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Three Dimensional Self-Assembled Blue Phase Liquid Crystalline Photonic Crystal

Meng Wang , He Song , Yewen Li

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (8) : 1734 -1747.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (8) : 1734-1747 DOI: 10.7536/PC211013
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Three Dimensional Self-Assembled Blue Phase Liquid Crystalline Photonic Crystal
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Abstract

Blue phase (BP) liquid crystals have been regarded as one kind of the most promising candidates for tunable three-dimensional photonic crystals due to their unique 3D self-assembly nanostructures, photonic bandgaps in visible light range and characteristics of soft matter. They exhibit great potential in numerous applications such as next-generation ultra-fast displays, reflection-mode displays, tunable lasers and optical communication devices. Herein, we provide the research advancement in the self-assembly structures of blue phase liquid crystals in recent years. First, the latest research on the hierarchical self-assembly of three-dimensional micro/nano structural and phase transition behaviors between three sub-phases are introduced. Then, the control methods of their self-assembly behavior or three-dimensional periodic lattice structures are demonstrated in detail. The lattice nucleation growth and lattice orientation of BPI and BPII can be modulated by substrate surface orientation treatment or nano patterning treatment. Owing to the response characteristics of BPs to external electric field, monodomain cubic crystals of BP or new sub-phase with non-cubic nanostructure can be obtained by controlling applied electric field. Besides, BP photonic crystals with different plane orientations can be obtained in a short time by appropriate heat treatment. These studies provide theoretical foundation for the appreciable application of BP materials in three-dimensional photonic crystals and functional devices. For example, the applications of blue phase liquid crystalline photonic crystal materials in optical fields such as tunable laser and tunable grating are shown. At the end of this review, the challenges and possible development direction of blue phase liquid crystalline photonic crystal is prospected briefly.

Contents

1 Introduction

2 Self-assembled structures and phase transition processes of blue phases

2.1 Self-assembled structures and their characteristics

2.2 Phase transition processes between three sub-phases

3 Control methods of 3D self-assembled crystal structures of blue phases

3.1 Crystal orientation induced by substrate surface

3.2 Effects of electric field on blue phases

3.3 Crystal orientation induced by heat treatment

4 Applications of blue phase liquid crystalline photonic crystal in optical field

5 Conclusion

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Keywords

blue phase liquid crystal / self-assembly / photonic crystal / crystal structure

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Meng Wang, He Song, Yewen Li. Three Dimensional Self-Assembled Blue Phase Liquid Crystalline Photonic Crystal. Progress in Chemistry, 2022, 34(8): 1734-1747 DOI:10.7536/PC211013

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1 Introduction

In recent years, photonic crystals have attracted more and more attention due to their promising applications in many frontier and interdisciplinary fields, such as integrated optoelectronics, quantum optics and optical communication, and significant progress has been made in basic research and device development[1]. However, the fine microstructure and complex preparation process have brought great challenges to its development and wide application. Among many organic and inorganic materials, liquid crystal (LC) is an ideal material for tunable photonic crystals, which has both the partial molecular ordering of crystals and the macroscopic fluidity of liquid, can self-assemble to form periodic structures and maintain highly sensitive response to external stimuli[2⇓~4]. Among many liquid crystal phase materials, blue phase liquid crystal (BPLC) is considered to be one of the most potential liquid crystal photonic crystal materials due to its unique self-assembled three-dimensional periodic structure and selective light reflection properties[5,6].
The blue phase (BP) is a special phase between the isotropic phase (I) and the cholesteric phase (Ch) macroscopically, which often occurs in a narrow temperature range between the high chiral content of Ch and the clearing point of liquid crystals[7]. In 1888, when Reinitzer discovered liquid crystals, he had already observed the blue phase, but because its stable temperature range was too narrow, researchers neglected the research in this area. In 1973, Gray et al. Officially named this state of matter the blue phase because of its blue appearance[8]. In order to solve the problem of narrow temperature range of blue phase, scholars at home and abroad have studied the problem of narrow temperature range of blue phase in detail by using different methods and technologies in the past 20 years, and have achieved remarkable results.He has developed wide temperature range blue phase liquid crystal materials preparation methods including polymer stabilization method, dimer liquid crystal system, bent molecular liquid crystal system, hydrogen bond liquid crystal system and nanoparticle stabilization method, and published many research and review papers[9⇓~11][12,13][14⇓⇓~17][18][19,20][21⇓⇓⇓⇓~26]. It is worth mentioning that recently, the temperature range of small molecule blue phase liquid crystal system has been successfully extended to 280 ℃ through the optimization design of dimer biaxial molecule and uniaxial molecule mixed system, which lays a foundation for the practical application of blue phase liquid crystal materials[13]. In recent years, the unique self-assembly structure and excellent external field response performance of blue phase have attracted the attention of many researchers[27⇓⇓⇓⇓⇓⇓⇓⇓~36]. In the field of display, blue phase liquid crystal display is considered to be one of the most promising next-generation ultrafast flat panel display technologies because of its sub-microsecond response speed, support for field sequential driving and no need for color filters[37,38]. In the field of non-display optics, the excellent external field control performance of blue phase photonic bandgap is of great significance for the construction of functional optical devices, including tunable lasers, adaptive gratings, adaptive lenses and so on[39⇓~41][42][43]. As a kind of soft matter and self-assembled three-dimensional photonic crystal, the potential application of blue phase liquid crystal in the field of optoelectronics is expected by researchers all over the world. In this paper, the development status of blue phase liquid crystal photonic crystal materials in recent years is briefly summarized, focusing on the self-assembly structure and control of blue phase, the development and application of blue phase liquid crystal photonic crystal devices.

