Quasi-in-plane leaky modes in lasing cholesteric liquid crystal cells

Using a planar cell consisting of a prism and a flat glass, we have carried out precise measurements of the angles at which quasi-in-plane leaky (QIPL) laser modes propagate within a thin layer of a cholesteric liquid crystal (CLC) doped with a laser dye. Both the prism and the glass have refraction indices higher than relevant CLC indices, therefore, the waveguide effect in CLC is excluded. For this type of almost thresholdless lasing neither mirrors nor distributed feedback is necessary. The modes are generated in the amplifying CLC layer due to strong Fresnel reflections from the glass boundaries at propagation angles very close to 90° with respect to the cell normal. In the experiment, two modes polarized differently (s - and p -) have been found outgoing from the prism. Using equivalency between optical properties of a CLC and optically negative nematic liquid crystal at the propagation angles close to 90°, the analytical approach known for a uniform nematic liquid crystal was used for calculations of the threshold gain of the QIPL modes in a helical CLC. The gain has been found for the particular eigenmodes whose propagation angles have been measured. The experimental and calculated data on the propagation angles, polarization and threshold gain are in quantitative agreement.