Nonlinear photocurrents in topological semimetals encode quantum geometric characters of Bloch wavefunctions and band topology, and offer new opportunities for advanced photovoltaics. Wen group has characterized the nonlinear photoconductivities of chiral multifold semimetal CoSi through a refined THz emission spectroscopy. The results reveal a large linear shift conductivity (17 μA/V²), and confirm a giant injection conductivity (167 μA/V²) 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 not only addresses the nonlinear optical origin of photocurrents in chiral transition metal monosilicides, but also highlights their potential for mid-infrared photovoltaics via various nonlinear optical channels. This work was selected as Editor’s Suggestion in Physical Review B.

Interface-specific hydrogen (H)–bonding network of water directly controls the energy transfer and chemical reaction pathway at many charged aqueous interfaces, yet to characterize these bonded water layer structures remains a challenge. We now develop 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 a model surfactant-water interface reveals a hidden weakly donor H-bonded water species, suggesting an asymmetric hydration-shell structure of fully solvated surfactant headgroups. In another application to a zwitterionic phosphatidylcholine lipid monolayer–water interface, we find a highly polarized bonded water layer structure associating to the phosphatidylcholine headgroup, while the diffuse layer contribution is experimentally proven to be negligible. Our all-optic method offers an in situ microscopic probe of electrochemical and biological interfaces and the route toward future imaging and ultrafast dynamics studies.

Optical Physics Lab (Wen group) is a multidisciplinary research team at Institute of Physics, Academia Sinica, Taiwan. We developed advanced nonlinear/ultrafast optical spectroscopic techniques, aiming to elucidate the light-matter interactions at interfaces and novel quantum materials. We particularly focus on physical chemistry phenomena at liquid water surfaces – the most important platform for chemical and biological processes. We probe vibrations of the interfacial molecules selectively by nonlinear optical means, offering the unique opportunity to establish the structure-property-function relation of the water interfaces on the molecular level. We are also interested in nonlinear optics of quantum materials, particularly the effect of quantum geometry of the band structure on the nonlinear photocurrent generation. The study promotes the development of next-generation optoelectronic and solar-energy applications.