2 Self-assembly structure and phase transition process of blue phase

2.1 Blue phase self-assembly structure and its characteristic

The blue phase structure is a kind of molecular multilevel self-assembly structure. Microscopically, the liquid crystal molecules are twisted and rotated along two screw axes at the same time, showing a double twisted columnar arrangement in space, as shown in Figure 1A. Double-twist cylinders (DTCs) construct a certain crystal structure in three-dimensional space through supramolecular self-assembly. Because of this double twist arrangement, the molecular arrangement is discontinuous in three-dimensional space, resulting in the formation of defects in three-dimensional long-range space, which are called disclination lines. Therefore, compared with the molecular arrangement structure of other liquid crystal phases, the BP structure has poor stability and is easy to dehelix to form the Ch phase. According to the spatial structure symmetry, the blue phase can be divided into three sub-phases, which are blue phase I (BPI), blue phase II (BPII) and blue phase III (BPIII) with the increase of temperature. Among them, BPI and BPII are three-dimensional periodic structures in the director field, with body-centered cubic symmetry (defect space group I4132) and simple cubic symmetry (defect space group P4232), respectively, and the lattice sites are occupied by bidirectional dislocation lines[44]. The three-dimensional self-assembled periodic structure enables BPI and BPII to have selective light reflection characteristics, and the optical texture appears bright color under the polarization optical microscope (POM) in the reflection mode, as shown in Figure 1b and C. BPIII is in an amorphous state with a disordered arrangement of double twisted cylinders, similar to the isotropic state. The optical texture of BPIII has no obvious plates and appears as a dark blue or purple fog, as shown in Figure 1 d[45]. Whether a substance has a blue phase and the temperature range of the blue phase can be judged and determined by using a polarizing microscope combined with a variable temperature hot stage[46].
The three-dimensional self-assembled periodic structure of BPI and BPII makes them have special selective light reflection properties, and then there is an incomplete photonic band gap in three-dimensional space[47]. The Bragg reflection of the lattice satisfies the following relation:
λ= 2 n a h 2 + k 2 + l 2
Where, λ, n and a are the wavelength of the incident light, the average refractive index of the liquid crystal and the BP lattice constant, respectively; H, K and l are the Miller indices of the crystal plane. In BPI, H + K + l is even, and usually the diffracting crystal bands are { 110 }, { 200 }, { 211 } and { 220 }. In BPII, H + K + l is odd, and the common diffraction bands are { 100 }, { 110 } and { 111 }. At present, the lattice structure type and crystal plane of BP can be determined by Kossel diffraction technique, small angle X-ray diffraction technique, transmission electron microscopy (TEM) and other characterization techniques[48][49][50,51]. In addition, the BPI structure can be observed nondestructively by confocal laser scanning microscope[52].

