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【Abstract】

This promotion talk summarizes our recent nonlinear optical studies on two condensed matter systems. First is about the electrical double layer that controls the energy transfer and chemical reaction pathway at many aqueous interfaces. We have developed a sum-frequency spectroscopic scheme with varying photon momenta as an all–optic solution for retrieving the vibrational spectra of the bonded water layer and the ion diffuse layer, and hence microscopic structural and charging information about an interface.
Application of the method to ionic structure at the air/water interface and interfacial hydrogen-bonding structures at model surfactant–water interface and zwitterionic lipid monolayer–water interface will be discussed.
Secondly, nonlinear photocurrents in topological semimetals encode quantum geometric characters of Bloch wavefunctions and offer new opportunities for advanced photovoltaics. In this direction, we have characterized the nonlinear photoconductivities of chiral multifold semimetals through a refined THz emission spectroscopy. The results reveal a large linear shift conductivity, and confirm a giant injection conductivity as a consequence of strongly interfered non-quantized contributions from the vicinity of multifold nodes with opposite chiralities. Bulk transverse injection currents and weak nonlocal photon drag effect are also identified. This work highlights the potential of chiral transition metal monosilicides for mid-infrared photovoltaics via various nonlinear optical channels.