THz Emission Spectroscopy of Surface Photogalvanic Effects in the Chiral Weyl Semimetal RhSi

One of the signature characteristics of Weyl semimetals is the topologically protected open Fermi arc surface states that connect surface projections of pairs of opposite parity bulk Weyl points. Although photoemission has confirmed the existence of these exotic states, investigation of their electronic and optical properties is complicated by the overwhelming response from the bulk. This provides a unique opportunity to second order nonlinear optical spectroscopies, such as the linear and circular photogalvanic effects, that can selectively couple to surface responses when the bulk response is forbidden by lattice symmetry. In this talk, we will describe our measurements of these laser driven currents on the Fermi arc surface states of the chiral Weyl semimetal RhSi, which we measure through the THz radiation these impulsively driven currents emit. While our data broadly match the theoretical prediction for the amplitude of the current as a function of photon energy, we will discuss our observations of a surprising departure from theory on the direction that this current flows, instead being reminiscent of a higher degree of symmetry than permitted by the surface.