, a new graduate student in our group is specializing in making a large variety of samples so that he can study how the dopant and polymer host affect the healing process. Ben is doing a a series of experiments to optically image the damaged areas to better pin down the population dynamics, and he is building a new experiment that will allow us to determine the absorption spectrum at each point in the damage region. The combination of new samples and new measurements will provide valuable complimentary data that will undoubtedly aid us in unraveling the puzzle of self healing.
Nathan is getting additional data on the photomechanical response that appears to be consistent with our models. While the results are giving us insights into the new class of liquid crystal elastomeric materials, our conclusions appear to be at odds with those of our collaborators, who supplied us with the samples. I am confident that we will eventually reach a consensus because the truth always bubbles to the top. Even if we are proven right, we most likely have only part of the answer. More interesting mysteries are undoubtedly lurking at the next layer of depth.
Xianjun is in the process of calculating the response of Photomechanical Optical Devices (PODs), with the goal of predicting how they will behave when acting in series. This is a highly nonlinear problem, with complex solutions. At this stage, we are still struggling with the relatively simple things, like the response of a single nonlinear etalon. More complex systems will require us to consider more subtle issues and to be clever in our approximations to solving the full problem. In parallel, Xianjun is starting experiments to burn Bgragg gratings in polymer optical fibers with the goal of making and characterizing PODs. Measurements will play an indispensable part in developing our numerical models.
I hve more to write, but our flight to Amsterdam is boarding. This long trip will eventually lead us to Budapest, where I am giving a plenary lecture on self healing and photo mechanical effects. I apologize for any typos that resulted from my haste, and will write about the meeting upon my return.
As I discussed in a previous post, the case of the quantum rotor is a specific example that had been treated rigorously by Stavros Fallieros. I had an idea of how to apply a similar argument to a straight section of wire. The upshot is that along a wire, the sum rules hold. The problem with an idealized one-dimensional wire is that the wave function is by definition confined to the wire, and therefore vanishes outside. By the Heisenberg uncertainty principle, a particle that is confined in that way must have an infinite transverse momentum, implying an infinite energy state.
If these infinite energy states are included, the sum rules are obeyed. I came up with a simple textbook approach that models transverse confinement with a Dirac delta function potential in the limit when the strength of the delta function is infinite. While I had not solved the full problem, I wrote up the concept in a file LaTeX where I wrote out the form of the solutions. Then, I passed the document along to my students for them to do the hard part: evaluating infinite sums of complicated expressions in the limit when various parameters are large and small. Since there are no loopholes in the way the sum rules are derived, I am confident that this approach will work.
The other day, after I emailed this file to the students, we had a spirited debate. They disagreed with my approach and gave all sorts of counterarguments to prove me wring. They constructed special cases that seemed airtight arguments against my approach. But slowly, they became convinced; not because I am the expert, but because my argument is sound. This is one of the most satisfying aspects of the community of science. In the end, reason wins. This time, I may have been vindicated, but I have made enough mistakes in the past to not be overly dejected when I am proven wrong. Being scientists requires us to admit error.
As an update to one of our papers that was initially rejected, in the process of responding to a substantive comment made by one of the reviewers, David Watkins found an intrinsic hyperpolarizability that exceeds unity - an impossibility, according to my theory. Though my theory has been tested over an over again using different computational techniques under a broad range of conditions, I panic when it appears that I might have missed something. David and I sent many emails back and forth on the topic, trying to understand if somehow the sum rules were being violated by the new case under study. To my delight, David found and fixed a couple of bugs in his code, which solved the problem. The results for this new case is now consistent with all our other calculations.
On another front, three students from my nonlinear optics class and I had finished a nice paper on cascading at the beginning of the summer. As I had reported in a previous post, just prior to submitting the paper, we had found a case where the fundamental limits were exceeded. Since then, we have tried all sorts of approaches to reconcile the problem, but to no avail. Nathan, the lead student on the project, believes that cascading is a way to beat the limits. However, based on general principles, I know the limits must hold. And it's not that I want my theory to be true, but, based on general arguments, the cascading results - which are a special case - must agree with the more general theory. If a specific case appears to violate the more general one, it is incumbent upon us to track down the source of the inconsistency. In other words, we have to specifically show how this case falls outside the realm of the theory. At this point, cascading seems to be formulated in a way that makes it a simple subset of the more general theory.
