All-optical control of light, which refers to direct manipulation of light with light, has attained much attention in both scientific studies and practical applications. Conventional all-optical control devices are realized with nonlinear optical effects, whose performances are generally limited by the weak nonlinearities of common optical materials. In recent years, the use of graphene as a nonlinear optical material has opened up many new possibilities for controlling light with light. In particular, the integration of graphene into optical waveguides has shown great promise in harnessing its strong nonlinearity. Among the various nonlinear effects exhibited by graphene, the photothermal effect, which refers to the effect of converting light absorbed by graphene into heat, offers a particularly effective physical mechanism for the realization of on-chip all-optical control devices.

This thesis consists of a series of theoretical and experimental studies on the exploration of the photothermal effect of graphene in graphene-buried polymer waveguides for the realization of various all-optical control functions, where signal transmission can be controlled either by pump light at a different wavelength or by the signal light itself. Low-index-contrast polymer waveguides allow the use of TE-polarized light to maximize graphene’s absorption and TM-polarized light to suppress graphene’s absorption. Such a feature is not available with high-index-contrast waveguides. The large thermo-optic coefficient and the low heat conductivity of polymer can help to reduce the input power required for the observation of the photothermal effect of graphene.

In the first study, we propose a low-power all-optical switch based on the structure of a graphene-buried balanced Mach-Zehnder interferometer. Our experimental device fabricated with polymer waveguides buried with 5-mm long graphene shows a pump absorption of 10.6 dB (at 980 nm) and a graphene-induced signal loss of 1.1 dB (at 1550 nm) and can switch the signal light with a pump power of 6.0 mW at an extinction ratio of 36 dB. The rise and fall times of the switch are 1.0 and 2.7 ms, respectively. Our switch can be butt-coupled to single-mode fibers and could find applications in fiber-based and on-chip all-optical signal processing.

In the second study, we demonstrate all-optical mode switching with an asymmetric directional coupler (DC) buried with 6.2-mm long graphene. Our device can spatially switch between the fundamental mode and the higher-order mode with extinction ratios larger than 10 dB (at 1580 nm) and switching times slightly shorter than 1 ms at a pump power of 36.6 mW. The graphene-induced signal loss is only 0.1 dB. This new all-optical control function could find applications in mode-multiplexing systems.

In the third study, we realize an all-optical tunable broadband filter based on the structure of a long-period waveguide grating (LPWG) formed in a graphene-buried polymer waveguide. Our experimental gratings designed for operation at ∼1550 nm and pumped at 980 nm, which contain ∼6-mm long graphene, achieve a tuning efficiency of approximately −0.7 nm/mW, which corresponds to a blue shift of ~20 nm in the resonance wavelength with a pump power lower than ∼30 mW. Our grating can also serve as an all-optical switch when operated at a fixed wavelength, which can provide an extinction ratio larger than 20 dB with a pump power lower than 30 mW at a response time of ∼2.0 ms. The graphene-buried grating platform could be further explored for the development of a wide range of wavelength-sensitive all-optical control devices.

In the above studies, the TM-polarized signal light is controlled by the TE-polarized pump light at a different wavelength. In the last series of studies, we demonstrate nonlinear mode-coupling effects in graphene-buried polymer waveguides, where the TE-polarized signal light is controlled by itself. We implement three graphene-buried waveguide structures: a graphene-buried LPWG, a symmetric DC with graphene buried in two cores, and a symmetric DC with graphene buried in one core. We develop physical models for these nonlinear structures based on the coupled-mode theory, which serve well to explain the experimental results. The nonlinear mode-coupling effects generated in our 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.