University of California, Berkeley
Shining Light on Topological Insulators and Weyl Semimetals
The last decade has witnessed an explosion of research investigating the role of topology in band-structure, as exemplified by the wealth of recent works on topological insulators (TIs) and Weyl semimetals (WSMs). In this talk I hope to convince you that optical probes of solids give unique insight into these topological states of matter. First, I will discuss how we can probe the special low-energy electrodynamics of 3D TI thin films of Bi2Se3 using time-domain THz spectroscopy. I will then discuss our work following the evolution of the response as a function of magnetic field from a semi-classical transport regime  to a quantum regime . In the later case, although DC transport is still semi-classical, we find evidence for Faraday and Kerr rotation angles quantized in units of the fine structure constant . This is consistent with the long-sought “axion electrodynamics” and the topological magneto-electric effect of 3D TIs. Among other aspects this give a purely solid-state measure of the fine structure constant based on a topological invariant . I will also discuss how optics can observe quantized Hall conductance without involving the edge states . Finally, I will present our most recent discovery of the largest 2nd harmonic generation in transition monopnictide Weyl semimetals such as TaAs  and talk about a new perspective of nonlinear optics in term of probing the Berry connection/curvature in momentum space . (The focus of my talk will be on Refs. [3, 4].)
1. Wu, et al, Nat. Phys. 9, 410-414 (2013).
2. Wu, et al, Phy. Rev. Lett. 115, 217602 (2015).
3. Wu, et al, Science 354, 1124-1127 (2016).
4. Wu, et al, Nat. Phys. (2016). doi:10.1038/nphys3969