In this award, funded by the Experimental Physical Chemistry Program of the Chemistry Division and the Solid State Chemistry Program of the Division of Materials Research, Prof. Keith A. Nelson of the Massachusetts Institute of Technology and his postdoctoral and graduate student colleagues will work to further develop methods for exciting and controlling the vibrational and electronic degrees of freedom in condensed phase materials. Prof. Nelson and his group will use optical excitation and control of lattice vibrations (optic phonons) and coupled lattice vibrational/electromagnetic waves (phonon-polaritrons) motions to learn details about condensed phase order, dynamics and structure, as well as to generate tailored terahertz (THz) frequency fields to be used in THz spectroscopy and coherent control. The methods that Prof. Nelson and his group develop should become useful tools for other scientists interested in developing a firmer understanding of the properties of liquids and solids.
Besides the broader scientific and technological impacts of the research being supported, Prof. Nelson continues to introduce high school students to the excitement of cutting edge research in a dedicated laboratory, through his "Lambda Project."
Light can affect materials in many different ways. It can start photochemical reactions or photosynthesis, it can produce heat, and at high intensity it can be used to inscribe patterns or cut and shape industrial components. It can be used to monitor chemical reactions, to read and write DVDs, and to measure distances. In our NSF project we extended the ways that light can be used to control and study complex materials in three ways. We developed new ways in which light can generate ultrasonic (sound) waves at frequencies far higher than our ears can hear. Human hearing extends to kilohertz frequencies (thousands of cycles/second); the acoustic waves we generated and monitored have megahertz and gigahertz frequencies, millions and billions of cycles/second. The measurements told us a great deal about complex material behavior. For example, in mechanical engineering it is taught that the difference between a solid and a liquid is that if you try to push one part of the material past another (a shear stress), the liquid flows while the solid does not. But even the liquid resists shear for a short time, and this can be discovered by seeing whether a shear acoustic wave propagates in the liquid or not. If the acoustic frequency is very high, so the shear wave oscillates back and forth faster than the molecules in the liquid can flow past each other, then no flow occurs and the liquid acts like a solid: the shear wave propagates. If the acoustic oscillations are slower than the time needed for the molecules to flow past each other, then they never come back: there are no oscillations and there is no propagating shear acoustic wave. We studied acoustic wave propagation in viscous liquids, which flow very slowly at cold temperatures. (If you refrigerate honey and then try to pour it, you'll see how slow!) Our measurements showed that as the temperature is cooled, the time needed for molecular flow slows down in a very predictable way. That has practical applications for industrial processing of viscous materials like polymers, which are used to make plastics. Processing is done at high temperature so the materials can be poured and shaped as needed, then the formed parts are cooled and hardened. Our results suggest that we can predict the flow dynamics over wide ranges of temperatures and time scales, without needing to make detailed measurements at all those temperatures and time scales for every material we use. It also means we can model material performance under widely varying conditions including extreme temperatures. In related measurements, we used light to drive and monitor terahertz-frequency oscillations - trillions of cycles per second. These are vibrations of neighboring atoms against each other inside a crystalline solid. Computer simulations showed that if we can drive the atoms hard enough, we can move them permanently into new positions in the crystal lattice, generating an altered crystalline structure! If our ongoing experimental efforts succeed, we will have a novel way to control material structures and properties. Because electrons are much lighter than whole atoms, they move faster and their oscillation frequencies are even higher, typically hundreds of terahertz. Using new ways to control and monitor electronic oscillations in semiconductors, we found that the oscillation frequency and damping rate of one electron is changed by other nearby electrons. This happens because electrons are charged and they interact with each other. The oscillations of multiple electrons (up to six) can become strongly correlated, with highly synchronized motions that continue for many oscillation cycles. Highly energized states involving multiple electrons are useful for semiconductor lasers and related applications. To advance the use of light for material control, the key is often how we control the light itself. For example, to generate a high-frequency sound wave we use a sequence of short laser pulses. Each light pulse generates one cycle of the sound wave as shown in the figure. We tune the sound frequency by varying the time between pulses. The other oscillations we drive are also controlled by the timing and form of short laser pulses. Our NSF project had broad impact beyond our own research. The methods we developed are now used around the world by other researchers. They are also instructive. We bring high school students to the lab to conduct experiments using light to generate and monitor acoustic waves, as in our research. The students learn about light waves and sound waves, and they learn how to fabricate thin metal films for their samples. Their measurements of sound waves in a film reveal the film thickness. That noncontact measurement is made in microelectronics fabrication facilities using a commercial instrument that is based on the methods that we developed.
