Nonlinear mode coupling in graphene-buried optical waveguides

The photothermal effect of graphene, which refers to the effect of converting light absorbed by graphene into heat, offers an effective physical mechanism for the realization of all-optical control devices. In this paper, we explore this physical mechanism for the study of nonlinear mode-coupling effects with three graphene-buried waveguide structures: a graphene-buried long-period waveguide grating, a symmetric directional coupler with graphene buried in two cores, and a symmetric directional coupler with graphene buried in one core. We establish physical models for these graphene-buried waveguide structures based on the coupled-mode theory and experimentally implement these structures with polymer waveguides. Our experimental results agree well with the theoretical analyses. The nonlinear mode-coupling effects generated in the graphene-buried waveguide structures show similar characteristics as those achieved with Kerr nonlinearity, but the input powers required in our experiments are much lower (only several tens of milliwatts), which can be delivered by common continuous-wave lasers. The graphene-buried waveguide platform makes feasible the generation of strong nonlinear mode-coupling effects at low powers and offers much flexibility for nonlinearity engineering, which can greatly facilitate the investigation of nonlinear mode-coupling effects in different waveguide structures for practical applications.