are new types of semiconductors in which adjacent layers are held by van der Waals force. [1] TMDs have many rich physical properties such as extremely weak phonon-assisted photoluminescence (PL) in bulk structures, relatively strong PL yield in monolayer structures, [2][3][4][5] strong valley-dependent absorption in visible range, [6][7][8] and strong nonlinear optical response. [9][10][11][12] By taking advantage of these properties, significant progresses have been made in the development of electronic and optoelectronic technologies by these materials, [13,14] such as photodetectors, [15] field-effect transistors, [16][17][18] solar cells, [19,20] light-emitting diodes, [21,22] valleytronics, [8,23] integrated circuits, [24] and so on.The exciton binding energy in the monolayer TMDs has been observed in the range of 0.1-1.1 eV, [25][26][27][28] which is larger than that in traditional bulk semiconductors and traditional quantum well. [29,30] This rich excitonic state makes it possible to observe the complex exciton dynamics at room temperature, [31] such as charged and neutral phase of carriers, [32] trions, [33] and biexciton. [34] Meanwhile, bounded exciton could not only dominate the optical response but also play an important role in the optoelectronic processes, such as photocurrent generation and photoconduction in TMD semiconductors. [33,35,36] As a typical TMD material, the bulk WSe 2 has an indirect bandgap of 1.2 eV, and it would transform to the ≈1.65 eV direct bandgap as decreasing to monolayer. [37,38] WSe 2 has been widely used in optoelectronic devices due to its high absorption coefficients in the visible range. Field-effect transistors based on single crystal WSe 2 have been achieved and a carrier mobility comparable to silicon at room temperature was demonstrated. [39] Besides, WSe 2 can also play an important role in the development of van der Waals heterostructures, [40][41][42] in which long-lived interlayer excitons have been observed.Recently, ultrafast time-resolved photoexcited carrier dynamics has been studied in layered TMDs by the most widely used tools, such as optical-pump optical-probe spectroscopy, [43] PL spectroscopy, [38] photocurrent spectroscopy, [36] and electroluminescence. [35,44] The prevalentThe dynamics of photoexcited species is quite important for the development of next-generation ultrafast optoelectronic devices based on transition metal dichalcogenides (TMDs). Herein, time-resolved optical pump terahertz (THz) probe spectroscopy, which is sensitive to both bounded excitons and free electrons/holes, is employed to study the dynamics of photo-induced carriers in the typical layered TMDs crystal tungsten diselenide (WSe 2 ). Initial photoexcitation could generate both free carriers and excitons. The free carriers decay followed by phonon-assistance (≈30 ps) and defect-assistance (≈200-280 ps). The excitons decay followed by the phonon-assisted recombination (≈100 ps) and the defect-induced exciton separation (≈40-200 ps). With the increasing of pump fluence, more f...

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Strain engineering is an attractive method to induce and control anisotropy for polarized optoelectronic applications with two-dimensional (2D) materials. Herein, we have investigated the nonlinear optical coefficient dispersion relationship and the second-harmonic generation (SHG) pattern evolution under the uniaxial strains for graphene, WS2, GaSe, and In2Se3 monolayers. The uniaxial strain can break the in-plane symmetry of 2D materials, leading to both trade-off breaking of the nonlinear coefficient and new emergent nonlinear coefficients. In such a case, a classical sixfold ϕ-dependent SHG pattern is transformed into a distorted sixfold SHG pattern under the strain. Due to the lattice symmetry breaking and the uneven charge density distribution in strained 2D materials, the SHG patterns also depend on the excitation photon energy. The results could give a guide for the SHG pattern analysis in experiments, suggesting strain engineering on 2D materials for the tunable anisotropy in polarized and flexible nonlinear optical devices.

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.

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.