Band-tunable color cone lasing emission based on a dye-doped cholesteric liquid crystal film

This investigation reports for the first time a novel phenomenon, called band-tunable color cone lasing emission (CCLE), based on a single-pitched one-dimensional photonic crystal-like dye-doped cholesteric liquid crystal (DDCLC) cell. The lasing wavelength in the CCLE pattern is distributed continuously at 676.7-595.6 nm as the oblique angle increases continuously from 0° to 50° relative to the helical axis. The variation of the lasing wavelength in the CCLE with the oblique angle is consistent with that of the wavelength at the long-wavelength edge (LWE) of the CLC reflection band (CLCRB) with the oblique angle. Simulation results obtained utilizing Berreman's 4x4 matrix method show that, at each oblique angle, the associated group velocity and density of photonic state (DOS) are near zero and large at the shortwavelength edge (SWE) and LWE of the CLCRB, respectively, and are in good agreement with experimental results. The particularly strong lasing ring emission at a cone angle of ∼35° can be explained to be likely due to a special effect that, under the condition of overlap between the LWE of the CLCRB measured at 35° and the SWE of the CLCRB measured at 0°, the LWE and SWE fluorescence propagating along 35° and 0°, respectively, may indirectly enhance each other due to individual enhanced rate of spontaneous emission. Furthermore, the lasing band of the CCLE can be tuned from long-wavelength (deep red-orange) to short-wavelength (orange-green) regions by changing the concentration of the chiral or by the photo-irradiation on a DDCLC cell with a photoisoemerizable chiral dopant.