A simple and versatile in situ fabrication of MAPbX nanocrystal-embedded polymer composite films is developed by controlling the crystallization process from precursor solutions. The composite films exhibit enhanced photoluminescence properties, improved stability, and excellent piezoelectric and mechanical properties. Applications of these composite films as color converters in liquid-crystal-display backlights are demonstrated, showing bright potential in display technology.

We investigated the fundamental properties of MAPbBr3 single crystal by applying temperature-dependent x-ray diffraction and photoluminescence (PL) measurements from 10 K to 270 K. The structural and spectral analyses illustrate the phase transitions of MAPbBr3 single crystal from cubic phase (-MAPbBr3) to tetragonal phases (-MAPbBr3 and -MAPbBr3), and then to orthorhombic phase (-MAPbBr3) at ~230 K, ~160 K and ~150 K, respectively. The Time-resolved PL results show that both free excitons (FE) and bound excitons (BE) have contributions to the PL emissions. The PL at high temperature mainly originates from the BE transitions, while the PL at low temperature (-MAPbBr3 phase) comes from both BE and FE transitions, and the FE related PL becomes more and more dominant at lower temperatures. The first-principle calculation at zero temperature reveals that the BE-related PL was caused by defects from the anti-site substitution of MA and Br ions.

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Polarized light is very necessary to achieve functional optical systems for display, imaging, and information storage. Luminescent materials with polarized emission are of great interest to achieve polarized light. Here, strong polarized photoluminescence from stretched perovskite‐nanocrystal‐embedded polymer composite films is reported by combining an in situ fabrication process with controllable mechanical stretching. The material characterizations show that perovskite quantum dots (QDs) in stretched composite films are oriented aligned into wires along the stretching direction. The optical measurements illustrate that the stretched composite films exhibit not only isotropic absorption but also polarized photoluminescence emission. This feature can be explained with their unique structure of “QD‐aligned wires”. The achieved polarization ratio is consistent with the calculated results by considering the dielectric confinement of optical electric field and exciton–exciton interactions. In addition, the optimized stretched composite films show strong photoluminescence emission with a polarization ratio of up to 0.33 and a quantum yield of 80%. The use of these composite films in liquid crystal display backlights has potential to increase the light transmittance of polarizers from 50% (without considering the optical loss) to 65%, which is of great significance to improve the energy efficiency.

We propose and experimentally demonstrate a refractive index (RI) sensor based on cascaded microfiber knot resonators (CMKRs) with Vernier effect. Deriving from high proportional evanescent field of microfiber and spectrum magnification function of Vernier effect, the RI sensor shows high sensitivity as well as high detection resolution. By using the method named "Drawing-Knotting-Assembling (DKA)", a compact CMKRs is fabricated for experimental demonstration. With the assistance of Lorentz fitting algorithm on the transmission spectrum, sensitivity of 6523nm/RIU and detection resolution up to 1.533 × 10(-7)RIU are obtained in the experiment which show good agreement with the numerical simulation. The proposed all-fiber RI sensor with high sensitivity, compact size and low cost can be widely used for chemical and biological detection, as well as the electronic/magnetic field measurement.