Terahertz pulse emission from photoexcited bulk crystals of transition metal dichalcogenides

Abstract

Multilayer MoS2, MoSe2, and WSe2 crystals were excited with femtosecond optical pulses of various wavelengths. The emitted terahertz (THz) radiation pulses were found to be the most intense at photon energies coinciding with the direct energy gap at the K points of the Brillouin zone of corresponding materials. It was shown that the transition metal dichalcogenides of different dopant type emit THz pulses of opposite polarity, suggesting that the surface field plays a role in THz emission. Nevertheless, the optical pump–THz probe experiments contradicted THz emission observations, demonstrating a decrease in photoconductivity at higher energy quanta. The proposed THz pulse generation mechanism, therefore, assumes the accumulation of excitonic electric dipoles during optical pulse absorption and a fast build-up of the internal electric field inside the samples. This assumption was supported by double optical pulse investigations.

An interplay between optical rectification and transient photocurrent effect on THz pulse generation from bulk MoS2 layered crystal

“…All these studies were carried out at a single excitation wavelength for the above bandgap photoexcitation case. Within the above-bandgap excitation case, the dependence of the THz emission at a few excitation wavelengths was reported by Nevinskas et al [40], where the highest THz emission was found for the direct energy-gap excitation at the K-point of the Brillouin zone. However, a comprehensive study in the determination of the origin and role of microscopic THz emission mechanisms both above and below bandgap excitation is missing.…”

Semiconductor Characterization by Terahertz Excitation Spectroscopy

“…As a contactless experimental technique, THz excitation spectroscopy is particularly useful for studying the electrical properties of various semiconductor nanostructures and layered Van der Waals bonded materials. The initial studies of these materials with TES methodology made it possible to estimate the direct bandgap dependence of bismuth layers on their thickness, the influence of surface electron accumulation on GaInAs nanowires’ electrical properties, and the formation of dark excitons in layered transition metal dichalcogenide samples [ 89 ]. TES with improved sensitivity and spatial resolution can become the main electrical and optical characterization technique of nanostructures and electronic and photonic circuits based on them.…”

Ultrafast Shift Current in SnS2 Single Crystals: Structure Considerations, Modeling, and THz Emission Spectroscopy

“…[9][10][11][12] Another class of materials where THz emission spectroscopy has been applied to probe nonlinear optical properties is transition-metal dichalcogenides (TMDs) of the formula MX 2 , layered 2D semiconductors with van der Waals (vdW) interactions between the layers. [13][14][15][16][17] In a prototypical TMD material, MoS 2 , trigonal prismatic coordination exists between the Mo and S atoms. It does not have inversion symmetry in a monolayer form and exhibits second-order nonlinearities.…”

“…[17][18][19] In a naturally found and most commonly observed phase of MoS 2 , known as 2H-MoS 2 , bulk inversion symmetry is restored by layer stacking and experimentally observed shift current response arises solely in the surface layer. [13][14][15][16][17] In other members of layered MX 2 TMDs, such as TiS 2 and TiSe 2 , ZrS 2 , VSe 2 , and SnS 2 , chalcogenide atoms in a single layer do not stack directly above one another but are staggered, resulting in octahedral rather than trigonal coordination. [18,[20][21][22] In this case, monolayers possess inversion symmetry, ruling out second-order nonlinear effects such as the shift current or optical rectification.…”