Recently, Researcher Wang Jingxia and her team from the Institute of Physics and Chemistry at the Chinese Academy of Sciences have solved the problem of laser emission of blue-phase liquid crystals at temperatures below 0°C, enabling the blue-phase liquid crystal lasers to achieve an ultra-wide temperature range, with the lowest temperature reaching -180°C and the highest temperature reaching 240°C.

This not only opens the door to the development of low-temperature blue-phase liquid crystal lasers but also provides important insights for the design of new types of organic optoelectronic devices.

The blue-phase liquid crystal lasers created in this way are expected to promote the development of lasers in different temperature environments. This includes not only low-temperature environments such as polar regions, deep-sea, and aerospace, but also high-temperature environments such as deserts, military space activities, and metallurgy.

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At the same time, with the flexible characteristics of blue-phase liquid crystals, they can be made into movie screens, flexible laser display panels, and wearable laser display devices.

When designing and encoding blue-phase liquid crystals, a type of laser anti-counterfeiting label can also be created to achieve the transmission and exchange of information.In addition, based on the circular polarization emission of blue phase liquid crystal lasers, if a certain optical field design scheme is adopted, it is also expected to achieve naked-eye 3D laser display.

The "low-temperature pain" of blue phase liquid crystal lasers

Blue phase liquid crystal is a special type of liquid crystal with a three-dimensional periodic structure. With its unique optical properties and bright structural colors, it has been widely used in the preparation of new flexible optoelectronic devices.

Based on the periodic structure and the three-dimensional narrow photonic bandgap, blue phase liquid crystal can capture and confine light, and then regulate the optical field, thereby preparing various optical components, such as phase modulators, laser resonant cavities, etc.

For lasers made from blue phase liquid crystal, they not only have the advantages of flexibility and simple preparation, but also the emitted laser has a narrower linewidth and a lower laser threshold. At the same time, it also has the advantages of three-dimensional multi-directional emission and circular polarization of laser emission.Over the years, the academic community has conducted numerous studies on blue-phase liquid crystal lasers. Due to the narrow temperature window of blue-phase liquid crystals (usually 2-3°C), this severely limits the practical application of blue-phase liquid crystal lasers.

Later, the academic community proposed polymer-stabilized blue-phase liquid crystals, which significantly broadened the temperature window of blue-phase liquid crystals. Correspondingly, the laser temperature range of blue-phase liquid crystals was also expanded.

Previously, through a stable polymer support system, the team had expanded the temperature range of blue-phase liquid crystal lasers to 25-230°C.

However, within the blue-phase liquid crystals, the flowing liquid crystal molecules undergo random crystallization, making it difficult to emit lasers at temperatures below 0°C. This limits the application of blue-phase liquid crystal lasers in low-temperature environments.

Four students, six years of researchFor many years, Wang Jingxia's research group has primarily focused on the application of blue phase liquid crystals in lasers [1]. In 2019, the team, in collaboration with Professor Yang Zhou from the University of Science and Technology Beijing, jointly cultivated a doctoral student, Yang Jiajia, who, after more than two years of dedicated research on blue phase formulations, developed a wide temperature range (-180 to 350°C) blue phase liquid crystal film.

In the same year, the team's graduate student, Liu Jie, discovered that the aforementioned blue phase liquid crystal film exhibited unique low-threshold, high-quality laser emission characteristics.

In 2021, Liu Jie used the high-quality laser emission behavior of blue phase liquid crystals to in-situ characterize the phase transition process of blue phase liquid crystals.

In 2022, Liu Jie modulated the laser emission mode from single-mode, dual-mode, triple-mode to quadruple-mode by adjusting the photonic bandgap of blue phase liquid crystals.

In response to the 2022 paper, the reviewers asked the research group to study how the blue phase laser emission behavior changes with temperature. They replied to the reviewers' comments by providing additional experiments and supplementary data.The reviewer's comments have also provided inspiration for their new research. Although this paper was published in 2022, the team has seriously considered the lasing temperature range of blue-phase liquid crystals, as well as the related influencing factors, and carefully compared them with the wide temperature range of blue-phase lasers and their related influencing factors.

Later, two Ph.D. students from the research group, Chen Yujie and Zheng Chenglin, conducted research on the aforementioned issues, thereby gaining a deeper understanding of the differences between the temperature range affecting blue-phase liquid crystals and the temperature range of blue-phase liquid crystal lasers.

By adjusting the proportion of polymerizable components in blue-phase liquid crystals, they achieved a wide temperature range blue-phase liquid crystal laser covering -25 to 250°C.

On this basis, Chen Yujie noticed that the lasing behavior of blue-phase liquid crystal lasers below 0°C does not exist, and it is precisely this gap that limits the application of blue-phase liquid crystal lasers in low-temperature environments.

Therefore, in this study, she set the realization of blue-phase liquid crystal lasers below 0°C as the main goal. Later, Chen Yujie and others found that in most blue-phase liquid crystal laser systems, the component of polymerizable liquid crystal monomers is less than 10wt%.Achieving Blue Phase Liquid Crystal Lasers Below 0°C

After analysis, they inferred that the narrow temperature range for lasing in blue phase liquid crystals might be due to the low content of polymerizable components in the blue phase liquid crystals, leading to poor structural stability.

Therefore, they increased the content of polymerizable liquid crystal monomers in the blue phase liquid crystal system to 30wt%, thereby forming a robust polymer scaffold structure, which expanded the temperature range for blue phase liquid crystal lasers to 25 - 230°C.

