Nonlinear Photocurrent Spectroscopy and Polarization-Tunable Shift Current in the Layered Semiconductor $\mathrm{CuScP_2S_6}$
The class of layered metal thiophosphates and selenophosphates are emerging as promising candidates for non-linear photonic and optoelectronic applications. Here we report one-photon absorption (1PA) and two-photon absorption (2PA) induced photocurrents in $CuScP_2S_6$, establishing it as an optoelectronically-active non-centrosymmetric semiconductor. Spectrally-resolved 1PA and 2PA photoresponsivity paired with photoluminescence and differential reflectance measurements reveal a band gap of 2.35 eV, several higher energy interband transitions, and sub-band gap multiphoton excitonic transitions. Vertical graphite/$CuScP_2S_6$/graphite devices exhibit zero-bias photocurrent with a polarity controlled by both photon energy and optical polarization orientation within the ab-plane. This sign-dependence combined with a characteristic power-law transition from linear to square-root scaling are consistent with a shift-current origin to this bulk photovoltaic effect (BPVE). These findings establish $CuScP_2S_6$ as a multi-functional platform for polarization-sensitive non-linear optoelectronics.