Valley-orbit coupled excitons in 2D semiconductor superlattices

Abstract

In monolayer transition metal dichalcogenides (TMDs), the electron-hole exchange interaction couples exciton's valley pseudospin to center-of-mass momentum, splitting the exciton dispersion into two branches of in-plane valley pseudospins, coupled to photons of linear polarization longitudinal and transverse branch to the exciton momentum, respectively. The Coulomb interaction is spatially modulated by placing on a periodic dielectric substrate, leading to renormalized dispersion and spatial texture featured wavefunction of the longitudinal excitons. One-dimensional (1D) dielectric substrate renormalizes the longitudinal branch's dispersion, which features isotropic and anisotropic energy minima as well as saddle points at the center and edge of the superlattice Brillouin zone. The longitudinal branch exciton’s wavefunction exhibits 1D features, confined to either the low- dielectric or high-dielectric regions, while transverse branch excitons retain the dimensional (2D) nature. The transmission and reflection of the longitudinal branch exciton at a dielectric-defined lateral interface can be described by a Snell-Descartes type law, analogous to that of the optical system, thus valley-polarized excitonic waveguide can be realized by using two parallel interfaces, while the transverse branch exciton is not affected by such interfaces. When the monolayer semiconductor is placed on 2D dielectric superlattice, we find the excitonic Bloch function features a spatial texture of valley polarization in the supercell, which is pattern locked to the propagation direction, enabling nano-optical excitation of directional exciton flow in the 2D plane. The phenomenon is reminesent of the chiral light-matter interaction in 1D nano-photonics structures, where the role of the spin-orbit coupled electromagnetic wave is now played by the valley-orbit coupled exciton Bloch wave. The left-right directionality of exciton current is controlled by the circular polarization of excitation, while the angular directionality is controlled by the excitation location, exhibiting a vortex pattern in a supercell. We also investigated exciton-polariton formed by layer hybridized exciton in TMDs heterobilayers which has a large optical dipole and permenant electric dipole. Placed in an optical cavity, the exciton-polariton can inherit the gauge structure from the photons, resulting in a spin/valley Hall effect, which presents electrical tunability through the control of exciton-cavity detuning.