Room-temperature valley-selective emission enabled by planar chiral quasi-bound states in the continuum

Optically addressable spin-photon interfaces in monolayers of transition metal dichalcogenides (TMDCs) are pivotal to realizing classical and quantum operations using photons. Valley pseudospin in TMDCs allows circularly polarized light to be coupled with electron (hole) spin, thus enabling initialization and readout of both classical and quantum information. Rapid valley-dephasing processes have impeded the development of scalable, high-performance valleytronic devices operating at room temperature. Here we demonstrate that a chiral resonant metasurface can enable room-temperature valley-selective emission. This platform, driven by chiral quasi-bound states in the continuum, provides circular eigen-polarization states featuring a high quality factor (Q-factor) and strong chiral near-field enhancement, and results in unitary emission circular dichroism (i.e. single-handed circularly polarized emission). Our fabricated high-Q-factor (> 200) Si chiral metasurfaces at visible wavelengths strongly enhance valley-selective optical transitions in devices incorporating MoSe2 monolayers under linearly polarized light excitation, achieving a high degree of optical circular polarization (DOP) from 100 K to 294 K and reaching nearly 0.5 at 294 K. The high DOP is attributed to enhanced exciton/trion radiative recombination rates for a specific valley. Our work could facilitate the development of compact chiral classical and quantum light sources and the creation of molecular chiral polaritons for quantum enantioselective synthesis.