Based on the selective reflection of cholesteric liquid crystals and the high photoluminescence efficiency of perovskite quantum dots, Hu Xiaowen and others from South China Normal University developed a new strategy to directly emit high asymmetry factor circularly polarized lasers. Research results show that in a Fabry–Pérot (F–P) resonant cavity constructed based on a double-layer right-handed cholesteric liquid crystal, under 800nm femtosecond laser pumping, right-handed amplified spontaneous emission with an asymmetry factor of up to 1.4 was detected. Each layer of the entire device is processed through solution, making the device display ultra-flexible. After 100 times of 180-degree bending, the amplified spontaneous emission intensity remains unchanged. This result realizes for the first time the high asymmetry factor circularly polarized amplified spontaneous emission of a flexible film based on perovskite quantum dots and cholesteric liquid crystals, opening up a new path for the development of perovskite materials in the field of circularly polarized lasers.
This paper adopts a full solution process to prepare independent flexible thin film laser devices through layer-by-layer (CLCs/PVA/PNC/PMMA/CLCs) coating and peeling processes, in which double-layer cholesteric liquid crystals (CLCs) serve as Fabry -Perot cavity (F-P) mirror, CsPbBr3 nanocrystal (quantum dot) (PNC) layer as gain medium, this laser device is highly flexible and translucent.
The right-handed CLCs film is used as the upper and lower reflective layers to selectively reflect the right-handed CPL and finally achieve polarization selectivity. The PNC is excited with an 800 nm femtosecond pulse laser based on two-photon absorption. Excited electrons can decay to a metastable or ground state through spontaneous emission. When the excitation intensity is large enough, particle number inversion occurs, resulting in amplified spontaneous emission (ASE) in PNC. Since the PNC film is sandwiched between two right-handed CLCs reflective layers, only the spontaneous emission of right-handed CP (R-CP) is continuously reflected, oscillated and amplified in the cavity, while the spontaneous emission of left-handed CP (L-CP) is emitted outward. , the final output is R-CP ASE/lasing.
For optoelectronic devices based on perovskite materials, achieving long-term stability is a very big challenge. This article also characterizes the stability. The device described in this article does not require further packaging, and after 3 months of storage at a humidity of 60% and a temperature of 25°C, its emission intensity decreased only slightly (~ 5%). This is attributed to the protection of the polymers PVA and PMMA, which insulate the PNC from moisture and oxygen. In addition, the device exhibits high operating stability under continuous excitation by ambient pump pulse laser. After continuous excitation for more than 20 minutes, the laser efficiency of the device remains above 80% of the initial value. The slight decrease in laser efficiency may be due to heat accumulation caused by continuous excitation, causing the CLC resonant cavity to be disturbed and thus degrading device performance. The laser intensity of the film remains unchanged even after 100 bending (180) cycles, indicating excellent mechanical flexibility. Under the excitation of 365 nm ultraviolet light, the film can adhere well to spherical, sharp corners, and curved surfaces and emit uniform green light, showing great application prospects in the field of flexible optoelectronics.
The above research results have been supported by the National Natural Science Foundation, Guangdong Natural Science Foundation, Guangzhou Science and Technology Plan Project, Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, Ministry of Education Optical Information International Cooperation Joint Laboratory and National Higher Education Institutions Discipline Innovation and Talent Introduction Financial support for projects such as Plan 111 Intelligence Introduction Base. The co-authors of the paper, Dr. Wang Kai from Pennsylvania State University in the United States, Professor Lakshminarayana Polavarap from the University of Vigo in Spain, and Associate Researcher Lu Ying from Changchun Institute of Optics and Mechanics, Chinese Academy of Sciences, provided strong support in the discussion and writing of the paper's results.
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