Coherent Elliptically Polarized Terahertz Wave Generation in WSe₂ by Linearly Polarized Femtosecond Laser Excitation

Abstract

Coherent polarization control of terahertz (THz) wave radiation in both the time-domain and the frequency-domain is significant in information technology, material science, and spectroscopic analysis. Elliptically polarized THz radiation is generally limited to chiral materials induced by circularly polarized light excitation. Herein, we demonstrate the coherent elliptically polarized THz radiation from few-layer tungsten diselenide (WSe2) in both the time-domain and the frequency-domain under linearly polarized femtosecond laser excitation. This coherent elliptical THz radiation is mainly dominated by in-plane anisotropic shift current and out-of-plane drift current, which is verified by the THz radiation dependence on the pump laser polarization angles, incident angles, and sample azimuthal angles systematically. The ellipticity and major axis direction of the elliptical THz wave can be efficiently controlled by either pump light polarization or sample azimuthal angle due to the controllable amplitudes and phases of two coherent orthogonal THz wave components. Our finding provides a method to distinguish drift and shift photocurrents in different directions and offers a unique design concept for elliptical THz generation with two-dimensional (2D) material physics.

Interplay between Ultrafast Shift Current and Ultrafast Photon Drag Current in Tellurium Nanotubes

“…As such, the bandwidth in the frequency-domain spectrum of the Te nanotubes (∼1.1 THz; Figure 3c,d) is narrower than that of the vertically grown graphene 40 and WSe 2 . 8 Such narrow bandwidth also excludes the influence of the coherent optical phonon (2.9 THz) that exist in the single-crystal Te. 25 As for the different polarization states of the incident laser, it is obvious that both the E THz-p and E THz-s components have the larger THz amplitude under the E in-p excitation than that under the E in-s excitation, and the polarities of the generated THz waveforms are reversed under the E in-p and the E in-s excitations.…”

“…The incident angle (θ) of the pump laser and the refraction angle (θ THz ) of the generated THz wave follows the general Snell's law as 39 The time duration of the E THz-p component is also larger than that of some nanomaterials and layered materials, such as vertically grown graphene (∼3 ps) 40 and WSe 2 (∼3 ps). 8 This might be due to the disordered nanotube morphology as the carrier transport needs to pass across the grain boundaries along different orientations, thereby slowing down the photocurrent dynamics and decreasing the THz frequency bandwidth. We have also obtained the absolute value of the THz electric field as 478 V/m, which is larger than the absolute THz electric field of the vertically grown graphene of 142.46 V/m 40 and WSe 2 of 23.15 V/m 8 (see detailed calculation process in Supporting Information).…”

“…The nonlinear optical effect is the manifestation of the nonlinear optical response for various materials, including the photon drag effect (PDE) 6 and the photogalvanic effect (PGE). 7,8 The PDE is caused by the incident photon momentum transfer to the carriers within the light penetration depth. 9 The carriers are "dragged" in the direction of the light propagation, resulting in the generation of photon drag current.…”

“…17 Especially, the THz emission spectroscopy makes unique advances in distinguishing linear and nonlinear photocurrents that concurrently generate in a single material. For instance, the contribution ratio of the shift current to drift current is 2:3 by analyzing the THz radiation mechanism of few-layer WSe 2 , 8 from 40% to 90% after decreasing the incident angle from −40°to 0°. 18 In these scenarios, the linear and nonlinear optical effects are not coupled together, which can be readily separated via analyzing the experimental data.…”

“…This is beneficial to minimize the energy loss, improve the light-to-current conversion efficiency, and surpass the Shockley-Quiesser limit, thereby taking a crucial step toward high-efficiency photovoltaic devices. The nonlinear optical effect is the manifestation of the nonlinear optical response for various materials, including the photon drag effect (PDE) and the photogalvanic effect (PGE). , The PDE is caused by the incident photon momentum transfer to the carriers within the light penetration depth . The carriers are “dragged” in the direction of the light propagation, resulting in the generation of photon drag current.…”