Color control of natural fluorescent proteins by photonic crystals.

The emission of fluorescent proteins inside photonic crystals is studied. It is demonstrated that the apparent emission color of the fluorescent protein can be controlled externally by the photonic crystal. With increasing crystal lattice parameter, the appearance of the proteins turns from orange to red, and suddenly to green. The dramatic color changes agree with the theoretically expected redistribution of light escaping around the stop band of the photonic crystal. Our experiments show the potential of combining biological systems with nanophotonics. This "biophotonic engineering" may be extended to control emission rates and complex Förster energy-transfer systems obtained by protein engineering.