Optical properties of real photonic crystals: anomalous diffuse transmission

Unavoidable structural disorder in photonic crystals causes multiple scattering of light, resulting in extinction of coherent beams and generation of diffuse light. We demonstrate experimentally that the diffusely transmitted intensity is distributed over exit angles in a strikingly non-Lambertian manner, depending strongly on frequency. The angular redistribution of diffuse light reveals both photonic gaps and the diffuse extrapolation length, as confirmed by a quantitative diffusion theory that includes photonic band structures. Total transmission corrected for internal reflection shows that extinction increases slower with frequency than Rayleigh's law predicts. Hence disorder affects the high-frequency photonic bandgap of fcc crystals less severely than expected previously.