Slope Efficiency Improvement in Mode-Hop Driven Tunable Single-Mode Cholesteric Liquid Crystal Laser

Abstract

We report the improvement in the slope efficiency of tunable single-mode cholesteric liquid crystal (ChLC) lasers with a three-layered structure. The device consists of one photopolymerized ChLC layer with a wide reflection band, another ChLC layer with a notch reflection band, and a rhodamine-6G-doped ionic liquid layer acting as the gain medium. Single-mode lasing is obtained from this device structure because the ChLC layer with the notch reflection band strongly reflects only one of the Fabry–Perot cavity modes, and the mode-hop-driven tuning of the lasing wavelength is possible because the reflection band of the notch ChLC can be tuned using external stimuli, such as heat. Our device showed a maximum slope efficiency of 16% (considering emission only in the forward direction), and was found to be approximately 1.5 times larger than that of ordinary ChLC lasers doped with the pyrromethene 597 laser dye. The improvement in the slope efficiency is believed to be attributed to the high quantum efficiency of the rhodamine 6G dye, which cannot be used in conventional ChLC lasers because of its low solubility, but can be used in our device because of the isotropic gain medium employed.