  In this paper we study thermo-optical effects in gallium phosphite photonic
crystal cavities in the visible. By measuring the shift of narrow resonances we
derive the temperature dependency of the local refractive index of gallium
phosphide in attoliter volumina over a temperature range between 5 K and 300 K
at a wavelength of about 605 nm. Additionally, the potential of photonic
crystal cavities for thermo-optical switching of visible light is investigated.
As an example we demonstrate thermo-optical switching with 13 dB contrast.