2.2 Phase transition process of blue phase trion phase

Due to the narrow temperature range of BP, the study of its phase transition process has remained in the stage of theoretical analysis and simulation calculation. Recently, Zheng Zhigang of East China University of Science and Technology and Wang Jingxia of the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences used a BPLC material system containing 65 wt% polymerizable monomers to fix the microstructure changes of BPs phase transformation during cooling by rapid photopolymerization curing method, and characterized the microstructure by TEM and synchrotron small angle XRD scattering (syn-SAXS)[53].
The study first divided the cooling from phase I to BPI into five stages by POM observation, as shown in Figure 2A. The first stage is the phase transition from I to BPIII, in which BPIII dark blue misty microdomains are formed in the black field of I phase. In the second stage, a bright blue region appears in the center of the dark blue region, that is, a BPII crystal nucleus is formed inside the BPIII region, and a core-shell structure surrounded by an inner BPII crystal nucleus and an outer BPIII is formed, and the BPII crystal core can grow rapidly in a few seconds. It is observed that there is always a thin layer of BPIII phase between I phase and BPII phase, and BPII can not be obtained directly from I phase, that is, the transformation from I to BPII must pass through the transition phase BPIII. The green plate structure appears in the POM photos of the third stage, that is, the phase transition of BPII ~ BPI occurs. The selective reflection peaks of BPII (100) and BPI (110) can be measured simultaneously through the reflection spectrum, and the central wavelengths are 477 nm and 550 nm, respectively. In this state, there is a mixed phase of untransformed BPIII, BPII and newly formed BPI in the system, and BPI can nucleate and grow directly from the dark blue BPIII center, forming a core-shell structure surrounded by an inner BPI nucleus and an outer BPIII. With the further decrease of temperature, the outer part of BPIII is transformed into BPII, forming a more complex three-layer core-shell structure of BPIII/BPII/BPI. In this process, the BPIII ~ BPI phase transformation occurs before the BPIII ~ BPII phase transformation, which may be due to the fact that the thermodynamically stable BPI prefers to nucleate preferentially. Similar to BPII, BPI is unable to nucleate directly from the I phase. In the fourth stage, all BPIII/BPI and BPIII/BPII core-shell structures were transformed into BPIII/BPII/BPI mixed phases, and only BPII and BPI nucleated in the third stage continued to grow. With the increase of holding time, the number of nuclei of BPI and BPII decreases gradually, and the mixed phase domain region becomes larger. Finally, BPI domains with a size of more than 1 mm can be obtained by slowly cooling the BPIII/BPII/BPI mixed phase in the above stages, that is, all BPIII and BPII phases are transformed into BPI in the fifth stage, and single-phase multi-domain BPI is obtained, and the system only shows a single reflection peak of BPI. The samples at different stages were polymerized by UV curing method, and the samples obtained had a wide blue phase temperature range and maintained the microstructure and optical properties of the original BPLC.
TEM and syn-SAXS were used to analyze the diffraction images and structural parameters of the phase change region after light curing, as shown in Figure 2b. Although there is an obvious interface between the blue BPII and the green BPI in the POM image, the syn-SAXS diffraction speckles near the interface overlap, and the lattice structure obtained by TEM is coherent. The DTCs at the interface belong to both the adjacent BPII and BPI lattices, which proves that there is no diffusion of DTCs during the BPII ~ BPI phase transition. As far as crystallography is concerned, the microstructure of BPII ~ BPI will change from simple cubic crystal to body-centered cubic crystal, and the surface fluctuation caused by twin lattice can be observed in the BPI obtained by phase transformation. Repeated rapid temperature rise and drop, BPI ~ BPII rapid reversible change and no re-nucleation process. Therefore, it can be considered that the BPII ~ BPI phase transformation process is similar to the thermoelastic martensitic transformation of metal materials, and DTCs can be regarded as structural units similar to atomic crystallization, which produce changes in lattice self-assembly structure through large-scale collective reorganization.
Similarly, it is found that during the phase transformation of BPIII ~ BPI and BPIII ~ BPII, the arrangement of DTCs at the interface is consistent with that in the adjacent BPI or BPII crystal region, and there is no diffusion of DTCs. In addition, because BPI or BPII cannot be directly nucleated from phase I, the phase transition process must pass through BPIII, so the phase transition of BPIII ~ BPI/BPII is also attributed to the reorganization of DTCs, that is, the amorphous scattered DTCs in BPIII can self-assemble to form the simple cubic lattice structure of BPII or the body-centered cubic lattice structure of BPI without diffusion.
Through the above study, it can be seen that DTCs are regarded as structural units, the phase transformation process of BP trion phase belongs to diffusionless phase transformation, and the microstructure is determined by the reconstruction of DTCs in three-dimensional space. In this study, the phase transition process of each subphase of BP was analyzed in detail by microscopic characterization, which provided a theoretical basis for further understanding and controlling the self-assembly structure of blue phase.

3 Blue phase self-assembly crystal structure control

Compared with other liquid crystal phases, the molecular arrangement and three-dimensional self-assembly structure of BPLC are more complex, and the self-assembly behavior and crystal structure of BPLC are affected by the phase type, the surface state or morphology of the substrate, the heat treatment process, and the external electric field. At present, BP single crystals with high orientation consistency can be obtained by substrate surface orientation treatment or nano-patterning induction, electric field induction, heat treatment induction and other methods. The study of BP self-assembly behavior mainly includes the following three aspects.

