It all stared as a final project in nonlinear optics class. Each group of students were assigned a project that applied to what they learned in class to an interesting question. One group was given the task to investigate whether or not cascading, the use of two lower-order nonlinearities to mimic a higher-order one, could be used to beat the fundamental limits. When I was first asked this question by a colleague, I responded in the negative based on the fact that my calculations were general, and applied to a pair of molecules that interact through cascading as well as to a single molecule.
However, upon reflection, I wondered if we could learn something new while studying this process. To get to the punchline, we found that cascading does not break the limit. However, what started as an average-length paper grew into two huge papers. As often happens, our first calculation missed an important case, which presented an assortment of issues that each required deep thought. In the end, we learned a great deal that we felt was of sufficient significance to warrant publication in the Physics Review.
We sent the papers to Physical Review A, and waited. Because our papers are complex, the journal had difficulty finding multiple qualified reviewers. Instead, they chose a highly distinguished individual (we believe that we know his identity based on the style and tone of the review), who reviewed the two papers together. The review started off a bit negative,
"These two manuscripts form a series devoted to the theoretical analysis of the contribution of cascading to second hyperpolarizability of molecules. Cascading is for sure a very interesting phenomenon in non linear optics, far from being well understood and easy to rationalize, and therefore the effort made by Kuzyk and co-workers is for sure welcome. Nevertheless, after reading the two manuscripts, having appreciated the mastering of the microscopic theory for the limit cases studied, with all the approximations involved, I am not sure that I have discovered something either new or unexpected applying to the real world. To be more precise, I am not sure I was given a clue of why experiment on real molecules behaves as it does."
The next paragraph offered that, "The papers report a nice theoretical study, well written and also entertaining of some very very special cases (point like molecules, with one-dimensional symmetry, interacting only in very special arrangements, and with many other approximations) with no direct (at least transparent) insight into the complicacies of real systems...," followed by some detailed criticisms.
The review concluded with, "Now, to make a long story short. The papers are in my view publishable, since the science is solid, the topic is interesting and the general field of research relevant. In order to enhance the chance that these papers do not remain nice case studies of academic interest, the authors should in my view..."
Since the reviewer brought up some valid points, we made extensive revisions to address each and every point, at times developing new and more general theories and running more simulations. The size of the papers grew, but I beleive that the final product is much improved.
The reviewer and the editor agree. This morning, we got an email that opened with: "We are pleased to inform you that your manuscript has been accepted for publication as a Regular Article in Physical Review A. We would also like to bring the appended referee comments to your attention," which reads, "I am fully satisfied with the changes made by the authors. I believe that the papers convey now the information much more efficiently. I have no further comments."
Congratulations to Nathan, Ben, and Jennie for a job well done! Ironically, these papers were accepted on the day of Nathan's wedding. What a great present!
Showing posts with label Cascading. Show all posts
Showing posts with label Cascading. Show all posts
Saturday, September 3, 2011
Monday, August 1, 2011
Wasting time, in a good way
Today my morning started early; responding to emails at 6:30 am and an 8:00 am search committee meeting. Various other administrative tasks delayed my arrival in the lab until about 9:45am. After doing the rounds in the lab, and then signing some more paperwork in the Physics office, I made it to my desk, where I spent the rest of the morning answering emails - with a short diversion to chat with the guys fixing our sprinkler system.
After lunch, I finally got back to the task of working on Nathan's cascading paper, which incidentally, I worked on a bit last night. As I was revising text in response to the reviewer's comments, I had a stroke of genius which I imagined would make a significant impact on the world of physics.
Without going into details, cascading is a process by which two molecules cooperate by exchanging a real photon. My insight provided the means for making the exchanged photon virtual. As a consequence, this photon's energy would not need to be conserved as long as the process were fast enough not to violate the uncertainty principle. This made the problem richly beautiful; and more importantly, it meant that a large area of nonlinear optics was flawed. I couldn't resist thinking about this problem with my full attention, so I placed my long "to do" list on the back burner.
I drew Feynman diagrams of the process and immediately realized that if the virtual photon did not conserve energy, it forced the cascading process to also not conserve energy. Thus, the photon must be real and my line of reasoning flawed. I am no genius after all!
