Relaxing Symmetry Rules for Nonlinear Optical Interactions in Van der Waals Materials via Strong Light–Matter Coupling
Mandeep Khatoniar, Rezlind Bushati, Ahmed Mekawy, Florian Dirnberger, Andrea Alù, Vinod M. Menon
Abstract
Transition metal dichalcogenides (TMDCs) have been in the limelight for the past decade as a candidate for several optoelectronic devices and as a versatile test bed for various fundamental light–matter interaction phenomena thanks to their exceptional linear optical properties arising from their large binding energy, strong spin–orbit coupling, and valley physics in the monolayer (ML) limit. They also boast strong nonlinear properties fortified by excitonic responses in these systems. However, the strong second-order nonlinear responses are mostly restricted to the ML limit, owing to crystal symmetry requirements, posing several limitations in terms of smaller interaction length and lower damage threshold. Here we demonstrate a self-hybridized exciton–polariton system in bulk WSe2 that allows us to relax the crystal symmetry rules that govern second-order nonlinearities. The demonstrated polariton system shows intense second harmonic generation (SHG) when the fundamental wavelength is resonant with the lower polariton, with an efficiency comparable to the one from a ML WS2 when excited at the same fundamental wavelength and intensity. We model this phenomenon by considering a system with alternating second-order susceptibilities under an asymmetric electric field profile determined by the polariton mode. Helicity-resolved polarization experiments show a very similar nonlinear response as the one from a ML, where the helicity of the SHG flips with respect to the fundamental harmonic. This polaritonic system offers a platform to leverage a robust second-order nonlinear response from centrosymmetric systems, while at the same time allowing access to third-order nonlinearity inherent in strongly coupled systems.