Density of states and lasing at the edge of a photonic stop band in dye-doped cholesteric liquid crystals

We present simulations of optical propagation in cholesteric liquid crystal (CLC) films and find a stop band in which the wave is evanescent and the density of states is zero. Sharp structure is found in the calculated transmission spectra near the band edge. This corresponds to enhanced residence times and lowered group velocities, as well as to significant enhancements of the energy density and of the density of states within the sample. These simulations are consistent with our measurements of suppressed emission within the stop band and of enhanced emission at the band edge from dye molecules doped into CLCs. We also observe spatially coherent emission from the CLC sample with narrow spectral lines at the edge of the stop band and a distinct threshold behavior for the coherent emission. The results of computer simulations together with observations of emission and lasing demonstrate that the optical properties of CLCs, including laser action at the modes closest to the band edge, are consequences of its band gap structure. The compact nature of these structures and the ease with which they can be fabricated suggest that they may be useful for producing integrated lasers and photonic devices.