Novel dye-doped cholesteric liquid crystal cone lasers with various birefringences and associated tunabilities of...

This study is the first to investigate novel cone lasers and the tunabilities of their lasing feature and performance based on dye-doped cholesteric liquid crystal (DDCLC) films with various LC birefringences (Δn). A unique conically-symmetric lasing ring with a low energy threshold occurs at a specific nonzero oblique angle (θ(ring)). The low energy threshold is comparable to those for common lasing signals occurring simultaneously at the short- and long-wavelength edges (SWE and LWE) of the CLC reflection band (CLCRB) for 0°. The lasing ring is induced by the enhancement in the density of photonic state for the fluorescence with a wavelength of λ(ring) based on an edge-overlapping effect, in which λ(ring) is just located at an edge-overlapping spectral position of the SWE of the CLCRB for 0° and the LWE of the CLCRB for θ(ring). The lasing feature (i.e., the lasing wavelengths of the three lasing signals and the emitted angle of the lasing ring) are tuned by varying Δn. The simulated relationship of an oblique angle with Δn, in which the SWE of the CLCRB for that oblique angle just overlaps the LWE of the CLCRB for 0°, can be obtained by calculating the dispersion relation of a planar CLC structure with various values of Δn based on Berreman's 4 × 4 matrix approach. The result of the calculation is highly consistent with the experimental data for the dependence of θ(ring) on Δn. Furthermore, the dependence of lasing performance (energy threshold and relative slope efficiency) on Δn for the three lasing signals is also measured, which findings can be used to qualitatively identify positive interaction or competition among the three lasing signals.