In the study of low-temperature blue phase liquid crystal lasers, they believe that the random crystallization of liquid crystal molecules at low temperatures is the reason why blue phase liquid crystals are difficult to emit lasers at low temperatures.Therefore, they prepared a series of coumarin 6 doped dyes, using C6M as the polymerizable liquid crystal monomer, and a blue phase liquid crystal system with different degrees of polymerization content (referred to as the C6M+C6 system), hoping that such fully polymerized samples could effectively broaden the laser temperature range of low-temperature blue phase liquid crystals.

However, at low temperatures, the surface of the sample began to frost, making the collection of laser signals extremely difficult.

So, they improved the detection device by designing a vacuum-sealed device, sealing the sample in a vacuum, and then testing it to prevent the frost on the surface of the low-temperature sample from affecting the collection of laser signals.

In addition, they found that for the C6M+C6 system samples with different degrees of polymerization, their low-temperature laser temperature range did not widen with the increase in polymerization degree.

During the process of increasing the polymerization degree from 15wt% to 40wt%, the lowest emission laser temperature was slightly broadened from -10°C to -20°C.Thus, they continued to increase the degree of polymerization, yet even when the blue phase liquid crystal bandgap was matched with the fluorescence peak, the samples with 60wt% and 100wt% polymerization still could not emit laser light.

From this, it can be seen that the reason hindering the low-temperature laser emission of blue phase liquid crystals is not only the crystallization of liquid crystal molecules at low temperatures. So, why can the samples with 60wt% and 100wt% polymerization also not emit laser light?

For this question, they temporarily could not find the reason. In order to obtain a wider low-temperature blue phase liquid crystal laser temperature range, the team used different liquid crystal monomers and laser dyes, and adopted different degrees of polymerization to prepare a large number of blue phase liquid crystal films.

These films contain different system-doped dyes. Subsequently, the research group began to test the laser temperature range and laser threshold for these films.

To better achieve low-temperature blue phase excitation, they mainly focused on optimizing from two aspects: the dye system and the liquid crystal monomer system.By comparing the laser temperature range and laser threshold of different doped blue-phase liquid crystal systems, they found that under the same system, the laser threshold of the DCM dye doped samples is always lower than that of the C6 doped samples. At the same time, the former's laser temperature range is also wider.

Therefore, they decided to prefer the DCM dye. In the experiment, the research group found that when using a system with a mass ratio of liquid crystal monomer RM105:RM257 of 3:1, the laser temperature range at low temperatures can be significantly widened to -160°C. Therefore, they decided to prioritize the use of materials from the RM105:RM257 system.

Later, the research group realized that the compatibility of each component in the blue-phase liquid crystal system is one of the important factors affecting the laser temperature range, thereby confirming that RM105 and DCM have better compatibility.

The compatibility between C6M and C6 is poor, which is also why as the degree of polymerization increases, the 60wt% polymerization and 100wt% polymerization C6M+C6 samples cannot emit laser.

So far, the two major factors affecting the laser temperature range of blue-phase liquid crystals have been identified: one is the system compatibility, and the other is the crystallization of liquid crystal molecules at low temperatures.Next, they chose the liquid crystal monomer with chain flexibility, RM105, and paired it with a small amount of RM257 (RM105:RM257=3:1), to facilitate the easier formation of blue phase liquid crystals.

In terms of dyes, they selected the DCM molecule with a lower threshold, thereby giving the entire blue phase liquid crystal better compatibility.

At the same time, by using the full polymerization method, it is possible to prevent the crystallization of liquid crystal small molecules at low temperatures and increase the structural stability of the system, thus obtaining the optimal laser temperature range and laser threshold.

Through this, the team successfully prepared a blue phase liquid crystal laser with a laser temperature range of -180-240°C. At the same time, they revealed for the first time the emission behavior of blue phase liquid crystal lasers below 0°C: that is, as the temperature decreases, the laser wavelength will show a slight red shift, and the laser threshold and laser line width will also increase.

When the temperature is within the range of -180-240°C, the laser wavelength begins to show a blue shift, and the laser threshold and laser line width exhibit a "U-shaped" pattern.Through further characterization, they found that this special laser behavior is attributed to the anisotropic deformation of the blue phase liquid crystal lattice induced by temperature.

Undoubtedly, this ultra-wide laser temperature range of over 400°C, especially the realization of high-quality laser emission below 0°C, is a significant breakthrough in the field of blue phase liquid crystal lasers.

Eventually, the related paper was published in Advanced Materials with the title "Super-Wide Temperature Lasers Spanning from -180 to 240°C Based on Fully-Polymerized Blue Phase Superstructures" [2].

Chen Yujie, a doctoral student at the Institute of Physics and Chemistry of the Chinese Academy of Sciences, is the first author, and Wang Jingxia, a researcher at the same institute, serves as the corresponding author. During the course of the project, Associate Professor Jin Feng and Senior Engineer Li Jing from the Institute of Physics and Chemistry provided a lot of help in the laser testing.

Professor Shi Lei from the Department of Physics at Fudan University provided assistance in characterizing the blue phase liquid crystal photonic bandgaps, and Researcher Jiang Lei offered a lot of important guidance and help in the development of the project. Thanks are due to Professor Yang Huai from Peking University for his guidance.Next, they will summarize experiments on the laser temperature range and laser threshold of different blue-phase liquid crystal systems, allowing the design strategy for wide-temperature-range blue-phase liquid crystal lasers to be further perfected.

At the same time, they may also study the issue of the lifespan of blue-phase liquid crystal lasers, such as using more stable gain media and adopting a stacked device structure to further delay its service life.

In addition, they may also utilize the characteristics of blue-phase liquid crystal circularly polarized lasers, and achieve their unique functions through ingenious chiral design, such as realizing complex pattern laser display, advanced laser encryption technology, etc.