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Deep UV birefringence hydrogels based on two-dimensional liquid crystals


Deep UV light modulation devices based on birefringence play an important role in the field of optics.

Recently, the team of Ding Baofu, associate researcher and academician Cheng Huiming, Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Cai Wei’s team of Xi’an High-tech Research Institute and Professor Liu Bilu of Shenzhen International Graduate School of Tsinghua University, developed and prepared a new optical hydrogel based on inorganic two-dimensional cobalt-doped titanium oxide liquid crystals with wide bandgap, which can realize continuous and stable modulation of deep ultraviolet light as a transmissive deep ultraviolet birefringence device.

The results were published under the title “Deep ultraviolet hydrogel based on 2D cobalt-doped titanate”Light: Science & Applications

The corresponding authors of this paper are Ding Baofu, associate researcher of Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Professor Cai Wei of Xi’an Institute of High-tech Research and Associate Professor Liu Bilu of Shenzhen International Graduate School of Tsinghua University, and the first author is Xu Youan, a doctoral student of Shenzhen International Graduate School of Tsinghua University and Xi’an Institute of High Technology. The research was supported by the National Natural Science Foundation of China, the Science and Technology Commission of the Military Commission, the Ministry of Science and Technology, the Department of Science and Technology of Guangdong Province, and the Science and Technology Innovation Commission of Shenzhen Municipality.

Birefringence-based transmissive deep ultraviolet modulators (operating wavelength λ<350 nm) have important applications in solar blind zone optical communication, beam waveform correction, high-density data storage, semiconductor micro-nano processing and lithography. After continuous exploration, scientists have developed deep ultraviolet birefringence single crystals such as α-BBO, MgF2 and Ca(BO2)2, which exhibit good deep ultraviolet light modulation performance, but the fixed birefringence value limits its application in some specific fields. Liquid crystal materials can be arranged in an orderly manner under the action of magnetic field or electric field, with adjustable birefringence, which has attracted widespread attention. However, traditional organic liquid crystal materials are prone to photochemical degradation under deep ultraviolet irradiation, and will induce the polymerization reaction of free radicals in some organic groups, disrupt the orderly arrangement of liquid crystal molecules, and then lead to birefringence instability, so it is difficult to work in the deep ultraviolet band. At the same time, high deep ultraviolet absorption is also an important factor that restricts the inability of traditional organic liquid crystal materials to work in the deep ultraviolet band. In order to break through the bottleneck of this technology, the authors prepared inorganic two-dimensional cobalt-doped titanium oxide liquid crystals by ion intercalation method, which can realize the transmission continuous stable modulation of deep ultraviolet light under the action of magnetic field. The liquid crystal material has an optical band gap close to 4 eV, and the average transmittance in the deep ultraviolet band of 300-350 nm is higher than 70%, which effectively solves the problem of high deep ultraviolet absorption of traditional liquid crystals. The doping of the magnetic element cobalt brings great anisotropy of the magnetic susceptibility inside and outside the plane, and after the application of an external magnetic field, the liquid crystal molecules tend to be arranged in an orderly manner in the direction of the magnetic field and thus produce birefringence. The birefringence value can be adjusted by a magnetic field to achieve continuous and precise modulation of deep UV intensity and phase. Magnetic anisotropy, great diameter-to-thickness ratio (ratio of transverse size to thickness) and intrinsic optical anisotropy jointly give the material ultra-sensitive magnetic field response, and experiments show that its magneto-optical Cton-Mouton coefficient is two to three orders of magnitude larger than that of traditional organic liquid crystal materials, so the driving magnetic field required to achieve the directional arrangement of liquid crystal molecules is greatly reduced. In this work, the author realizes the continuous modulation of the deep ultraviolet phase through a permanent magnet maintained by small volume and zero energy consumption, which has the advantages of energy saving, portability and convenient operation, which is conducive to practical application. (Figure 1)

Figure 1: Deep UV modulation based on inorganic two-dimensional cobalt-doped titanium oxide liquid crystals

Figure 2 shows the excellent deep UV modulation performance of this magneto-optical device. Hysteresis test, transient magneto-optical test, cycling test, and fatigue test under long-term UV light irradiation show that the device has good reversibility, millisecond fast response, excellent durability and UV stability.

Figure 2: Reversibility, response time, durability, and UV stability tests

The team members successfully prepared the first optical hydrogel with adjustable birefringence operating in the deep ultraviolet spectral region by using the ultrasensitive magnetic field response and excellent optical modulation performance of two-dimensional cobalt-doped titanium oxide liquid crystal.

The preparation process is mainly prepared by adding polymer monomer and photoinitiator to the two-dimensional liquid crystal, combined with the in-situ photopolymerization reaction assisted by magnetic field. The magnetic field drives the orderly arrangement of liquid crystal molecules, and the polymer crosslinking network stores this ordered structure intact in the hydrogel and gives it good elasticity. By compressing or stretching the hydrogel in the direction of the optical path, the ordered parameters of the optical path and liquid crystal molecular arrangement can be quantitatively manipulated, so as to achieve accurate modulation of the phase delay of deep ultraviolet light.

This mechanical modulation method has the advantages of lightweight, efficient and fast and stable cycling, and can be used as the core optical component of many deep ultraviolet optical devices such as deep ultraviolet optical waveplates or light open light. (Figure 3)

Figure 3: Deep UV birefringence hydrogel based on two-dimensional cobalt-doped titanium oxide liquid crystals

In this work, the authors develop an optical hydrogel that can stably operate in the deep ultraviolet band and is birefringence tunable to achieve continuous stable modulation of deep ultraviolet light in both transmission and mechanical ways.

This study lays the foundation for the construction of new deep ultraviolet light modulation devices based on birefringence, and opens the door to the application of liquid crystal optics currently used in visible and near-infrared bands to the deep ultraviolet band. (Source: China Optics WeChat public account)

Related paper information:https://doi.org/10.1038/s41377-022-00991-6

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