3.1 Ubstrate surface induced crystal plane orientation

Different from nematic liquid crystal, BPLC is difficult to control its orientation because of the scattered distribution of liquid crystal molecules and the lack of easy orientation axis at the substrate interface, resulting in the formation of randomly distributed crystal plane orientation. Many studies have shown that BP single crystals with consistent orientation can be obtained by certain surface treatment of the substrate.
Liu et al. Used a parallel-aligned liquid crystal cell with a substrate surface coated with a polyimide (PI) layer and subjected to rubbing alignment treatment to compare with a liquid crystal cell without surface treatment, and found that during the cooling process from the I phase to the BP phase, the Kossel diffraction parameter of the BP (200) crystal plane of the liquid crystal cell without surface treatment became larger, and the reflection wavelength of the sample moved to a longer wavelength[54]. However, the diffraction pattern of the crystal plane of the liquid crystal cell with parallel orientation does not change significantly, and the reflection wavelength hardly changes during the cooling process of the sample, so the parallel grooves on the surface of the substrate are used to anchor the BPLC molecules (Fig. 3). Joshi et al. Studied BP samples prepared by parallel orientation and non-orientation liquid crystal cells, and found that the parallel orientation layer could assist BPII to form uniform orientation during cooling from I phase to BPI phase, but the effect on BPI orientation was not significant after phase transformation[55]. Kim et al. Compared the alignment effect of BPII by rubbing parallel alignment liquid crystal cells with different cell thicknesses, and found that the uniform alignment effect became worse with the increase of cell thickness[56]. Kikuchi et al. Studied in detail the influence of substrate surface free energy on the orientation of BP self-assembled crystal planes through six liquid crystal cells with polyimide coating and six organosilane molecular coatings[57]. The surface free energy is understood to consist of two distinct components: a dispersive component γd and a polar component γp. By comparison, it is found that during the cooling process from the I phase to the BPI phase, whether there is a BPII-BPI phase transition or not, the BPI obtains a well-oriented (110) crystal plane in the liquid crystal cell with a small polar component of the substrate surface free energy. The smaller the substrate surface free energy, the smaller the polar component, and the better the orientation uniformity of BPI (110).
Lin et al. Studied the effect of the substrate surface orientation layer on the formation process of BP crystals[58]. By using three kinds of liquid crystal cells with different boundary conditions (ITO cell, PA cell and VA cell), it is found that the BP texture of the sample with PA cell has uniform reflection color and no obvious domain boundary, the BP texture of the sample with VA cell has the better regularity, and the BP fabric of the sample without orientation has random orientation of polycrystalline planes. To explore the possible influence mechanism of the orientation layer on the growth of BPI crystal, the schematic diagram is shown in Figure 4. In the orientation-free cell, due to the formation of many small nucleation centers with different orientations during the cooling process, there is a fierce competition between homogeneous nucleation and heterogeneous nucleation, and the crystal orientation of BPLC is random. The interaction between the orientation layer and the LC molecules contributes to the orientation of the LC molecules. The parallel orientation box substrate has a small contact angle and a large adhesion free energy with LC molecules, and the LC molecules tend to spread and diffuse on the parallel orientation box substrate rather than exist in the form of small droplets, so that the orientation of the BP crystal is more uniform during growth; For the vertically oriented cell, the contact angle of LC molecules on the substrate is larger than that of the non-oriented cell, so the nucleation centers near the substrate tend to nucleate inhomogeneously and grow along the same orientation under the chemical force of the substrate, while the nucleation centers in the middle of the substrate tend to nucleate homogeneously and form grain boundaries between different crystal domains due to different orientations. Therefore, the growth of the blue phase crystal in the parallel orientation box is more uniform; In the vertical orientation box, the blue phase nuclei near the substrate grow in the same direction, while the middle region of the box tends to form uniform boxes, and different crystal domains form grain boundaries due to different orientations, so the regularity is secondary. The blue phase texture in the disoriented box is a random orientation of polycrystalline planes.
Orzechowski et al. Studied the polarization characteristics of the outgoing light of BPLC in parallel and perpendicular orientations[59]. For both single-domain and multi-domain BP samples, the optical isotropy is observed macroscopically, but the incident light near the selective reflection wavelength changes from linear polarization to elliptical polarization after passing through the BP layer, and the optical rotation is weaker than that of Ch phase. Due to the existence of the parallel orientation layer, the selective reflection wavelengths of BPII and BPI are changed during the cooling process. Lee et al. Compared the electro-optical properties of BP samples prepared from two kinds of liquid crystal cells with and without surface treatment, which were coated with PI layer on the substrate surface and subjected to rubbing parallel orientation treatment.By comparison, it is found that the threshold voltage, operating voltage and hysteresis voltage of the rubbed parallel aligned liquid crystal cell sample are significantly lower than those of the unaligned BP sample, and the Kerr constant and light transmittance of the rubbed parallel aligned liquid crystal cell sample are higher than those of the unaligned BP sample[60].
Ot Otón et al. found that BPII and BPI with single orientation could be obtained by rapid cooling from the I phase after using different types of nylon coatings and rubbing parallel orientation, and the single crystal growth process took only 23 s (Fig. 5)[61]. However, only the polycrystalline texture with random orientation can be obtained when the temperature is increased from the Ch phase to the BPI phase. It is found that the anchoring energy of the orientation layer plays an important role in the self-assembly process of BP, and the weak anchoring energy is more conducive to the growth of BP crystals along a single specific orientation direction, thus preparing large and uniform BP single crystals. Compared with the PI alignment layer with large interfacial anchoring energy, the anchoring energy of the nylon coating layer is small, and the BP sample prepared by the liquid crystal cell coated with the nylon coating layer has significant improvement in the self-assembly speed and the stability of the single crystal structure.
