Ultra-thin WS₂ waveguide for efficient light guiding and modulation

Optical waveguide has been studied to guide light over long distances with low losses while confining photon energy to a small area. In particular, the plasmonic waveguide has been spotlighted due to its ability to confine photon energy in a much smaller area. However, a severe propagation loss of a plasmonic guided mode due to the scattering at a rough metal surface and ohmic loss remains a crucial issue. Here, we propose exciton-polariton waveguide made of WS₂ ultra-thin film (20 nm). Using theoretical simulations, we directly compared the WS₂ waveguide with two conventional waveguides (silver waveguide and Si₃N₄ waveguide). Simulation results showed that the effective mode area of the WS₂ waveguide is about half size of the Si₃N₄ waveguide whereas the propagation loss is about the half value of the silver waveguide. The characteristics of the WS₂ waveguide was confirmed experimentally by coupling a white light to the waveguide. Finally, we demonstrated a WS₂ waveguide modulator based on pumping-power dependent polariton dispersion relations. Because of the large nonlinearity of the polariton system, efficient light modulation can be achieved even with a short length of the modulation (2 μm). The advantage of the WS₂ waveguide are expected to contribute to the miniaturization of photonic devices in the future.