Prashant Padmanabhan (Los Alamos National Laboratory)

Nonlinear optical processes occur when a materials polarization becomes a nonlinear function of one or more applied light-fields, leading to optical self-interaction and wave-mixing effects. These processes are not only foundational to a wide range of applications, including tunable laser sources and quantum information processing, but have recently emerged as versatile tools to reveal broken symmetries, intrinsic orders, and exotic properties in quantum materials. In this talk, I will begin by highlighting our recent efforts to harness these techniques to explore the nonlinear optical responses of polar van der Waals (vdW) crystals. Here, I will focus on an emerging class of bismuth telluro-halides where we are able to unambiguously link large enhancements and textures in their second harmonic output to their polar structure. I will then turn to nonlinear frequency-mixing processes in vdW crystals driven by “twisted” light carrying finite orbital angular momentum. Here, we demonstrate the free tuning of the wavelength, topological charge, and radial index of vortex light-fields, via three- and four-wave mixing processes, entirely supported by an atomically thin semiconductor monolayer. Finally, I will conclude with a discussion of how terahertz emission derived from nonlinear photocurrents can provide direct insight into the topological properties of chiral crystals. Taken together, our work emphasizes the immense promise of nonlinear optical spectroscopies in uncovering the fundamental nature of quantum matter.