The Division of Chemistry supports Bradford Perkins of the Massachusetts Institute of Technology as an American Competitiveness in Chemistry Fellow. Dr. Perkins will work with Prof. Keith Nelson to investigate the flow of vibrational energy in condensed-phase systems. In particular, the research will use ultrafast terahertz methods to probe the flow of energy from low-energy phonon modes into higher-energy molecular vibrational modes, which ultimately leads to bond-breaking events (chemistry). The work will be carried out in collaboration with scientists at Los Alamos National Laboratory. For his plan for broadening participation, the PI will engage Boston-area high school students, particularly those from groups underrepresented in science, in terahertz imaging research as part of the "Lambda Project."
Research like that of Dr. Perkins is aimed at developing a better understanding of the mechanism of how energy flows in molecular materials. In particular, the research carried out with support from this award will reveal the step-by-step mechanisms of how energy moves from a low-energy form to a higher energy form that is capable of leading to chemical reaction -- research that is important in understanding thermally-initiated processes. Results from research like that supported here will lead to better computational models and new kinds of materials, e.g. explosives, outcomes which are of importance to national security. The efforts at broadening participation being pursued by Dr. Perkins are aimed at encouraging young people, especially those from groups underrepresented in the sciences, to pursue higher education in science, by providing exposure to the excitement of laboratory research in a modern research environment.
The primary scientific goal outlined in this proposal involves the study of energy transfer in molecular systems to understand reactive chemical mechanisms in highly energetic materials. In general, these types of crystals react when stimulated by mechanical shock. To study these mechanisms, we employed laser spectroscopy that utilized ultrashort (~1 ps) pulses of far infrared light (frequency ~ 1 THz) focused down into the crystal to simulate mechanical shock to the system. We then used additional laser pulses to probe ways in which the crystal changed due to the intense light field. Initial experiments revealed a need for stronger THz light fields used to simulate the mechanical shock. We explored several methods to generate stronger electric fields before discovering that novel metalmaterials provide an efficient way of amplifying the incident light field. Metamaterials are patterned arrays of split ring resonators that work by absorbing energy from the incident light field and turning it into oscillating charge. The electric field enhancement arises as large amounts of charge transiently accumulate at the gaps in the split rings. Specifically, we discovered that electric fields are enhanced by factors of 30 in the gaps of these metamaterial structures. While we studied metamaterial responses to strong light fields, we discovered interesting optical properties of metamaterials made from high temperature superconductive thin films. Specifically, we studied yttrium barium copper oxide (YBCO) split ring resonators, and we learned that the incident light could be used to control the transmission through these thin films. Above the superconductive critical temperature, the YBCO metamaterial acted like a poor metal conductor, which allowed partial transmission of the light. As the film cooled below the critical temperature, the YBCO turned into an excellent conductor, which strongly absorbed light at the resonance frequency of the split ring circuit. Interesting, transmission increased as the intensity of the incident light increased. To help understand the mechanism that leads to this behavior, we turned our attention to simpler YBCO thin films (50 and 100nm thick). In similar laser spectroscopy studies, we illuminated the YBCO films with THz light as we varied the temperature of the crystal from 30 K to room temperature. The temperature at which the material switches from an insulator to a superconductor is known as the critical temperature (Tc). Previous characterization at Los Alamos National Laboratory showed that Tc to be 90 K for these particular thin films. At low illumination intensities, the transmission of light through the crystal increased as the temperature rose from 30 to 300K. This can be understood by the fact that electrons in the crystal are particularly efficient at absorbing light at THz frequencies. In the superconducting phase, electrons are essentially frictionless particles that can interact and absorb incident light. The density of these types of electrons increases as the temperature drops. Therefore, we expect to see minimum transmission at the lowest temperatures, and then an increase in transmission as the crystal warms. We also studied the effects of incident intensity by increasing the THz electric field strength impinging the surface. Interesting, we observed that the amount of transmitted light increases as the THz field strength increases. This phenomenon is observed for all temperatures below Tc, while above Tc, the transmission is constant for increasing electric fields at a particular temperature. This observation suggests that the light breaks superconductivity at low temperatures. We also measured the time scale for the recovery of superconductivity in these films, which happens within several picoseconds after the pulse. With the successful completion of these experiments, future work will focus on additional superconductive materials. In parallel, the metamaterials will be used to drive nonlinear phenomena and chemistry in molecular crystals in ways that utilize the large field enhancement of the split ring resonators. In parallel with these experimental studies, efforts to increase community outreach at MIT focused on laboratory experiences for high school teachers and students. During each summer, the Nelson Group hosted a high school teacher from the local Boston area in an organized internship that allowed the teacher to learn advanced optical laboratory skills. In this program, the teachers spent time developing a research project that involved growing novel thin metal films, and then characterizing these films with a laser-based experiment. Results of these summer projects were presented at a poster session, which also provided the opportunity to develop contacts for a follow-up project. The teachers who worked in our laboratory recruited students from the local Boston and Cambridge area in an effort to develop a year-long research program.