Wu Shicong et al., University of Central Florida, USA, prepared a polymer stabilized blue phase (PSBP) full-color reflective display device using a parallel orientation cell[62]. The display has a narrow reflection bandwidth, a sub-millisecond response time, and a natural gray scale without a polarizer and a color filter, which makes it possible for the application of flexible displays.
In addition, it is found that different parallel orientation methods also have a significant effect on the self-assembly behavior of BP crystals. Zheng Zhigang et al. Realized the high-resolution micropatterning preparation of BP samples by using photoorientation technology combined with photomask[63]. A liquid crystal box substrate is coat with a photoalignment agent, and linearly polarized ultraviolet light irradiate that liquid crystal box through a patterned photomask, so that a light passing area forms planar alignment due to the photoalignment agent, and an area not irradiated by the linearly polarize ultraviolet light is still in a non-alignment state. After BPLC infusion, the sample showed a patterned state of periodically alternating crystal orientations, the BP crystals in the irradiated parallel orientation region were uniformly oriented and reflected a single color, and the BP crystals in the non-irradiated orientation region were scattered. As shown in fig. 6, this alternating patterned structure can also be repeatedly erased and rewritten by applying an external electric field and linearly polarized ultraviolet light irradiation.
Takahashi et al. Used Kossel diffraction technique to compare the changes of BP crystal structure in the heating and cooling process of samples prepared by two kinds of liquid crystal cells with rubbing orientation and light orientation, and found that when heating from Ch phase to I phase, the two kinds of liquid crystal cells were arranged with the (100) crystal plane of BPI or BPII parallel to the substrate[64]. When the temperature is reduced from the I phase to the Ch phase, the rubbed parallel oriented liquid crystal cell exhibits BPII (110) and BPI (211) crystal planes in turn with the decrease of temperature, while the photooriented cell exhibits BPII (100) and BPI (110) crystal planes (Fig. 7). This study proves that different parallel orientation coatings have a key influence on the easy orientation crystal plane of BP during cooling process.
Recently, Nealey et al. Successfully induced the self-assembly of BPLC molecules to form ideal single crystals by preparing chemically patterned substrates with specific nanostructures[65]. According to the Landau-de genne theory, the scalar order parameter (S) and continuum simulation were studied to explain the contribution of enthalpy change, elasticity and surface to the free energy. Through the simulation calculation of BP topological defects and S, it was found that different BP crystal planes corresponded to different patterned order parameter distribution maps on the surface of vertically oriented substrates. In the vertical region with large S value, BPLC molecules tend to align along the vertical direction in the immediate vicinity of the interface; In other regions, the BPLC molecular alignment gradually deviates from the interface-imposed orientation and tends to be parallel. Based on the simulation results, the researchers designed three kinds of patterned surface substrates with both vertical and planar anchoring regions, which can induce the (100), (110) and (111) crystal plane orientations of BPII, respectively. Then, the planar silicon substrate coated with PMMAZO was processed by photolithography, electron beam exposure and oxygen plasma etching, and the nano-patterned surface substrate with the same size as the simulation calculation was prepared[66]. A 3. 5 μm thick liquid crystal cell was prepared with the substrate and a vertically oriented substrate, and then filled with BPLC materials with specific lattice parameters (BPI lattice constant is about 255 nm, BPII lattice constant is about 150 nm) for observation. As shown in Fig. 8 a, the substrate with one-dimensional stripe pattern surface structure has no obvious effect on the structure and thermal behavior of BPI and Ch, but can induce the (100) crystal plane of BPII to form a large area of single crystal, which is observed by POM combined with Kossel diffraction. When the substrate surface is rectangular or circular, the (110) and (111) crystal planes of BPII are induced to form single crystals, respectively. The above experimental results are consistent with the theoretical simulation results. In the same way, BPI (110) crystal plane can also be formed into single crystal by using rectangular nano-patterned surface with specific size. The researchers constructed one-dimensional stripe nanopatterns of different sizes at different positions on the same substrate, and changed the free energy by changing the width ratio of parallel and perpendicular regions, so that BPLC molecules self-assembled on micro-nano surfaces of different scales to form single crystals of different orientations (Fig. 8B)[67]. Because BP self-assembled crystals have submicron lattice parameters, the nucleation and growth of BPs can be affected by changing the size of micro-nano patterns on the substrate surface. In addition, BPLC molecules can self-assemble into single crystals with different orientations in different regions of the sample by constructing different nanopatterns at different positions of the same substrate, and clear grain boundaries can be formed at the interface. The crystal plane structure or phase state can also be changed by temperature or electric field control, thus changing the macroscopic optical properties of the sample (Fig. 8C)[68]. By preparing local patterned surfaces, the researchers further studied the nucleation and growth process of BPII in different heating and cooling processes, different time, and different chemical pattern scales.It is found that the single orientation of BPII (100) induced by patterning in a certain scale can be epitaxially grown along the direction parallel or perpendicular to the patterned area, which makes it possible to prepare large area single crystal BP samples[69]. Based on the theoretical calculation, the above research has realized the preparation of diversified nanopatterned substrates with different BP crystal plane orientations by using microlithography technology, and the experimental results are consistent with the theoretical calculation results, which provides a new idea for the research of BPLC photonic crystals and the multi-function of its optical devices.

