Multiwave out-of-normal band-edge lasing in cholesteric liquid crystals

Abstract

We have investigated the lasing effect in dye-doped cholesteric liquid crystals, where the photonic stop-band for light propagating along the helical axis coinciding with the layer normal is strongly shifted to longer wavelengths with respect to the luminescence band of the dye. In the absence of overlapping of the photonic and luminescence bands, the well-known lasing effect along the normal at the stop-band edges is forbidden. However, the lasing is still allowed for out-of-normal directions, for which the photonic band is shifted to shorter wavelengths corresponding to the luminescence band. Despite of the out-of-normal light propagation, the positive feedback necessary for the lasing is still available in a case of the monodomain cholesteric layer, when its thickness is much less than the optical pumping aperture. We have observed a significantly increased energy threshold for the out-of-normal lasing modes despite they are characterized by lower threshold gain compared to the modes propagating along the helix axis. The increased energy threshold is explained in terms of continuous distribution of the lasing emission over a large angular sector and energy leakage caused by the leaky lasing into the substrates.