Graphene oxide (GO) is a two-dimensional (2D) material that has attracted significant interest because of its many attractive properties such as broadband photoluminescence [1, 2], high Kerr nonlinearity [3, 4], strong material anisotropy [5, 6], broadband light absorption [7, 8], and tunable material properties in wide ranges [9, 10]. In addition, with its facile fabrication processes, GO has a strong capability for large-scale manufacturable on-chip integration [6, 11, 12].
The incorporation of GO into integrated photonic devices has led to the birth of GO integrated photonics, which has become a very active and fast-growing field [13]. Integrated photonic devices incorporating GO films have been demonstrated for a range of applications, such as light absorbers [7, 8, 14], optical lenses and imaging devices [10, 11, 15], polarization-selective devices [6, 16], sensors [17, 18], and nonlinear optical devices [19–23].
Since GO can be converted to a reduced form with graphene-like properties under strong light irradiation or high temperature [24, 25], it has long been used as a precursor for the preparation of graphene [13, 26, 27]. Given the difference between the material properties of GO and reduced GO (rGO) [3, 11, 28], investigating the mechanisms and conditions for GO reduction in hybrid integrated photonic devices is of fundamental importance for applying this functional 2D material to integrated photonic devices [13, 29].
Previously, we observed power-sensitive photo-thermal changes in GO films coated on integrated photonic waveguides [19] and ring resonators [30] in nonlinear four-wave mixing experiments. In this paper, we provide a more detailed characterization for such changes arising from a range of effects such as photo-thermal reduction, thermal dissipation, and self-heating in GO layers. We experimentally investigate photo-thermal tuning (PTT) of 2D GO films coated on integrated optical waveguides. We measure the light power thresholds for reversible and permanent GO reduction in silicon nitride (SiN) waveguides integrated with 1 and 2 layers of GO. We identify three reduction stages according to the existence of reversible versus permanent reduction. Raman spectra at different positions of a hybrid waveguide with permanently reduced GO film are also characterized, showing the inhomogeneous nature of GO reduction the direction of light propagation through the waveguide. Finally, we compare the PTT induced by a continuous-wave (CW) laser and a pulsed laser with the same average power, and observe negligible difference between them. This confirms that the PTT mainly depend on the average power rather than the peak power of input light. These results reveal interesting features for the reduction of GO induced by the photo-thermal changes, which are useful for controlling and engineering GO’s material properties in hybrid integrated photonic devices.