However, by taking this detour, I found myself thinking about various cases where virtual processes contribute. To cut to the chase, my understanding of nonlinear interactions took a quantum leap. It made me appreciate the clever minds of great physicists such as Feynman, whose work embodies incredibly deep reasoning.
While most detours on the road waste time and make drivers frustrated, this kind was enjoyable and fulfilling. As I sit at my desk plowing through my work, I remain permeated with a calm happiness.
Until next time...
After lunch, I finally got back to the task of working on Nathan's cascading paper, which incidentally, I worked on a bit last night. As I was revising text in response to the reviewer's comments, I had a stroke of genius which I imagined would make a significant impact on the world of physics.
Without going into details, cascading is a process by which two molecules cooperate by exchanging a real photon. My insight provided the means for making the exchanged photon virtual. As a consequence, this photon's energy would not need to be conserved as long as the process were fast enough not to violate the uncertainty principle. This made the problem richly beautiful; and more importantly, it meant that a large area of nonlinear optics was flawed. I couldn't resist thinking about this problem with my full attention, so I placed my long "to do" list on the back burner.
I drew Feynman diagrams of the process and immediately realized that if the virtual photon did not conserve energy, it forced the cascading process to also not conserve energy. Thus, the photon must be real and my line of reasoning flawed. I am no genius after all!
However, by taking this detour, I found myself thinking about various cases where virtual processes contribute. To cut to the chase, my understanding of nonlinear interactions took a quantum leap. It made me appreciate the clever minds of great physicists such as Feynman, whose work embodies incredibly deep reasoning.
While most detours on the road waste time and make drivers frustrated, this kind was enjoyable and fulfilling. As I sit at my desk plowing through my work, I remain permeated with a calm happiness.
Until next time...
Thursday, January 27, 2011
Me as a reviewer and a complainer about modern-day publication practices
In the past, I have complained about reviewers who have evaluated my papers. However, it is a time-consuming job with almost no rewards, so I do appreciate their efforts. It is a service that we are all expected to provide. Given the time others have spent on my papers, I feel obligated to return the favor.
The most rewarding reviews are those from which I learn. I spent this morning reviewing a paper by some very distinguished scientists in my field. While I admit to the possibility that I may be wrong, I believe that there are serious issues with their paper, which will require attention before it is suitable for publication.
As usual, one activity leads my mind jumping around to other thoughts. This paper is an example of one in a series that is trying to simultaneously correct errors in the literature while introducing new science. This got me thinking again about the curse of information overload.
I am concerned that modern-day science, with the huge number of venues available for disseminating research results, is producing too much information along with lots of junk. The signal to noise levels are dropping while the whole system is bursting at the seems. People are becoming more specialized and less aware of other work. Since researchers are being judged on numbers of publications and citations, they overload existing top journals with so many papers that editors often cut good papers based on arbitrary guidelines. More second-tier journals are popping up to meet the growing demands by authors.
It's getting difficult for me to find useful information in the literature. For example, when searching electronically using very specific keywords, I get too many irrelevant hits that take forever to sort. It is also frustrating to have done what I believe to be great work in the past, only for it to be ignored for 20 years. Even more annoying is seeing the same identical topic of my research appearing many years later in Nature or Physical Review Letters with no citations to my papers. It is even more irksome when the modern work is but a subset of my original research, yet gets lots of recognition.
Sometimes, I send these modern-day authors reprints of my older papers. Some will respond apologetically pleading ignorance of my research, then continue not to cite my work. Others ignore my emails. These are indicators of a system that is not serving its purpose in producing research that serves society.
My review reminded me of the past era of more responsibility in publishing. Perhaps I view the past with unfounded fondness. However, I can atest to the fact that the authors of the manuscript that I have just reviewed are interested in the seeking truth. I therefore feel confident that they will carefully consider my comments and will only move forward with a revised manuscript if they are certain that they can make a real contribution to the field.
Below, I include a copy of my review for all to read. I, of course, will not reveal the identity of the authors, nor the journal to which this paper has been submitted. I take the risk of being exposed as the reviewer, but, I am sure that they will have already guessed my identity based on the flavor of my review; and, I will not deny being the reviewer if asked. Having gotten this off my chest, I need to get back to writing a proposal and grading homework. Perhaps I can then squeeze in a few moments to think about physics, and achieve the bliss that accompanies such thoughts.
