We are interested in the use of ultrafast optical pulses to study dynamics and phenomena in a variety of complex materials over a broad frequency range (terahertz to x-rays). Some recent examples multiferroics and ferroelectric/ferromagnet oxide heterostructures, ultrafast dynamics in two-dimensional nanosystems (e.g. graphene, topological insulators, transition metal dichalcogenides), and spatiotemporally resolved carrier transport in silicon core/shell nanowires. We further develop new ultrafast optical techniques for interrogating materials over a broad wavelength range with high temporal and spatial resolution (e.g. ultrafast wide-field optical microscopy, infrared pump/optical probe techniques etc.). Finally, we are also interested in combining metamaterial and plasmonic structures with complex materials to control their properties and reveal new phenomena.
Time-resolved second harmonic generation measurements on a ferroelectric/ferromagnet oxide heterostructure reveal coupling between magnetic and electric order developing within tens of picoseconds, limited by spin-lattice relaxation in the ferromagnetic layer (Y.-M. Sheu et al, Nature Communications, in press).
Metamaterials and Terahertz
Our research interests include fundamental and applied aspects of metamaterials, plasmonics, and electromagnetic phenomena. Recent examples include ultrathin metamaterial terahertz absorber, optically reconfigurable metamaterial, ultrafast surface plasmon polariton, and broadband plasmonic solar absorber. We are also interested in studying strongly coupled metamaterial system and their interaction with natural/artificial media. We have employed strongly coupled systems to enable electrically small efficient microwave antenna and tunable terahertz modulators. For this research, we extensively use nano/microfabrication facilities and a variety of spectroscopic techniques including time resolved terahertz and optical pump-probe systems. Additionally, we are involved in developing new measurement capabilities including time-resolved terahertz spectroscopy, terahertz ranging system, and microwave anechoic chamber for antenna characterization.
Ultrathin broadband solar light absorber
X-ray Imaging and Spectroscopy of Materials Dynamics
We are developing X-ray imaging and spectroscopy techniques for studying materials dynamics on a variety of systems. We use various sources from synchrotron and X-ray free electron laser based facilities to tabletop high harmonic generation sources to probe materials' structural, charge, and spin changes on the shortest of time scales (femtosecond to attosecond). We use these X-ray sources to study materials' electronic dynamics with time-resolved angle resolved photoemission, time resolved spectroscopy, and image nanometer-scale structural or magnetic spin dynamics with coherent X-ray imaging.
Diagnostics for Materials in Extremes
Our team develops various optical diagnostics to study materials in extreme conditions including shock, detonation, and plasma environments. These diagnostics have taken various forms over time, but include laser velocimetries (VISAR and photonic Doppler velocimetry - PDV), time resolved spectroscopies, THz spectroscopy, X-ray imaging, and optical fiber Bragg grating sensor development. These diagnostics have been used to support a wide array of NNSA and National Security missions.