Graphene-Buried Polymer-Waveguide Platform for the Study of Nonlinear Mode Coupling

Nonlinear mode coupling, the phenomenon of power-dependent coupling between two modes in an optical waveguide or fiber structure, has been studied extensively over the last 40 years for the achievement of nonlinear optical functions, such as nonlinear switching. The conventional approach of realizing nonlinear mode coupling is based on the Kerr effect. However, the Kerr nonlinearity of common optical materials is weak and it is necessary to use short laser pulses with high peak powers (several hundred watts or above) for the observation of nonlinear mode coupling effects, which imposes huge restriction in practical applications. In recent years, we have proposed a graphene-buried polymer-waveguide platform for the study of all-optical control of light. Here we further explore this platform for the realization of nonlinear mode coupling based on graphene’s photothermal effect, i.e., the effect of converting light absorbed by graphene into heat. We study two graphene-buried waveguide structures: a long-period waveguide grating for the demonstration of nonlinear coupling between a guided core mode and a cladding mode, and a symmetric directional coupler for the demonstration of nonlinear coupling between the modes of two separate waveguides. We develop physical models for these structures based on the coupled-mode theory and fabricate these structures with our in-house microfabrication facilities based on spin-coating and photolithography. Our experimental results agree well with the theoretical analyses, as shown by the typical results in Fig. 1. Our graphene-buried waveguide structures show Kerr-like nonlinearity, but the input powers required are much lower (within several tens of milliwatts), which are available with common continuous-wave lasers. The graphene-buried waveguide platform allows the generation of strong nonlinear mode-coupling effects at low powers and provides much flexibility in nonlinearity engineering, which can greatly facilitate the exploration of nonlinear mode-coupling effects in different waveguide structures for practical applications.