3.2 Effect of electric field on BP

In the early study, it was found that the selective reflection wavelength of BP would shift under the action of external electric field, and the subsequent study found that the electric field had an impact on the dielectric constant, birefringence coefficient and lattice constant of BPLC materials[70,71]. Under the action of electric Field, BP usually has the following effects: Kerr effect, Crystal reorientation, Electrostriction and field-induced phase transitions[72,73][74][75][76]. Among them, Kerr effect is the basic principle of BPLC material to prepare display devices: when there is no electric field, BP is optically isotropic on the macroscopic scale, the incident light does not undergo birefringence, and the sample presents a black field under the action of a polarizer; Under the action of the electric field, the BPLC molecules are oriented by the electric field force and show anisotropy and birefringence, and the incident light can pass through the sample to show a bright field, so that the incident light can be controlled by the electric field to induce the BPLC to produce birefringence[77]. The response time of Kerr effect is about 0.1 ms, which lays the foundation for the application of BPLC materials in the field of ultrafast display. Low electric field can induce BPLC molecules to form a uniform orientation along a certain crystal plane, for example, BP samples with random orientation tend to reorient along BPI (110) and BPII (100) crystal planes under the action of electric field. In addition, the lattice constant of BPs can be changed by an external electric field, that is, lattice distortion occurs, which leads to the change of selective reflection wavelength. This phenomenon of electric-field-induced lattice distortion is called electrostriction. In general, the reflection wavelength of BPLC with positive dielectric anisotropy value shifts to long wavelength (red shift) with the applied electric field, while the reflection wavelength of BPLC with negative dielectric anisotropy value shifts to short wavelength (blue shift) with the applied electric field. Under the action of a large external electric field, BP may also change its phase. For example, BPI and BPII can be transformed into BPX metastable phase of tetragonal lattice structure and BPH metastable phase of hexagonal crystal structure respectively under the action of electric field, that is, new crystal structures can be induced under the action of electric field. With the increase of the electric field strength, the phase transition from BP to Ch or I will occur. The study of blue-phase electric field effect lays an important theoretical foundation for the application of BPLC materials in the field of optoelectronic functional devices (Fig. 9).
Wu Shicong et al. Obtained BPI single crystals with uniform orientation by applying AC electric field to BP samples prepared by unoriented liquid crystal cells[74]. For the PSBP samples with different contents of chiral additive and cell thickness of 5 μm, after cooling to the BP temperature, the BPI crystal plane was reoriented by applying an alternating electric field (1 kHz, 2 V/μm) for 1 s, and the uniform orientation state of (110) crystal plane perpendicular to the electric field was obtained. When the electric field is removed, the sample can not return to the initial random orientation state due to the high viscosity of BP, and only the (110) crystal plane is tilted, while the uniform orientation texture does not change significantly. At this time, the uniform orientation state can be fixed by ultraviolet irradiation with a certain intensity for a period of time. It is found that the BP sample oriented by the parallel substrate exhibits specular reflection, while the BP sample oriented by the electric field has a serious divergence of the angular intensity distribution of the reflection spectrum, and the longer the emission wavelength is, the more significant the divergence effect is. This divergent reflection effect can help broaden the viewing angle of a reflective BP display device (fig. 10).
Neyts et al. Found that the electric field could help BPII to form single crystals with uniform orientation during rapid heating and cooling by applying a continuous AC electric field during heating and cooling of BP samples[79]. For the sample with large thickness, it is difficult for the orientation box to induce the formation of good crystal plane orientation due to the large distance between the two substrates and the weak influence of the surface anchoring effect on the molecules far away from the substrate, while the electric field orientation method can effectively solve the above problem.
In addition to the change of orientation, the structure or phase of BPLC can also change under the action of electric field. Tiribocchi et al. And Alexander et al. Calculated the changes of BP lattice space and defect network under the action of electric field by simulation method, and found that the BPI system could be induced to form a long-term metastable structure by applying a square pulse electric field, and the defect structure was different from the initial state[80][81]. The disclination lines in BPII converge at the body-centered position to form a defect network, which is simply deformed by the electric field until the non-cubic phase is formed. When the electric torque and the elastic torque are balanced, the defect network quickly reaches equilibrium, so BPII always recovers the initial simple cubic crystal structure state when the electric field is removed. However, the disclination lines in BPI are separated from each other and undergo distortion, coalescence and dissociation during electrostriction, so they show more evolution paths during field-induced deformation-reorganization, resulting in the emergence and persistence of metastable States. Therefore, BPI can relax to a metastable state when the electric field is removed, and the defect arrangement depends on the direction of the applied electric field and the dielectric properties of BPLC. Recently, Lin Zongxian et al., Sun Yat-sen University, Taiwan, China, successfully realized the reconstruction of BP three-dimensional periodic structure by repeatedly applying electric field pulse technology (Fig. 11)[78]. Different from the conventional long-term electrification, the BPI defect can continue to deform on the basis of metastability by repeated electric field switching, so that the BPI defect can deform along different paths to form different distortion States. In addition, prolonged continuous energization and the use of high electric field strength can cause significant thermal effects, which can lead to the phase transition of BP to Ch or I. The energization method using repeated short electric field pulses induces lattice distortion during the electric field duration, and the electric field removal interval eliminates the heating effect caused by the energization. The body-centered cubic lattice of BPI is transformed into orthorhombic lattice and tetragonal lattice by the repeated application of electric field pulses, and these reconstructed non-cubic crystal structures can be maintained after the electric field is removed. Polymer-stabilized BPX single crystal with tetragonal lattice structure can be further prepared by ultraviolet polymerization, and the thermodynamic stable temperature can be increased from 3 deg C to 30 deg C. This study provides a new research direction for the theoretical and experimental study of BP crystal structure and its application in the field of three-dimensional optoelectronic functional devices.

