Optical Creation and Probing of Non-Equilibrium Superconductors and Topological Semimetals

The high intensity and versatility of optical pumping allows the creation of new states in a material that may not exist anywhere in the equilibrium phase diagram. This talk focuses on electronic phases and on pump pulses, or alternately current drives, that are chosen to modify the symmetry of a material. For example, circularly polarized light provides a means to induce broken time-reversal symmetry in a non-magnetic material, while current drive breaks both time-reversal and inversion symmetries. Examples discussed include current-induced linear optical effects in semimetals and superconductors, where they provide important probes of topological band structure and order parameter symmetry respectively. These effects can be viewed as part of the nonlinear optical tensor, and we then turn to other kinds of nonlinear optical effects and a unique quantization effect obseved in certain Weyl semimetals. The closing discussion will attempt to illustrate the value that the general class of beyond-linear-response measurements is bringing to the investigation of quantum materials.<br/><br/>(The work reported includes previous collaborations with Michal Papaj, Kazuaki Takasan, Takahiro Morimoto, Tianrui Xu, and others, as described in the talk.)