Ultrafast Terahertz Emission in Centrosymmetric Films Driven by Nonlinear Photon-Drag Injection Currents
Long Geng, Wenjie Zhang, Qiushi Ma, Peng Suo, Kaiwen Sun, Chen Wang, Jun Peng, Yuqing Zou, Wei Wang, Xian Lin, Xiaodong Zeng, Guohong Ma
Abstract
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′-MoTe 2 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.