I have been writing much about theory, but our experimental work has been going well. Shiva has built a beautiful temperature-controlled chamber that will allow him to do experiments from temperatures well bellow ambient to over 100 C. Since the temperature-dependence of a measurement provides a window into the energetics of a process, we hope the new experiments will provide us with a clue as to the metastable species involved that usher self heal self healing of a molecule upon photodegradation. In parallel, I am trying to work out a general theory of self-healing based on our past observations. The real test of this theory will be its power to predict the behavior of new observations as better experiments push the envelope of our knowledge. As a sneak preview, the theory includes a recovery process that is akin to stimulated emission, but in the case of dye recovery, has to do with coupling between the guest molecules and phonons in the host polymer.
Prabodh, a new graduate student in our group is specializing in making a large variety of samples so that he can study how the dopant and polymer host affect the healing process. Ben is doing a a series of experiments to optically image the damaged areas to better pin down the population dynamics, and he is building a new experiment that will allow us to determine the absorption spectrum at each point in the damage region. The combination of new samples and new measurements will provide valuable complimentary data that will undoubtedly aid us in unraveling the puzzle of self healing.
Nathan is getting additional data on the photomechanical response that appears to be consistent with our models. While the results are giving us insights into the new class of liquid crystal elastomeric materials, our conclusions appear to be at odds with those of our collaborators, who supplied us with the samples. I am confident that we will eventually reach a consensus because the truth always bubbles to the top. Even if we are proven right, we most likely have only part of the answer. More interesting mysteries are undoubtedly lurking at the next layer of depth.
Xianjun is in the process of calculating the response of Photomechanical Optical Devices (PODs), with the goal of predicting how they will behave when acting in series. This is a highly nonlinear problem, with complex solutions. At this stage, we are still struggling with the relatively simple things, like the response of a single nonlinear etalon. More complex systems will require us to consider more subtle issues and to be clever in our approximations to solving the full problem. In parallel, Xianjun is starting experiments to burn Bgragg gratings in polymer optical fibers with the goal of making and characterizing PODs. Measurements will play an indispensable part in developing our numerical models.
I hve more to write, but our flight to Amsterdam is boarding. This long trip will eventually lead us to Budapest, where I am giving a plenary lecture on self healing and photo mechanical effects. I apologize for any typos that resulted from my haste, and will write about the meeting upon my return.
Thursday, July 15, 2010
The consistency of physics
Most of my entries are not quite in the form of a diary as I had planned. However, my posts reflect the fact that I spend lots of time thinking about things, so I argue that these are genuine diary entries. But at least for today, my entry will be more diary-like
When I made my rounds in the lab this afternoon, I noticed a common theme in my discussions with the students -- the amazing consistency of physics. Physics covers a broad range of phenomena (everything in the universe obeys the laws of physics!) that mesh together in the most beautiful ways.
My first stop was the department office on the 12th floor to sign paperwork. In the most recent power change, I once again avoided chairing the department, but as a comprise, I agreed to be the associate chair for a year -- the same deal I had made 8 years ago. But, my true pleasures awaited me on the 7th floor, the location of my labs.
First, I spent time talking with Julian, an undergraduate student, and Shoresh, a graduate student, who are using sum rules as a guide to developing theories of the relationship between the geometry of quantum nanowires and their nonlinear-optical properties. What started out as a simple class project has evolved into a huge undertaking. New subtleties keep popping up. Last week we had to revisit the basics of expectation values of quantum mechanics when applied to loops of wires. The fact that the answers that we are getting continue to be inconstant with the sum rules is a sure sign that we are getting something wrong.
Since sum rules are derived directly from the Schrodinger equation with no approximation, they cannot be violated. About an hour of discussions on the topic led to some new ideas and new approaches that will help us better understand our problems, which hopefully will lead to a solution. While constantly tracing errors in theoretical models is highly unpleasant, the exhilaration of learning makes it all worthwhile. Michael Cohen, a student of Feynman's and a professor of mine at U Penn showed me the value of going back to the basics and testing the assumptions, no matter how trivial. The process always leads to a deeper understanding.
Next I went to Nathan's office, a student who is doing work that we hope will someday lead to ultra-smart materials. After I helped him and Xianjun clean some laser optics, Nathan described his ambiguous experimental results of the previous evening. Then we returned to his office to discuss another project that we had started in nonlinear optics class. He and two of his classmates did calculations to understand if the process of cascading could be used to break the fundamental limits of the nonlinear optical response that I had calculated 10 years prior. Since my calculations were fairly general, I did not believe this to be possible; but, we are obligated to test the hypothesis. Cascading appears to be a topic of growing interest in the nonlinear optics community because of the new avenues it may provide for making better materials. Our work will assess its usefulness and may lead to design guidelines for new nonlinear-optical material paradigms.