3.3 Heat treatment induced crystal plane orientation

Lin et al. Found that the unit cell of a certain crystal direction could form a large domain region by repeatedly heating and cooling the BPLC sample, and the PSBP sample obtained after polymerization had no significant electro-optical hysteresis problem[82]. During the rapid cooling from I phase to BPII phase, LC molecules self-assemble to form multi-oriented crystal nuclei. With the increase of holding time, the crystal nuclei grow up and reorient to eliminate the grain boundary and reduce the free energy, so large single-domain BPII samples can be obtained by holding for a long time. However, BPI single crystal can not be obtained by holding for a long time because the disclination lines are independent of each other, there are many local minimum free energy, and there are stable defects in the microdomain regions of various crystal plane orientations without merging.
In 2017, Lin Zongxian et al. Successfully prepared a large BPI single crystal sample with a thickness of 100 μm and a width of 1 mm by using gradient temperature technology to control the BPLC self-assembly process, and the preparation time was only 3 H[83]. As shown in Fig. 12, two hot stages with different temperatures are used in the study, and the distance between the two hot stages is about 2 mm. By controlling the set temperature of the hot stage, the sample is in the BPII phase in the region on the high temperature hot stage and in the BPI phase in the region on the low temperature hot stage. In addition, the liquid crystal cell with parallel orientation was rubbed by PI, so that the BP phase of the sample had a certain pre-orientation on both hot stages. Because the liquid crystal cell is thick (100 μm), there is a certain temperature difference along the cell thickness direction, so the BPI forms a uniform orientation crystal nucleus near the surface of the lower substrate. When the sample is moved from the high temperature stage to the low temperature stage at a rate of 0. 1 μm/s under this temperature gradient, the BPII phase of the sample is rapidly cooled to BPI phase, thus changing the formation mechanism of BPI from homogeneous nucleation and slow growth to thermodynamically controlled heterogeneous nucleation. By adjusting the proportion of chiral additives, BPI single crystals with reflection wavelengths in the red, green and blue bands were successfully obtained, and the optical parameters such as optical reflectivity and half-peak width of the reflection peak were significantly optimized. At the same time, another temperature control method was mentioned in this study. By controlling the set temperature of the hot stage, the sample was rapidly cooled from phase I to phase BPII at a rate of 10 ℃/min, and kept in phase BPII for 36 H, so that BPII crystals could grow spontaneously through platelets. The as-grown BPII single crystal was then used as the precursor to rapidly cool from the BPII phase to the BPI phase at a rate of 10 ° C/min to form a large BPI crystal. The sample was then held for 24 H, and the striations were annealed to release the mechanical stress generated during the pseudomorphic transition. In addition, large single crystal PSBP samples can be prepared by adding photopolymerizable monomers into the material system, and the reflection wavelength of the PSBP samples can be reversibly red-shifted under the action of a DC electric field. This method lays a certain experimental foundation for the rapid preparation of large single crystal BPLC photonic crystal materials.

4 Applications of Blue Phase Liquid Crystal Photonic Crystals in Optics

BPLC materials have important scientific research value in the preparation of small volume, low threshold and high intensity laser sources. Compared with the traditional laser, the blue phase laser is a kind of Distributed feedback laser, and its three-dimensional self-assembled microstructure can play the role of a resonant cavity, and can also be used as a band-edge laser[6]. Because BP has the characteristics of external stimulus response, the wavelength of laser radiation can move under the action of external fields such as temperature, pressure, electric field, ultraviolet light and so on, so the blue phase laser has excellent tuning performance and broad application prospects in the field of tunable lasers.
In 2002, Cao et al. First observed the three-dimensional laser radiation phenomenon in BPII[39]. Compared with the one-dimensional Ch-phase laser under the same experimental conditions, the laser emitted by the BPLC laser is close to the diffraction limit, the excitation threshold is small, and the beam shape is a cone with an apex angle of about 0.12 rad, as shown in Figure 13. Kikuchi et al. Prepared fluorescent dye-doped PSBP samples, which have both wide-temperature range BP and low-threshold circularly polarized laser emission characteristics[40]. Coles et al. Prepared a template film with a BPI three-dimensional periodic structure by washing out small molecule LC after polymer stabilization, and the BP temperature range covered -125 ~ 125 ℃ after reinfusion of small molecule nematic LC[41]. A BP template laser can be obtained by doping a small amount of laser dye into a small molecular nematic LC. Zhu Jiliang et al. tested the laser radiation performance of fluorescent dye-doped PSBP samples in the three-dimensional direction under the action of an AC electric field, and found that the laser radiation intensity in the three-dimensional direction was basically the same without the action of an electric field, while the laser radiation intensity along the direction of the electric field decreased, while the laser emission intensity in the other two perpendicular directions increased[84]. Hamdi et al. Obtained room temperature BPI microcapsules with a size range of 60 ~ 100 μm by adding LC materials into glycerol matrix, and studied their AC electric field response and laser radiation properties. It was found that with the increase of electric field intensity, the laser peak moved to the short wavelength direction and the half-peak width narrowed significantly, and the wavelength shift range was about 11 nm[85]. After the electric field is removed, the initial state without electric field is restored.
Our research team used the polymer stabilization method to prepare a three-dimensional BPLC photonic crystal film with electric field control characteristics, which showed the electric field response characteristics of asymmetric bidirectional movement of photonic band gap under the action of DC electric field (Fig. 14A)[86]. The electrically tunable BPLC laser with low excitation threshold was prepared by adding laser dye into the material system, and the laser wavelength can be controlled up to 22 nm. It is found that the existence of the polymer network and its morphology in the liquid crystal cell play a key role in the electric field response characteristics of the sample. The SEM image of the film shows that the film has a heterogeneous polymer network structure with a gradually decreasing density from top to bottom. Under the action of a DC electric field, the polymer network that adsorbs cations and is positively charged moves by the force of the directional electric field and the direction of motion is controlled by the direction of the electric field, resulting in the stretching or compression of the blue phase lattice structure, thus showing the bidirectional shift of the photonic band gap (Fig. 14b). In order to avoid the influence of the presence of laser dye on the structure and morphology of BP in the polymerization process, the researchers prepared a polymer stabilized blue phase liquid crystal sample with a wavelength range controlled by an electric field covering the visible light band.Furthermore, a template BPLC laser was obtained by infusing dye-doped small molecular nematic liquid crystal into the BP polymer template by the "wash-out and infusion" method. The selective reflection peak shift range of the sample covers the visible light band (Fig. 14C), and the laser radiation wavelength has the tunable performance of DC electric field induced bidirectional shift. The laser radiation peak moves to the long wavelength under the positive DC electric field, and moves to the short wavelength under the negative DC electric field, and the laser wavelength control range can reach 48 nm (Fig. 14d)[87].
In terms of lens, BPLC can be used to prepare adaptive lens, which has the advantages of small size, light weight, easy integration, excellent electric field tunability, wide adjustable focal length range, and faster response speed than other LC phases. The device driven by a vertical electric field also has the characteristic of polarization independence. Wang Qionghua of Beijing University of Aeronautics and Astronautics has made a detailed study on the application of BPLC in optical devices such as Fresnel lens and beam controller through the design and simulation of electrode structure[88,89]. In terms of gratings, Wu Shicong et al. Fabricated tunable phase gratings with high diffraction efficiency and sub-microsecond response speed using PSBP samples[42]. Li Qing et al. Of Southeast University fabricated a dual-period tunable PSBP phase grating by designing electrodes and driving methods, and the grating period and diffraction angle can be switched by controlling the driving method[90]. Li Yan et al. Used interference visible light as the polymerization light source to expose the prepolymer sample to the interference light pattern, stabilize the BP structure and realize the periodic change of polymer concentration, and prepared the holographic PSBP grating[91]. Hu Wei et al. Of Nanjing University prepared BPLC Dammann vortex grating by designing periodic fork electrodes, and proved that it can be used as an equal energy distribution light array generator[92]. Zheng Zhigang et al. Prepared BP samples with periodically arranged orientation regions and random orientation regions on the basis of photoalignment patterning technology, and studied their application in the field of fork gratings[63]. As shown in fig. 15, due to the selective reflection of the BP orientation region, the incident light near the center wavelength of the reflection peak is reflected by the sample and interacts with the incident light to form a two-dimensional diffraction spot, while the incident light at the wavelength position of the non-reflection peak is not diffracted. The reflection wavelength can be moved by the external electric field, and then the diffraction wavelength can be controlled. The disappearance of even order (second order) diffraction spots was also observed in the experiment. Kim et al. Compared the performance of single-domain PSBP and multi-domain PSBP grating samples with frictional parallel orientation, and found that the diffraction efficiency could be increased by 9% after single-domain orientation, while the threshold voltage and driving voltage were significantly reduced[93]. Due to its unique three-dimensional self-assembly structure, macroscopic optical isotropy and fast response, BP has shown broad application prospects in the field of photonic devices.

