A chiral waveguide directional coupler using transition metal dichalcogenide monolayers

Recent discoveries and advances in transition metal dichalcogenide monolayer materials have brought new possibilities and research interests. When the materials come to atomically thin monolayers, their indirect energy bandgaps in bulk become direct band. The most unique feature of these TMD monolayers is the valley dependent property based on inversion symmetry breaking and spin-orbit coupling. That is, circular polarized light with opposite angular momentum can only excite excitons in a certain valley. This spin-valley coupling gives rise to a new degree of freedom of the material, enabling new applications and the emergence of spin-valleytronics. In this work, we demonstrate an on-chip broadband polarization-dependent directional coupler based on coupling between a glide-plane photonic crystal waveguide and WSe₂ monolayers. The direction of propagation and output in the device depends on the helicity of the input signal with a high directionality close to 0.5. This demonstrated coupling of spin-valley degree of freedom to photonic devices could serve as building blocks in integrated optical networks, and enlighten future works on utilizing this new degree of freedom in fundamental photonic elements such as diodes and transistors.