And now, finally, the review:
The authors do some combinatorial wizardry to determine the coefficients of the various orders of the nonlinear birefringence. I am not willing to check all of the math, but from what I have checked, I trust that this is done correctly. However, I have a serious concern that may invalidate the approach, as I describe below.
The fundamental property of a material is its nonlinear susceptibility, not the nonlinear birefringence. The nonlinear susceptibility is what governs the physics of light/light interactions while the nonlinear birefringence is the quantity measured. They are related through the constitutive relationship D = epsilon E = E + 4 pi P. In the process of relating the two, one takes a square root of a power series in the field with the susceptibilities as coefficients. The crux of what I believe to be the fallacy of this paper is that n_m is related only to chi^(n+1). In the process of doing the expansion of the square root, one gets cross terms that are products of various lower-orders of the nonlinear susceptibilities that coincidentally may look like expressions that one sees in cascading calculations. The authors have in effect only expanded the square root to the first term. I believe that if the calculations are done properly, then it may be impossible to define unique constants of proportionality. However, under certain approximations, it may be able to define unique constants in the spirit of the authors' original intention.
A second problem along these lines is the neglect of the imaginary parts of the susceptibility. While experiments are off-resonance, there is always a small imaginary part. The cross-terms that I mention above can include products of imaginary parts that give a real response. Since it is possible that effects due to the imaginary part may get large for higher-order susceptibilities, they also need to be considered in the calculation. The fact that in practice, higher-order susceptibilities are by necessity more resonantly enhanced is a problem with applying this theory to real experiments at ultra-high intensities, and should be mentioned.
Nonlinear dichroism can also lead to polarization rotation, a common way of measuring the nonlinear birefringence. This contribution might also be large in practical experiments. While a good experimentalist would take this into account, I am concerned that a blind application of your results could lead to the unintended consequence of more junk in the literature. Thus, if not accounted for specifically, I would suggest adding at least a cautionary note.
I believe that the above issue needs to be carefully addressed before the manuscript is reconsidered for publication.
As a more minor point, In the introduction, the authors mention that the proportionality constant depends on the number of eigenmodes. I usually associate this factor with the number of degenerate frequencies. Is it true that if the frequencies are the same in a pump-probe geometry one gets the factor of 2/3? I thought that if the prorogation directions are different in the non-collinear polarization geometry, the effect shows up in the tensor properties of the susceptibility. That is, if the polarizations are different, then one is probing that particular component of the nonlinear susceptibility tensor. In any case, the meaning of eigenmode needs to be clarified or the sentence needs to be reworded. The use of the expression "eigenmode" in the rest of the paper may also need to be reconsidered.
I find the issue of cross terms to be a major one. If the authors choose to argue that the results hold in some limiting cases, that might diminish the relevance of the paper in loss of generality. If the calculation includes the terms that I believe to be missing, the coefficients will no longer be well defined. In either case, I believe that this paper may need major revision before it is suitable for publication. If I am wrong in my assessment, then the paper may be suitable for publication after minor revisions. In this scenario, it would be useful if the authors provided a more detailed explanation of the relationship between the nonlinear birefringence and the nonlinear susceptibility.
The most rewarding reviews are those from which I learn. I spent this morning reviewing a paper by some very distinguished scientists in my field. While I admit to the possibility that I may be wrong, I believe that there are serious issues with their paper, which will require attention before it is suitable for publication.
As usual, one activity leads my mind jumping around to other thoughts. This paper is an example of one in a series that is trying to simultaneously correct errors in the literature while introducing new science. This got me thinking again about the curse of information overload.
I am concerned that modern-day science, with the huge number of venues available for disseminating research results, is producing too much information along with lots of junk. The signal to noise levels are dropping while the whole system is bursting at the seems. People are becoming more specialized and less aware of other work. Since researchers are being judged on numbers of publications and citations, they overload existing top journals with so many papers that editors often cut good papers based on arbitrary guidelines. More second-tier journals are popping up to meet the growing demands by authors.
It's getting difficult for me to find useful information in the literature. For example, when searching electronically using very specific keywords, I get too many irrelevant hits that take forever to sort. It is also frustrating to have done what I believe to be great work in the past, only for it to be ignored for 20 years. Even more annoying is seeing the same identical topic of my research appearing many years later in Nature or Physical Review Letters with no citations to my papers. It is even more irksome when the modern work is but a subset of my original research, yet gets lots of recognition.
