  Photonic crystal nanocavities at visible wavelengths are fabricated in a high
refractive index (n>3.2) gallium phosphide membrane. The cavities are probed
via a cross-polarized reflectivity measurement and show resonances at
wavelengths as low as 645 nm at room temperature, with quality factors between
500 and 1700 for modes with volumes 0.7(lambda/n)^3. These structures could be
employed for submicron scale optoelectronic devices in the visible, and for
coupling to novel emitters with resonances in the visible such as nitrogen
vacancy centers, and bio- and organic molecules.