5 Conclusion

In this paper, the self-assembly behavior of BPLC materials, the control methods of crystal structure and its application in the field of optoelectronics are summarized. High-resolution TEM and synchrotron small-angle X-ray diffraction (SR-SAXS) were used to study the phase transformation of BP. It was found that the phase transformation of each subphase of BP was determined by the diffusionless reorganization of self-assembled structural units DTCs, and the BPII ~ BPI phase transformation was similar to the thermoelastic martensitic transformation of metal materials. The nucleation and growth of BP on the substrate surface can be controlled by different substrate surface orientation treatment methods or by preparing a chemically patterned surface with a specific nanostructure, and a single crystal BP sample with consistent crystal plane orientation can be obtained. According to the response characteristics of BP to electric field, single crystal or non-cubic structure of BP can be obtained under different external electric fields. BP single crystals with different crystal orientations can also be obtained by controlling the heating and cooling process. Due to its special three-dimensional self-assembly structure, photonic band gap in the visible light band, macroscopic optical isotropy and fast response performance, BP has been widely used in the fields of photonic devices such as tunable distributed feedback lasers, adaptive lenses and tunable gratings.
Compared with traditional semiconductor photonic crystals, BPLC is a kind of soft matter and self-assembled three-dimensional photonic crystal, whose optical band gap is just in the visible light band, and it has good fast response characteristics to external fields such as electric field, temperature, illumination and so on. In recent years, many studies on BP self-assembly behavior and structure control methods have made it possible to prepare large area BP single crystal samples, which is expected to provide new ideas for breaking through the bottleneck of the existing three-dimensional photonic crystal manufacturing technology, and also open up a new way for the application of BPLC materials in the field of optoelectronics. At present, the research of BPLC photonic crystal materials is still in its infancy. How to realize the reasonable design of BPLC optoelectronic device structure to reduce the driving voltage and improve its transparency is still the focus of current research. In addition, the design of BP photonic crystal device structure and the development of specific functional applications will also become an important research direction in this field in the future.

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Funding

National Natural Science Foundation of China(52003293)

National Natural Science Foundation of China(51927806)

Fundamental Research Funds for the Central Universities(2021YQJD16)

Training Program of Innovation and Entrepreneurship for Undergraduates of Beijing(202011413133)

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