There were several false starts - calculations that gave infinities or calculations that yielded limiting cases that were inconsistent with known physics. After a couple of weeks, and many discussions, we finally converged to what appeared to be a robust theory of cascading. After receiving a draft manuscript from the students, I spent several days adding lots of new material, expanding the bibliography, as well as correcting lots of little errors. Just as I was applying the finishing touches in smug satisfaction of the beauty of the final product, I noticed that we failed to consider one important case. So, I sent Nathan an email, and he went back to the drawing board.
Again, the infinities reared their ugly heads. Luckily, my 2006 paper provided us with some guidance. Nathan concluded that indeed, he could remove the infinities with this method; but, again, there are many subtleties and ambiguities regarding the formulation of the model. After a brief discussion, we agreed on an approach and Nathan immediately began implementing the calculations.
Almost 10 years ago, one of my students and I discovered that some materials self heal after photodegradation. This was an exciting discovery because of the apparent reversal of the arrow of time. The result is yet to be fully understood. Three students are presently working on the project. This work requires lots of samples to be made, long experiments that can take several days of continuous operation, and complex analysis of huge data files. Some of these experiments are currently up and running while we are in the planning phases of building a new instrument and upgrading some older experiments.
The experimental results are encouraging but continue to throw more puzzles our way than providing reliable tests of our hypotheses. But, I have a good feeling that we are expanding into new experimental techniques that will hopefully provide lots of answers as well enabling us to pose more interesting new questions.
When I got home from the lab, my inbox had a message labeled "important." Shoresh had found a paper that shed light on our observation of sum rule violation. The paper's abstract was short but to the point, "We discuss application of the Thomas-Reiche-Kuhn sum rule to simple quantum-mechanical models and its apparent violation by the rigid rotator."
Scanning through the paper, I noticed that the first reference in the bibliography was coauthored by Stavros Fallieros -- an incredible coincidence considering that my parents bought his house in 1968 when I was 10 years old. There must have been the essence of sum rules in the walls that were infused into my being during adolescence.
As it turns out, my in-laws were good friends with the Fallieros family, and continued to remain in touch. Several years after I wrote my 2000 paper on fundamental limits, my wife was visiting her mother in a suburb of Philadelphia, where she ran into Stavros. During the exchange of pleasantries, he learned about our common interest in sum rules, and relayed through my wife the message that he wanted a copy of my paper, which I happily mailed to him. Unfortunately, he passed away soon after, and we never had a chance to discuss the work. Ironically, the sum rule paper that he wrote, and which I recently found, was his last journal publication. With the perfect symmetry not often found in life, one of the last papers that he had in his possession was my paper on sum rules.
The interconnectedness of science is mirrored in the relationships between scientists. We are each small cogs in a gargantuan machine that produces new knowledge. One of my former graduate students, Xavi (PhD in 2007), who was the first person to appreciate my use of the sum rules in nonlinear optics will be visiting my group for 6 weeks this summer. Later in the summer, his co-advisor will also be visitng from Belgium. This combination of our two visitors, a new student in my group, Sengting, who is following in the footsteps of Xavi in his pursuit of a joint PhD degree between WSU and the University of Leuven, along with Shoresh and Julian provides us with a formidable crew that will undoubtedly add to the knowledge base of science.
As I sit here at my computer in this small pocket of nostalgic bliss, thinking about international collaborations and our connections with past and future science, I am looking forward to the satisfaction of learning new things as well as tracking divergences in calculations, fixing lasers, and deficit spending to buy components for a new experiments. Another typical day begins!
When I made my rounds in the lab this afternoon, I noticed a common theme in my discussions with the students -- the amazing consistency of physics. Physics covers a broad range of phenomena (everything in the universe obeys the laws of physics!) that mesh together in the most beautiful ways.
My first stop was the department office on the 12th floor to sign paperwork. In the most recent power change, I once again avoided chairing the department, but as a comprise, I agreed to be the associate chair for a year -- the same deal I had made 8 years ago. But, my true pleasures awaited me on the 7th floor, the location of my labs.
First, I spent time talking with Julian, an undergraduate student, and Shoresh, a graduate student, who are using sum rules as a guide to developing theories of the relationship between the geometry of quantum nanowires and their nonlinear-optical properties. What started out as a simple class project has evolved into a huge undertaking. New subtleties keep popping up. Last week we had to revisit the basics of expectation values of quantum mechanics when applied to loops of wires. The fact that the answers that we are getting continue to be inconstant with the sum rules is a sure sign that we are getting something wrong.
