Carrier dynamics in solid–state materials

Fundamental electrical, magnetic, and optical properties of solid–state materials are determined by the dynamical response of carriers to internal and external fields. The near–equilibrium properties of carriers in most materials are well understood from classical studies of transport and optical conductivity. However, due to strong interactions of carriers among themselves and with the lattice, studies of nonequlibrium dynamics on femtosecond time scales (10 – 15 s) are just emerging. In our group, a particularly versatile and powerful technique, time–resolved two–photon photoemission (TR–2PP) spectroscopy, has been developed for studying the carrier excitation and relaxation processes in solid–state materials. With this technique we are investigating the quantum mechanical phase and carrier population relaxation times in metals, and for intrinsic and adsorbate induced surface states on metals. Of particular interest are the physical processes that induce e–h pair decoherence, since they impose limits on time scales for quantum control of carriers through the optical phase of the excitation light. The manipulation of the carrier phase with light may lead to applications such as ultrafast (>10 THz) switching and information processing, as well as, atomic manipulation of matter, and therefore, it is of great interest for advanced technologies in the 21st century.

Ultrafast microscopy

Understanding of the carrier dynamics under quantum confinement is a key to advancing nanoscale science and technology. Although with the existing scanning probe techniques we can potentially study dynamics of individual nanostructures, there is also a clear need for ultrafast imaging microscopic techniques in studies of dynamics in complex systems of nanostructures that could comprise ultrafast electronic or optical device. Photoemission electron microscopy (PEEM) is a well–developed surface science technique for imaging nanostructures on metal and semiconductor surfaces. In combination with femtosecond pump–probe excitation, we intend to develop time–resolved PEEM with potentially <1 fs, <20 nm, <100 meV carrier energy resolution. This technique will be applied to fundamental studies of carrier dynamics in individual nanostructures and coupled nanocomposite systems, in order to understand the fundamental physics of hot carriers in low dimensional systems and to develop advanced device concepts.

Atomic manipulation with light

Electronic excitation of clean or adsorbate covered metal surfaces can impulsively turn–on large mechanical forces that lead to mass transport parallel or perpendicular to the surface. When such forces are harnessed properly, they can be used for atomic manipulation or even atomic switching. Although there are now several examples of atomic manipulation with STM techniques, much less is known about how equivalent, but much larger scale manipulation could be accomplished with light. The recent observation and demonstration of quantum control of motion of Cs atoms above a Cu(111) surface by our group provides a proof–of–principle for the atomic manipulation of surfaces with light. Such studies are being extended to identify the factors that govern the electronic relaxation of adsorbates on metal surfaces, which can effectively quench the nuclear motion.

Awards

• Ahmed Zewail Award in Ultrafast Science and Technology, 2019
• Fellow of the American Association for the Advancement of Science, 2016
• Morino Award, 2014
• Chancellor's Distinguished Research Award, 2005
• Fellow of the American Physical Society, 2002