Ultrafast Terahertz Emission in Centrosymmetric Films Driven by Nonlinear Photon-Drag Injection Currents

The nonlinear injection photocurrent, a second-order photogalvanic effect absent in centrosymmetric structures, can be revived by the photon-drag effect (PDE), leading to efficient terahertz (THz) radiation. Previous studies on THz emission from centrosymmetric films under oblique incidence attributed the effects to PDE-induced nonlinear photocurrents, and pump polarization-dependent THz emission was analyzed usually with the material’s point group. However, these studies overlooked two critical issues: (1) the distinction between the photon-drag shift and injection currents in transient THz radiation and (2) the irrelevance of pump polarization-dependent THz emission to the point group in centrosymmetric materials. Our theoretical analysis reveals that when the band velocity of conduction bands is a substantial difference from that of valence bands, the photon-drag injection current dominates in thin films with inverse symmetry following optical excitation, while the nonlinear photon-drag shift current is negligible. This theory is supported by ultrafast THz emission spectroscopy on a 1T′-MoTe₂ film and aligns well with existing literatures. This study introduces a new efficient THz emitter and enhances the understanding of nonlinear photon-drag currents in centrosymmetric materials, potentially guiding the design of THz radiation devices.