Thursday, November 18, 2010

Looking back

I was recently asked by a colleague to send him a short history of my work. While I do not feel particularly successful as measured by standard metrics, I felt satisfaction in the accomplishments of my career. More to the point, I recalled the thrills involved in the process of seeking understanding.

One such thrilling moment came in the late 1980s while I was working in a dark lab, illuminated only by the colorful lights on the panel of electronics and the pristine fine lines of the laser beam that bounced around the experiment. The idea of my work was simple. I wanted to understand how molecules embedded in a polymer were able to reorient. So, I passed a beam of light through the material to probe the molecular orientation and applied an electric field to coax the molecules to reorient.

On a microscopic scale, the polymer is like a bowl of cold spaghetti and the molecules like embedded toothpicks. The applied electric field acts only on the molecules while the polymer restricts the movement of the molecules. Before running the experiment, I turned up the voltage by hand, and watched the bar graph on my lock-in amplifier panel swell in response. This simple action may seem no more significant that watching the glowing bar grow when turning up the volume of a stereo; but, the realization that the bar graph represents the orientation of the tiny molecules sent chills down my spine. I spent several minutes turning the voltage up and down, picturing the rotation of the little toothpicks, and the stretching of the spaghetti.

I eventually got back to work and recorded reams of data, analyzed it, and developed a quantitative description of the process. The polymer acted as a large spring whose stiffness depends on temperature. The work eventually eventually appeared in journal, and formed the basis of future developments in our field. However, when I look at the plot in my publication that very coldly states, "figure so and so shows a plot of the intensity as a function of applied voltage," the image of the tiny molecules responding to my rude intervention brings back the thrill of doing my work.

Almost every piece of work tells a similar story. Recalling the work is like visiting with old friends and family and recalling the happiness of those moments that have been frozen in our memories. But, I delight in the fact that I continue to form new memories doing new experiments and developing new theories with my crew of students and colleagues. The life of science is truly privileged, and I am thankful that I live in a time when this selfish pursuit of passion is used by others in the future to the benefit of society.

I close with a short and terse summary of my work. Hopeful, I will have time in the future to share with you the stories that go along with each piece of work. For now, this is a bookmark to remind me of my past.

At Bell labs, with Ken Singer and Sohn, I developed the thermodynamic model of poling (now called the SKS model), which has since been vastly improved upon by Dalton and Jen. I also measured the electroscopic coefficient in corona-poled side-chain dye doped polymers to demonstrate that large poling fields were possible. During my time at Bell Labs, I also developed, with Carl Dirk, the missing state analysis, which is used to determine the importance of excited state contributions to the second hyperpolarizability as well as proposed and demonstrated that centrosymmetric molecules, such as the squaraines, should have the largest second hyperpolarizability. Also at that time, I showed experimentally and modeled theoretically the various mechanisms that contribute to the electrooptic effect (first and second order). The reorientational mechanism was used by W. E. Moerner to develop polymeric materials with large photorefractive effect. In addition, I showed that the tensor properties of the second-order susceptibility could be controlled by applying uniaxial stress to a polymer while poling it with an electric field.

At WSU, my group - in collaboration with Carl Dirk at UTEP and Unchul Paek of Bell Labs, was the first to fabricate single-mode polymer optical fiber doped with nonlinear-optical molecules, which have a large intensity-dependent refractive index. Later, we demonstrated a nonlinear directional coupler in duel-core fibers. In separate work, we used dye doped fibers with a large photo-mechanical effect to demonstrate the first all-optical circuit where sensing, logic, information transmission and actuation were all performed optically in one system to stabilize the position of a mirror to within 3 nm. This same system is found to be mechanically and optically multistable. After this proof of concept, we showed that this system could be miniaturized into a small section of fiber that combines all device functions into a single discrete device that can be easily integrated with many others. This work suggests that it may be possible to make ultra-smart morphing materials.

In other work using optical fiber, we demonstrated that we could write (and erase/rewrite) a hologram in a fiber waveguide and use it for image correction with phase conjugation. Similar fibers were used to demonstrate nonlinear optical processes using twisted light (i.e. a beam with orbital angular momentum) and showed the advantages of using such light to measure the nonlinear-optical properties of a material as well as its use for optical limiting applications.

More recent work has focused on using sum rules to build a broad understanding of the nonlinear-optical response. This work was motivated by my calculations that show that there is a fundamental limit of the nonlinear-optical response. This has lead to the concept of scale invariance and intrinsic hyperpolarizabilities, which can be used to directly compare the nonlinear-optical response of molecules of differing shapes and sizes. More importantly, these concepts have lead us to theoretical studies that have suggested new paradigms for large-non-linearity molecules - which have been experimentally demonstrated. Also, this work has shown that quantum systems whose nonlinear-optical response is at the quantum limit share certain common universal properties.

Our most recent work is focused on understanding our discovery of self-healing in dye-doped polymers. We find that when certain molecules are embedded in a polymer, the system recovers after being damaged by a high-intensity light source. These same molecules degrade irreversibly in liquid solution. This work has applications in making materials that withstand higher intensities.

No comments:

Post a Comment