  We experimentally investigate power sensitive photothermal tuning (PTT) of
two dimensional (2D) graphene oxide (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. Raman spectra at different positions of a hybrid waveguide with
permanently reduced GO are characterized, verifying the inhomogeneous GO
reduction along the direction of light propagation through the waveguide. The
differences between the PTT induced by a continuous wave laser and a pulsed
laser are also compared, confirming that the PTT mainly depend on the average
input power. These results reveal interesting features for 2D GO films coated
on integrated optical waveguides, which are of fundamental importance for the
control and engineering of GO properties in hybrid integrated photonic devices.
