Temperature dependence of the photonic bandgap and the orientational order parameter for a cholesteric photonic crystal

The reflection spectra of a cholesteric photonic crystal have been measured. The experimental spectra are described by a theoretical expression that follows from the analytical solution of the Maxwell equations. The photonic bandgap width Delta nu has been determined. The photonic bandgap width changes abruptly as the position of the diffraction band changes. The temperature dependence of the relative bandgap width Delta nu/nu(0) and the order parameter for a photonic crystal are described by Landau's theory of phase transitions.