Polarized THz Emission from In‐Plane Dipoles in Monolayer Tungsten Disulfide by Linear and Circular Optical Rectification

Abstract

Recent advances in the development of polarized terahertz (THz) emission from nanomaterials have not only opened up a new “TeraNano” interdiscipline but also provided a new tool for nonlinear optical process research. Herein, THz radiation mechanism of monolayer tungsten disulfide (WS2) is first investigated by both linear and circular polarization laser excitations at room temperature. The results reveal that polarized THz emission is dominated by the optical rectification based on in‐plane nonlinear dipoles, which is totally different from that of bulk WS2. The mechanism is verified by the azimuthal angle and pump polarization angle dependence of THz emission in both experiment and theory. Furthermore, controllably elliptically polarized THz emission is observed with the maximum ellipticity of ≈0.52 based on nonresonant nonlinear process under the circularly polarized excitation. A clear understanding of THz radiation mechanism of 2D materials will facilitate further design, optimization, and polarization control of integrable 2D THz optoelectronics.

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

“…This ellipticity is not only controllable but also larger than the maximum ellipticity from WS 2 (0.52) under the circularly polarized light excitation based on nonresonant nonlinear process. 12 Since the ellipticity is decided by the amplitude of E THz X and E THz Y components that contributed by shift and drift currents, the fitting of ellipticity as a function of polarization angle reflects the controllability of drift and shift currents with respect to the light polarization. In particular, this intrinsic THz emission polarization can be adjusted directly via rotating the polarization angle of the pump light without an external polarization device.…”

“…29 Additionally, the absorption spectrum of WSe for the synthetic THz polarization states. 12,14 Since the band gap of WSe 2 is below the photon energy of the pump laser to generate real photocarriers, the photocurrent effects are the main concern, as follows:…”

“…For example, elliptically polarized THz radiation is generated by air plasma with circular polarization at the fundamental and linearly polarization at the second harmonic frequencies . Some elliptically polarized THz waves are generated by some new optical effects such as circular optical rectification in WS 2 or circular photon drag effect in vertically grown graphene . One of the specific cases, which can generate elliptically THz radiation under the linear polarization laser excitation, is the Weyl semimetal TaAs .…”

“…Recently, transition metal dichalcogenides (TMDs) afford this opportunity to generate coherent transient photocurrent after the femtosecond laser excitation. For instance, the THz radiation generated from WS 2 , MoS 2 , MoSe 2 , and SnS 2 , suggest that there are at least two kinds of coherent orthogonal THz components with different ratios and different THz radiation mechanisms that can be achieved in TMDs. However, only one of the THz radiation mechanisms in these materials plays a leading role, which cast a shadow on the dawn for the tunability.…”

Broadband terahertz wave generation from an epsilon-near-zero material

“…However, the highly sophisticated nanofabrication process and the low laser damage threshold have largely hindered their wide adoption. Nanoscale THz emitters have also been realized with monolayer graphene 20 and tungsten disulfide 21 , although with limited efficiency. Furthermore, surface THz emission from semiconductors such as InAs, InSb, and GaAs has been investigated 22 , 23 , although efficient THz emission has only been shown in the reflection configuration.…”