This study investigates and develops for the first time an electrically tunable liquid-crystal-polymer composite laser with a symmetric sandwich structure. This structure includes two identical polymer-stablized cholesteric liquid-crystal (PSCLC) layers, and a dye-doped nematic LC (DDNLC) layer is sandwiched between them. The PSCLC and DDNLC layers act as distributed Bragg reflectors and half-wave plate with a gain medium, respectively. The entire cell behaves as an optical cavity, where the resonant modes occur at the maxima of the cell's transmission spectrum. Through the competition among the resonant modes by considering the gain and loss, the resonant mode with the lowest lasing threshold can be sieved out for lasing occurrence. When the same voltage is applied to the two PSCLC layers, the pitch gradients are simultaneously formed through the two layers because of the electrically induced ion-concentration gradients. When the voltage increases, the pitch gradients increase, thereby leading to the expansion of the cell photonic band gap (PBG) and blue shift of the lasing wavelength. The sandwich cell PBG can expand from 55 nm to over 170 nm, and the tuning range of the lasing wavelength is as high as 70 nm, which is the widest ever achieved in LC-related sandwich systems. The proposed configuration of the sandwich sample presents new insights into tunable LC PBG and laser devices, thereby providing potential applications in the form of sensors, medical imaging, displays and lighting, among others.
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