RIKEN Makes New Progress in Ultra-wide Temperature Range (-180~240 ℃) Blue Phase Liquid Crystal Laser

Blue-phase liquid crystals (BPLCs) lasers, with their low laser threshold, multi-stimulus response, multi-directional emission and real-time reconfigurability, have great application prospects in sensing, display and anti-counterfeiting. Currently, the research on blue phase liquid crystal lasers includes the tunability of the laser wavelength under external stimuli (e.g., light, electricity, heat, force, etc.), and the narrow temperature window of the BPLCs themselves has led to an increasing interest in the study of wide-temperature domain BPLCs lasers. The adoption of polymer stabilization systems has successfully broadened the temperature range of BPLCs to 500 °C, which also leads to the corresponding broadening of the temperature range of BPLCs lasers. However, compared with other organic lasers, the random crystallization of small molecules of the mobile phase in BPLCs at low temperatures and the poor compatibility between the dye and the system make it challenging to emit lasers below 0 °C in BPLCs. Moreover, the working mechanism of BPLCs lasers at low temperatures is still unclear. This severely limits the potential applications of BPLCs lasers in other low-temperature environments such as polar, deep ocean, and space. Therefore, the design of suitable BPLCs systems to meet good system compatibility and low-temperature antifreeze is important for the development of low-temperature BPLCs lasers.
In order to solve the above problems, the team of academician Jiang Lei and researcher Wang Jingxia from the Center for Bionanomaterials and Interface Science of the Institute of Physics and Chemistry, Chinese Academy of Sciences, prepared polymer-stabilized blue-phase liquid crystals with a wide range of temperatures (-190℃~360℃) in their previous work (Nat. Commun. 2021, 12 (1), 3477.); by adjusting the band gap centers and dye patterns of the blue-phase liquid crystals, we have been able to achieve the same results. By adjusting the prepared blue-phase liquid crystal bandgap center, dye ordering parameter, resonance cavity quality and pump energy, controlled one-to-four-mode surface emission lasing has been achieved in the resonance cavities of dye-doped blue-phase liquid crystals (C6-BPLCs) (Adv. Mater. 2022, 34 (9), 2108330.); the prepared blue-phase liquid crystals are used as templates for the preparation of highly resolved multicolor blue-phase liquid crystals. Using the prepared blue liquid crystals as templates, high resolution multi-color blue liquid crystals were prepared (Adv. Funct. Mater. 2022, 32 (15), 2110985.); and by regulating the polymer content of the blue liquid crystals, the polymer scaffolding system of the blue liquid crystals was obtained, and the temperature range of the BPLCs was extended to 25~230 ℃ (Adv. Mater. 2022, 34 (47), 2206580.). Mater. 2022, 34 (47), 2206580.
Recently, the research team has successfully realized a wide laser temperature range (-180-240 °C) below 0 °C by rational system selection and design, reducing the random crystallization of small liquid crystal molecules at low temperatures by all-polymerization, and selecting chain-flexible liquid crystal monomers (RM105) and dye molecules (DCMs) to improve the compatibility of the system. It was shown that the all-polymer BPLCs exhibited a narrow laser linewidth (0.0881 nm) and low laser threshold (37 nJ/pulse) due to good system compatibility; meanwhile, the all-polymerized system increased the photo-thermal stability of the samples, including sufficient reflectance/fluorescence signals, suitable quantum yields and fluorescence lifetimes, matched reflectance and fluorescence spectra, stable BPLCs fabrication and high decomposition temperature, which enabled the samples to emit laser light in -180-240 °C. In addition, the variation rules of the laser wavelength and threshold of BPLCs at low temperature (<0 ℃) are revealed for the first time, i.e., red-shifted laser wavelength and increasing laser threshold with decreasing temperature, resulting in a red-shifted laser wavelength and a "U"-shaped laser threshold in -180~240 ℃. These unique laser behaviors are related to the temperature-dependent anisotropic deformation of the BP lattice (-180-0 ℃: BPI lattice contracted along the (110) direction; 0-26.7 ℃: almost unchanged BPI lattice; 26.7-240 ℃: BPI lattice accelerated to expand along the (110) direction). This work not only opens the door to low-temperature BPLCs, but also provides important insights into the design of novel organic optical devices.
The results are presented as Super-wide Temperature Lasers Spanning from -180 °C to 240 °Cbased onFully-polymerized Blue Phase Superstructures, published in Advanced Materials.
The corresponding author of the article is Dr. Jingxia Wang of the Institute of Physics and Chemistry, Chinese Academy of Sciences. Yujie Chen, a PhD student at IUPAC, CAS, was the first author. Mr. Jing Li and Mr. Feng Jin from IUPAC helped the laser characterization of the blue-phase liquid crystals, Prof. Lei Shi from the Department of Physics, Fudan University helped the characterization of the photonic bandgap of the blue-phase liquid crystals, and Academician Lei Jiang from the Institute of Physics and Chemistry, Chinese Academy of Sciences, provided professional guidance and assistance for this study.
This research was supported by the National Natural Science Foundation of China and the Netherlands Research Program of the Chinese Academy of Sciences.

