Terahertz Surface Emission from Layered MoS₂ Crystal: Competition between Surface Optical Rectification and Surface Photocurrent Surge

Abstract

Terahertz (THz) radiation of layered molybdenum disulfide (MoS2) crystal under femtosecond laser irradiation was observed using THz surface emission spectroscopy under variable-angle transmission configuration. Although MoS2 demonstrates inversion symmetry, surface-symmetry breaking will introduce the resonant optical rectification, which is consistent with the incident polarization and azimuthal angle dependences of the THz radiation from MoS2. However, the surface depletion field induced THz radiation will make an important contribution under oblique incidence, which is consistent with the radiation saturation due to the electrostatic screening effect by photoexcited carriers. This pump-dependent saturable THz radiation can be fitted well by the calculation from Maxwell equations with electromagnetic boundary conditions. The maximum of the surface depletion field is estimated to be 1.45 × 104 V/cm with 130 nm in depth under −40° incidence. Interestingly, when the incident angle is tuned from −40° to 0°, the optical rectification contribution varies from 40% to 90%. In addition, MoS2 is diagnosed to be p-type from THz waveforms by comparison with GaAs(100). The results not only afford a comprehensive understanding of THz radiation from layered materials like MoS2 but also put forward THz emission spectroscopy for characterizing the surface and interface properties of two-dimensional materials.

Ultrafast photocurrents in MoSe2 probed by terahertz spectroscopy

“…Similar outof-plane emission has been previously observed in other thick TMDCs and ascribed to a drift of photoexcited charge carriers in the built-in electric field appearing close to the surface of the sample due to the surface charged states (Fig. 2c) [26][27][28]. Indeed, such a mechanism is out-of-plane, is very weak or absent in the BL sample, and should cause THz emission on a picosecond timescale [44,45].…”

“…Following photoexcitation, we find relatively slow (time scale ∼ 0.6 ps) out-of-plane photocurrents that are dominant in the ML samples and fast (∼ 50 fs) in-plane photocurrents that are dominant in the BL devices. We ascribe the out-of-plane photocurrent in the ML to drift of photoexcited carriers in the surface electric field [25][26][27][28] and the in-plane photocurrent in the BL to a resonant shift current [29][30][31][32][33]. Finally, the large bandwidth of our experiment allows us to detect oscillations in the THz emission which we attribute to quantum beats between inter-and intra-layer excitonic states in MoSe 2 [34][35][36].…”

High‐Order Shift Current Induced Terahertz Emission from Inorganic Cesium Bromine Lead Perovskite Engendered by Two‐Photon Absorption

“…Compared to the other photocurrent electrical detection with electrodes, THz emission spectroscopy is a nondestructive way with more information such as polarization, amplitude, phase, and polarity . THz emission spectroscopy has been used for the photophysical understanding of some advanced materials such as layered materials MoS 2 , WS 2 , WSe 2 , CH 3 NH 3 PbI 3 , and topological insulator . However, these is no evidence yet of high‐order shift current by TPA process involved in THz emission possibly due to low TPA cross‐sections in these materials.…”

2D THz Optoelectronics

“…[ 153 ] However, polarization‐resolved measurements of the THz transients produced in bulk layered MoS 2 point to an additional contribution from surface optical rectification. [ 154 ] Recent studies of THz generation in monolayer WS 2 exhibit a distinctly different polarization‐ and angle‐dependence, because in monolayer form, the vertical transient current is absent and the nonlinear response is thought to arise entirely from in‐plane carrier dynamics and the lack of inversion symmetry.…”