A Tale of Spin-Spin Entanglement—High-Quality-Factor Chiral Metasurfaces to Enhance Spin in Monolayer Materials

Spin is one of the most fundamental properties of elementary particles (e.g. bosons and fermions). Spin coherence describes the preservation of the phase relationship among spins over time. For electrons, long spin coherence time is required not only to create efficient spin-based devices (e.g. spin transistors and memories) but also to perform quantum operations using spin qubits. Circularly polarized light (CPL) is light with spin angular momentum. In transition metal dichalcogenides (TMDC) monolayers, CPL can excite excitons that possess spin angular momentum matched to that of the CPL by selectively addressing either the K or K’ valley in the TMDCs’ Brillouin zone. The control and manipulation of such chiral light-matter interactions could enable future solid-state, optically addressable quantum information systems (QIS). Unfortunately, intervalley dephasing processes in TMDCs, such as strong intervalley scattering and phonon-assisted valley coupling, can detrimentally affect chiral light-matter interactions and lead to fast spin decoherence. These fast dephasing processes prevent TMDCs from being employed as room-temperature on-chip valley-selective devices for wide applications in valleytronics and QIS.<br/>Here, we present a new design of high-quality-factor (Q-factor) metasurfaces with chiral “meta-atoms” that can strongly favor exciton transitions from a particular valley (K or K’) in TMDCs and thus enhance valley-polarized emission regardless of excitation polarization, i.e., chiral Purcell effects. These high-Q chiral metasurfaces are achieved by engineering a chiral quasi-bound state in the continuum (qBIC) at ~770 nm that matches the A-exciton resonance of MoSe2 monolayers. Our fabricated crystalline Si metasurfaces on a glass substrate yield Q-factors of several hundred at the qBIC resonance. Both our steady-state and time-resolved photoluminescence spectral measurements reveal strong valley-polarized emission at elevated temperatures from 100 K to 294 K with a high degree of circular polarization approaching 50%. The angle-resolved PL measurements show that our chiral metasurfaces localize valley-polarized emission in the vicinity of the Γ-point, a signature of the chiral qBIC mode for directional emission control. This chiral light-matter coupling can be leveraged to create chiral electro-optic devices (e.g., chiral electroluminescent devices and chiral light detectors) and quantum light sources encoded with spin angular momentum.