Unveiling Transition from 1-Photon to 2-Photon Induced Photon Drag Current in Vertically Grown GaS by Terahertz Emission Spectroscopy

The nonlinear photon drag current offers a promising platform for fundamental understanding of nonlinear optical physics and application of modern nonlinear optoelectronic devices. However, the fine differentiation of both first-order and second-order photon drag current from other photocurrents is in challenge and calls for a universal method and experimental design. Herein, a wavelength-tunable terahertz (THz) emission spectroscopy as an all-optical method is proposed to probe ultrafast photocurrents in vertically grown GaS with high nonlinear optical coefficients. Based on the rich THz spectroscopic information, a systematic method is established to differentiate the photon drag effect induced by single-photon absorption (SPA-PDE, first order) and that induced by two-photon absorption (TPA-PDE, second order) among other linear and nonlinear optical photocurrents. The intriguing transition occurs from SPA-PDE to TPA-PDE under above- to below-bandgap excitation, deepening the understanding of PDE in wide-bandgap semiconductors. Quantitatively, the contribution ratio of drift current to SPA-PDE is calculated as 0.71:0.29 with above-bandgap excitation and the ratio of rectification current to TPA-PDE changes to 0.24:0.76 under below-bandgap excitation. This work proposes a universal framework to disentangle different photocurrents, especially SPA-PDE and TPA-PDE in THz emission spectroscopy both qualitatively and quantitatively, pushing THz technology toward modern ultrafast higher-order nonlinear optics.