Sometimes, I send these modern-day authors reprints of my older papers. Some will respond apologetically pleading ignorance of my research, then continue not to cite my work. Others ignore my emails. These are indicators of a system that is not serving its purpose in producing research that serves society.
My review reminded me of the past era of more responsibility in publishing. Perhaps I view the past with unfounded fondness. However, I can atest to the fact that the authors of the manuscript that I have just reviewed are interested in the seeking truth. I therefore feel confident that they will carefully consider my comments and will only move forward with a revised manuscript if they are certain that they can make a real contribution to the field.
Below, I include a copy of my review for all to read. I, of course, will not reveal the identity of the authors, nor the journal to which this paper has been submitted. I take the risk of being exposed as the reviewer, but, I am sure that they will have already guessed my identity based on the flavor of my review; and, I will not deny being the reviewer if asked. Having gotten this off my chest, I need to get back to writing a proposal and grading homework. Perhaps I can then squeeze in a few moments to think about physics, and achieve the bliss that accompanies such thoughts.
And now, finally, the review:
The authors do some combinatorial wizardry to determine the coefficients of the various orders of the nonlinear birefringence. I am not willing to check all of the math, but from what I have checked, I trust that this is done correctly. However, I have a serious concern that may invalidate the approach, as I describe below.
The fundamental property of a material is its nonlinear susceptibility, not the nonlinear birefringence. The nonlinear susceptibility is what governs the physics of light/light interactions while the nonlinear birefringence is the quantity measured. They are related through the constitutive relationship D = epsilon E = E + 4 pi P. In the process of relating the two, one takes a square root of a power series in the field with the susceptibilities as coefficients. The crux of what I believe to be the fallacy of this paper is that n_m is related only to chi^(n+1). In the process of doing the expansion of the square root, one gets cross terms that are products of various lower-orders of the nonlinear susceptibilities that coincidentally may look like expressions that one sees in cascading calculations. The authors have in effect only expanded the square root to the first term. I believe that if the calculations are done properly, then it may be impossible to define unique constants of proportionality. However, under certain approximations, it may be able to define unique constants in the spirit of the authors' original intention.
A second problem along these lines is the neglect of the imaginary parts of the susceptibility. While experiments are off-resonance, there is always a small imaginary part. The cross-terms that I mention above can include products of imaginary parts that give a real response. Since it is possible that effects due to the imaginary part may get large for higher-order susceptibilities, they also need to be considered in the calculation. The fact that in practice, higher-order susceptibilities are by necessity more resonantly enhanced is a problem with applying this theory to real experiments at ultra-high intensities, and should be mentioned.
Nonlinear dichroism can also lead to polarization rotation, a common way of measuring the nonlinear birefringence. This contribution might also be large in practical experiments. While a good experimentalist would take this into account, I am concerned that a blind application of your results could lead to the unintended consequence of more junk in the literature. Thus, if not accounted for specifically, I would suggest adding at least a cautionary note.
I believe that the above issue needs to be carefully addressed before the manuscript is reconsidered for publication.
As a more minor point, In the introduction, the authors mention that the proportionality constant depends on the number of eigenmodes. I usually associate this factor with the number of degenerate frequencies. Is it true that if the frequencies are the same in a pump-probe geometry one gets the factor of 2/3? I thought that if the prorogation directions are different in the non-collinear polarization geometry, the effect shows up in the tensor properties of the susceptibility. That is, if the polarizations are different, then one is probing that particular component of the nonlinear susceptibility tensor. In any case, the meaning of eigenmode needs to be clarified or the sentence needs to be reworded. The use of the expression "eigenmode" in the rest of the paper may also need to be reconsidered.
I find the issue of cross terms to be a major one. If the authors choose to argue that the results hold in some limiting cases, that might diminish the relevance of the paper in loss of generality. If the calculation includes the terms that I believe to be missing, the coefficients will no longer be well defined. In either case, I believe that this paper may need major revision before it is suitable for publication. If I am wrong in my assessment, then the paper may be suitable for publication after minor revisions. In this scenario, it would be useful if the authors provided a more detailed explanation of the relationship between the nonlinear birefringence and the nonlinear susceptibility.
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.
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