We have experimentally studied the pump-probe Kerr rotation dynamics in WSe2 monolayers. This yields a direct measurement of the exciton valley depolarization time τv. At T = 4 K, we find τv ≈ 6 ps, a fast relaxation time resulting from the strong electron-hole Coulomb exchange interaction in bright excitons. The exciton valley depolarization time decreases significantly when the lattice temperature increases with τv being as short as 1.5 ps at 125 K. The temperature dependence is well explained by the developed theory taking into account exchange interaction and fast exciton scattering time on short-range potential.

Weyl semimetals are gapless topological states of matter [1][2][3][4][5][6][7][8][9][10][11][12] with broken inversion and/or time reversal symmetry, which can support unconventional responses to externally applied electrical, optical and magnetic fields. Here we report a new photogalvanic effect in type-II WSMs, MoTe2 and Mo0.9W0.1Te2, which are observed to support a circulating photocurrent when illuminated by circularly polarized light at normal incidence. This effect occurs exclusively in the inversion broken phase, where crucially we find that it is associated with a spatially varying

Topological photonics in strongly coupled light-matter systems offer the possibility for fabricating tunable optical devices that are robust against disorder and defects. Topological polaritons, i.e., hybrid exciton-photon quasiparticles, have been proposed to demonstrate scatter-free chiral propagation, but their experimental realization to date have been at deep cryogenic temperatures and under strong magnetic fields. We demonstrate helical topological polaritons up to 200 K without external magnetic field in monolayer WS2 excitons coupled to a non-trivial photonic crystal protected by pseudo time-reversal symmetry. The helical nature of the topological polaritons is verified where polaritons with opposite helicities are transported to opposite directions. Topological helical polaritons provide a platform for developing robust and tunable polaritonic spintronic devices for classical and quantum information processing applications.

Applications that use the orbital angular momentum (OAM) of light show promise for increasing the bandwidth of optical communication networks. However, direct photocurrent detection of different OAM modes has not yet been demonstrated. Most studies of current responses to electromagnetic fields have focused on optical intensity–related effects, but phase information has been lost. In this study, we designed a photodetector based on tungsten ditelluride (WTe2) with carefully fabricated electrode geometries to facilitate direct characterization of the topological charge of OAM of light. This orbital photogalvanic effect, driven by the helical phase gradient, is distinguished by a current winding around the optical beam axis with a magnitude proportional to its quantized OAM mode number. Our study provides a route to develop on-chip detection of optical OAM modes, which can enable the development of next-generation photonic circuits.

Topological photonics provides an ideal platform for demonstrating novel band topology concepts, which are also promising for robust waveguiding, communication and computation applications. However, many challenges such as extremely large device footprint and functionality at short wavelengths remain to be solved which are required to make practical and useful devices that can also couple to electronic excitations in many important organic and inorganic semiconductors. In this letter, we report an experimental realization of Z2 photonic topological insulators with their topological edge state energies spanning across the visible wavelength range including in the sub-500 nm regime. The photonic structures are based on deformed hexagonal lattices with preserved six-fold rotational symmetry patterned on suspended SiNx membranes. The experimentally measured energy-momentum dispersion of the topological lattices directly show topological band inversion by the swapping of the brightness of the bulk energy bands, and also 2 the helical edge states when the measurement is taken near the topological interface. The robust topological transport of the helical edge modes in real space is demonstrated by successfully guiding circularly polarized light beams unidirectionally through sharp kinks without major signal loss. This work paves the way for small footprint photonic topological devices working in the short wavelength range that can also be utilized to couple to excitons for unconventional light-matter interactions at the nanoscale. Keywords: photonic topological insulator, photonic crystal, visible-wavelength topological photonicsArising from the demonstrated generality of band topology concepts borrowed from solidstate electronic band structures, photonic topological insulators (PTIs) exhibit topologically protected edge states that lead to unique optical transport properties such as immunity to defects and lossless propagation through sharp turns, hence providing a promising platform to build robust photonic waveguides, communication lines and circuits. 1-3 Furthermore, given the flexibility of system design and fabrication with various optical materials, PTIs serve as testbeds in demonstrating concepts that are otherwise difficult to achieve in condensed matter systems, such as Floquet topological insulators 4-7 , higher order PTIs 8-14 , and PT-symmetric topological structures [15][16][17] . PTIs have now been demonstrated in various geometries, including gratings and arrays in one dimension 18-23 , photonic and plasmonic lattices and fiber arrays in two-dimensions 24-39 , as well as three-dimensional PTIs 40-42 , and with different mechanisms such as SSH insulators 19-23 , Chern 24-27 and valley Chern insulators [28][29][30][31][32] , and 2D and 3D Z2 topological insulators [34][35][36][37][38][39] . PTIs

Layered two-dimensional transition-metal dichalcogenides (2D-TMDs) are promising building blocks for ultracompact optoelectronic applications. Recently, a strong second harmonic generation (SHG) was observed in spiral stacked TMD nanostructures which was explained by its low crystal symmetry. However, the relationship between the efficiency of SHG signals and the electronic band structure remains unclear. Here, we show that the SHG signal in spiral WS 2 nanostructures is strongly enhanced (∼100 fold increase) not only when the second harmonic signal is in resonance with the exciton states but also when the excitation energy is slightly above the electronic band gap, which we attribute to a large interband Berry connection associated with certain optical transitions in spiral WS 2 . The giant SHG enhancement observed and explained in this study could promote the understanding and utility of TMDs as highly efficient nonlinear optical materials and potentially lead to a new pathway to fabricate more efficient optical energy conversion devices.

Two-dimensional semiconductors host excitons with very large oscillator strengthsand binding energies due to significantly reduced carrier screening. Two-dimensional semiconductors integrated with optical cavities are emerging as a promising platform for studying strong light-matter interactions as a route to explore a variety of exotic many-body effects. Here, in few-layered WS2 coupled with plasmonic nanoparticle lattices, we observe the formation of a collective polaritonic mode near the exciton energy and the formation of

Utilizing the spin or valley degree of freedom is a promising approach for realizing more energy-efficient information processing devices. Circularly polarized light can be used to generate spin/valley current in monolayer 2D transition metal dichalcogenides. We observe a geometrically dependent photocurrent in heterostructure M o S 2 / W S e 2 , where light with a different circular polarization generates photocurrents in opposite directions. Furthermore, we show that this photocurrent persists even at room temperature, and it can be controlled using an in-plane electric field and back gating. We explain the observed phenomena via valley-dependent valence band shift and the valley optical selection rule. This finding may facilitate the use of 2D heterostructures as a platform for opto-valleytronics and opto-spintronics devices.