Ultranegative angular dispersion of diffraction in quasiordered biophotonic structures.

We report that a three-dimensional quasiordered photonic structure, found in the cuticles of beetle H. sexmaculata, can diffract light in a "wrong" way and its angular dispersion is about one order of magnitude larger than that of a conventional diffraction grating. A new diffraction type of photonic bandgap (from an anticrossing of longitudinal and transverse modes) and additional disorder effect are found to play important roles in this phenomenon. Mimicking the structure could lead to novel optical devices with ultralarge angular dispersion.