My group’s research aims to deepen our understanding of new materials by studying how they interact with electromagnetic radiation, from the terahertz through the visible regions of the spectrum. In pursuit of this goal, we have developed and utilized advanced experimental techniques, including pump-probe optical spectroscopy, transient grating spectroscopy, magneto-optic Kerr effect measurements, nonlinear optics, and time-domain terahertz spectroscopy. Recently, we have also developed a scanning optical microscope that enables us to perform local time-resolved measurements of reflection, polarization rotation, and second harmonic generation. Using this suite of techniques, we are uncovering new phenomena in high-temperature superconductors, multiferroics, topological materials and frustrated magnets.
A major motivation in our current research is to discover how the quantum geometry and topology of electronic states in quantum materials controls their dynamical properties. We have recently explored topological materials in the family of Weyl semimetals, in which electrons effectively behave as massless relativistic Weyl fermions with a well-defined chirality. The nontrivial topological properties of these materials produce novel optical properties that are not only of fundamental interest but have exciting potential for applications in photonics and optoelectronics as well. For instance, we have discovered that the Weyl semimetals TaAs and NbAs exhibit extremely large nonlinear optical responses that manifest as exceptional figures of merit for second harmonic generation. Currently under investigation in our lab are Weyl semimetals that possess structural chirality (‘handedness’), for which theory predicts a quantized photocurrent whose direction can be reversed by switching the sense of circular polarization of incident light.
Many of the most exciting properties of quantum materials occur at boundaries that separate topologically distinct phases, where unique emergent electronic states arise. We are interested in investigating and ultimately controlling dynamical properties (motion) of topological interfaces through manipulation of these electronic states. Examples of such interfaces occur at Weyl semimetal/insulator boundaries and at domain walls in topological magnets. Each of these interfaces is predicted to host exotic electronic states that in principle have unique optical signatures. To enable unambiguous detection, understanding, and eventually control of these interfacial states, we are developing a next-generation time-resolved optical microscope with a spatial resolution on the 100 nanometer scale, pushing far-field optics to its ultimate limits.
Emergent Phenomena in Quantum Systems
University of California, Berkeley Department of Physics