Wavelength Hopping in Cholesteric Liquid Crystal Lasers

Due to their birefringence and periodic structure, cholesteric liquid crystals (CLCs) in the helical cholesteric phase are one-dimensional photonic band gap materials. Gain enhancement and distributed feedback effects give rise to low threshold mirrorless lasing at the band edges. Since the wavelength at the band edge is proportional to the cholesteric pitch, which is a smooth function of temperature, one would expect the lasing wavelength to vary smoothly with temperature. Observations show, however, that the lasing wavelength does not depend smoothly on temperature, but instead exhibits periodic jumps between regions of smooth monotonic behavior. We have determined the reflection band dynamics, observed multiple lasing peaks at the hopping wavelength, and see evidence of hysteresis on measuring the reflection band and lasing peaks at different heating rates. We compare our observations with theoretical models, and propose an explanation for the observed dynamics.