Since sum rules are derived directly from the Schrodinger equation with no approximation, they cannot be violated. About an hour of discussions on the topic led to some new ideas and new approaches that will help us better understand our problems, which hopefully will lead to a solution. While constantly tracing errors in theoretical models is highly unpleasant, the exhilaration of learning makes it all worthwhile. Michael Cohen, a student of Feynman's and a professor of mine at U Penn showed me the value of going back to the basics and testing the assumptions, no matter how trivial. The process always leads to a deeper understanding.
Next I went to Nathan's office, a student who is doing work that we hope will someday lead to ultra-smart materials. After I helped him and Xianjun clean some laser optics, Nathan described his ambiguous experimental results of the previous evening. Then we returned to his office to discuss another project that we had started in nonlinear optics class. He and two of his classmates did calculations to understand if the process of cascading could be used to break the fundamental limits of the nonlinear optical response that I had calculated 10 years prior. Since my calculations were fairly general, I did not believe this to be possible; but, we are obligated to test the hypothesis. Cascading appears to be a topic of growing interest in the nonlinear optics community because of the new avenues it may provide for making better materials. Our work will assess its usefulness and may lead to design guidelines for new nonlinear-optical material paradigms.
There were several false starts - calculations that gave infinities or calculations that yielded limiting cases that were inconsistent with known physics. After a couple of weeks, and many discussions, we finally converged to what appeared to be a robust theory of cascading. After receiving a draft manuscript from the students, I spent several days adding lots of new material, expanding the bibliography, as well as correcting lots of little errors. Just as I was applying the finishing touches in smug satisfaction of the beauty of the final product, I noticed that we failed to consider one important case. So, I sent Nathan an email, and he went back to the drawing board.
Again, the infinities reared their ugly heads. Luckily, my 2006 paper provided us with some guidance. Nathan concluded that indeed, he could remove the infinities with this method; but, again, there are many subtleties and ambiguities regarding the formulation of the model. After a brief discussion, we agreed on an approach and Nathan immediately began implementing the calculations.
Almost 10 years ago, one of my students and I discovered that some materials self heal after photodegradation. This was an exciting discovery because of the apparent reversal of the arrow of time. The result is yet to be fully understood. Three students are presently working on the project. This work requires lots of samples to be made, long experiments that can take several days of continuous operation, and complex analysis of huge data files. Some of these experiments are currently up and running while we are in the planning phases of building a new instrument and upgrading some older experiments.
The experimental results are encouraging but continue to throw more puzzles our way than providing reliable tests of our hypotheses. But, I have a good feeling that we are expanding into new experimental techniques that will hopefully provide lots of answers as well enabling us to pose more interesting new questions.
When I got home from the lab, my inbox had a message labeled "important." Shoresh had found a paper that shed light on our observation of sum rule violation. The paper's abstract was short but to the point, "We discuss application of the Thomas-Reiche-Kuhn sum rule to simple quantum-mechanical models and its apparent violation by the rigid rotator."
Scanning through the paper, I noticed that the first reference in the bibliography was coauthored by Stavros Fallieros -- an incredible coincidence considering that my parents bought his house in 1968 when I was 10 years old. There must have been the essence of sum rules in the walls that were infused into my being during adolescence.
As it turns out, my in-laws were good friends with the Fallieros family, and continued to remain in touch. Several years after I wrote my 2000 paper on fundamental limits, my wife was visiting her mother in a suburb of Philadelphia, where she ran into Stavros. During the exchange of pleasantries, he learned about our common interest in sum rules, and relayed through my wife the message that he wanted a copy of my paper, which I happily mailed to him. Unfortunately, he passed away soon after, and we never had a chance to discuss the work. Ironically, the sum rule paper that he wrote, and which I recently found, was his last journal publication. With the perfect symmetry not often found in life, one of the last papers that he had in his possession was my paper on sum rules.
The interconnectedness of science is mirrored in the relationships between scientists. We are each small cogs in a gargantuan machine that produces new knowledge. One of my former graduate students, Xavi (PhD in 2007), who was the first person to appreciate my use of the sum rules in nonlinear optics will be visiting my group for 6 weeks this summer. Later in the summer, his co-advisor will also be visitng from Belgium. This combination of our two visitors, a new student in my group, Sengting, who is following in the footsteps of Xavi in his pursuit of a joint PhD degree between WSU and the University of Leuven, along with Shoresh and Julian provides us with a formidable crew that will undoubtedly add to the knowledge base of science.
As I sit here at my computer in this small pocket of nostalgic bliss, thinking about international collaborations and our connections with past and future science, I am looking forward to the satisfaction of learning new things as well as tracking divergences in calculations, fixing lasers, and deficit spending to buy components for a new experiments. Another typical day begins!
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