We measured the effect of optical excitations on the back-gating of epitaxial quasi-free-standing single-layer graphene grown on a semi-insulating vanadium-doped 6H SiC. With the help of measurements of the electric field profiles in SiC, we show that graphene gating is directly related to the field effect at the graphene-SiC interface caused by changes in the distribution of the electric field in the SiC substrate under illumination. This provides a novel pathway for a comprehensive understanding of the photogenerated charge and the critical role of the substrate in optoelectronic devices. The change in graphene conductivity after the gating voltage is switched off is slow because of a very slow charge de-trapping from deep levels in silicon carbide. However, additional band-to-band optical excitation can significantly accelerate switching dynamics in graphene, especially when the light absorption is strongly localized below the graphene. Such an optically assisted field effect was enhanced by additional infrared light because of optical manipulation of the SiC deep level occupancy. • Optical assistance enables effective back-gating in epitaxial graphene on SiC. • Band-to-band illumination of the SiC substrate enhances gate-channel coupling. • Below-bandgap light eliminates slow gate response caused by deep-level traps. • Optical gating manipulates charge density without additional fabrication steps. • A physical model is supported by E-field measurements and drift simulations.