Chiral absorption enhancement via critically coupled resonances in atomically thin photonic crystal exciton-polaritons.

Atomically thin transition metal dichalcogenides (TMDS) offer a promising route to the scaling down of optoelectronic devices to the ultimate thickness limit. But the weak light-matter interaction caused by their atomically thin nature makes them inevitably rely on external photonic structures to enhance optical absorption. Here, we report chiral absorption enhancement in atomically thin tungsten diselenide (WSe) using chiral resonances in photonic crystal (PhC) nanostructures patterned directly in WSe itself. We show that the quality factors (Q factors) of the resonances grow exponentially as the PhC thickness approaches atomic limit. As such, the strong interaction of high Q factor photonic resonance with the coexisting exciton resonance in WSe results into self-coupled exciton-polaritons. By balancing the light coupling and absorption rates, the incident light can critically couple to chiral resonances in WSe PhC exciton-polaritons, leading to the theoretically limited 50% optical absorptance with over 84% circular dichroism (CD).