Single mode lasing in multilayer sandwiched systems consisting of cholesteric liquid crystals and dye solution

We have investigated defect mode lasing when a cell containing a dye solution is sandwiched between two cholesteric liquid crystal cells. When the cholesteric pitch of both cells is equal, the dye emission generates the typical multi-mode lasing peaks inside the photonic stop band. We prepared two cholesteric mixtures whose pitches were shifted one respect to the other in such a way that only the edges of band gaps of the cholesteric liquid crystals layers overlapped. Using these mixtures we have observed single mode lasing. Moreover the divergence of the laser beam spot is considerably lower than the one observed in dye doped cholesteric liquid crystal lasers.