Figure 1. Chemical structure and characterization of fully polymerized BPLCs. a) Chemical structural formulae of the substances used in the fully polymerized samples of doped dyes; b) Schematic diagram of the microstructural changes of the samples in the -180 ～ 240 °C laser temperature domain; c) TEM plots; d) Kossel plots; Variable-temperature e) Reflectance spectra and f) Fluorescence spectra of the samples of -180 ～ 240 °C; g) laser wavelength versus temperature; h) comparison of the present work with the operating temperature range of blue-phase liquid crystal lasers in the literature.

Figure 2. Comparison of the performance of this all-polymer system with other systems and dye compatibility test. a) Comparison of the laser temperature range; b) Comparison of the laser threshold at room temperature; c) Dye solubility test under POM c1) 90.0 mg RM105 + 4.5 mg DCM; c2) 90.0 mg C6M + 4.5 mg C6, at 120 °C. This indicates that DCM has better compatibility with RM105. d-f) Theoretical calculations of cohesive energy density (CED), experimental system: RM105 + RM257 + DCM; control system: C6M + C6. The experimental system has a larger CED and solubility parameter (δ) than the control system, which suggests that the all-polymer system has better compatibility than C6M + C6. g) D) Theoretical calculations of DCM, RM105 + 4.5 mg DCM; c2) 90.0 mg C6M + 4.5 mg C6 at 120 ℃. (g) DSC plot, there is only one glass transition temperature (Tg = 26.7 ℃) for the all-polymer sample, while there is not only a Tg (-42.94 ℃), but also a crystallization peak (Tc = -24.95 ℃) and a phase transition peak of the unpolymerized component (TBP = 77.35 ℃) for the sample with a polymerization degree of 25 wt%. (TBP= 77.35 ℃).
Figure 3. Laser properties of the all-polymer samples. a-b) Emission spectra, -180-240 °C; c-d) FWHM of the laser at room temperature; e) Laser threshold at room temperature; f) Threshold versus temperature in a "U " shape.

Figure 4. Photothermal property analysis of all-polymer samples. a) Thermogravimetric analysis; b-d) In situ variable temperature XRD; e) Relative positions of reflection peaks and fluorescence peaks at different temperatures; f) Reflection center wavelength/reflection intensity versus temperature; Variable temperatures g) Quantum yields and h) Fluorescence lifetimes; i) In situ variable temperature POM plots; j) In situ variable temperature angularly resolved spectra ( reflection mode).

Figure 5. In-situ Kossel variation during temperature change of all-polymer samples. a) Kossel plots; b) Kossel plots / BP lattice versus temperature; c) Kossel center circular radius (R) and reflection center wavelength (λ) versus temperature (T).

Figure 6. Microstructural changes and other laser properties of all-polymer samples during temperature change. a) Changes of BP lattice at different temperatures. a1) BPI lattice contracted along (110); a2) almost unchanged BPI lattice; a3) accelerated expansion of BPI crystals along (110); b) laser emission in three orthogonal directions of x, y, and z, pump energy: 0.205 μJ/pulse; c) polarization test of the laser, L/RCP: left/right circularly polarized light, pump energy: 0.205 μJ/pulse. pump energy: 0.205 μJ/pulse.