Liquid Crystals for Responsible Photonic Crystals Liquid Crystals for Responsible Photonic Crystals

The photonic crystals (PCs) exhibit photonic band gap (PBG), inhibiting specific wave - length of light decided by structural periodicity. PCs are a new class of periodic dielectric media that can provide novel ways to manipulate and control light. Researchers have recently devoted extensive efforts to fabricating PCs with controlled symmetry, size, and defects on a large scale and tuning of PBG. Liquid crystalline (LC) materials exhibit- ing self-organization, phase transition, and molecular orientation behaviors in response to external stimuli are attracting significant attention for the bottom-up nanofabrication and tuning of advanced photonic materials and devices. Here, we will introduce self-organization of PCs from LCs and photoswitching mechanism of PBG based on phase transition and anisotropic orientation of LCs.


Introduction
The structural coloration is basically caused by interaction between light and periodic nanostructured-materials, and light absorption with dyes or pigments is not required for the structural coloration. One can find structural coloration of various materials in nature, and the structural colors are iridescent and very durable, for example, opal, peacock wings, and so on. Recently, there is an increasing interest in developing bioinspired nanostructures with controlled symmetry, size, and defects on a large scale for photonic crystals (PCs) [1][2][3][4][5][6][7]. Generally, the fabrication processes are divided into bottom-up and top-down methods. The top-down methods are mostly based on lithography-related techniques, which are expensive and requiring high technology [8][9][10][11]. The bottom-up methods are primarily about the selfassembly of component units such as emulsions, colloidal particles, and block copolymers [12][13][14][15][16].
Liquid crystalline (LC) materials are attracting significant interest on the bottom-up nanofabrication of advanced photonic materials and devices with unique configuration and novel tuning mechanism. LCs undoubtedly represent such a marvelous dynamic system based on molecular reorientation behavior affecting macroscopic properties [17][18][19]. On the other hand, the responsiveness of LCs to various stimuli such as temperature, light, mechanical force, and electric and magnetic fields as well as chemical and electrochemical reactions can attribute to fascinating tunability of photonic band gap (PBG) in the periodic nanostructure, which provides numerous opportunities in all-optical integrated circuits and next-generation communication system. Here, several examples of self-organized LC nanostructures are introduced, and fabrication of three-dimensional (3D) PCs containing azobenzene LC molecules and photoswitching of PBG are discussed.

Self-organized LC nanostructures
We introduce several examples for the fabrication methods of precisely constructing of PCs using self-organized LC nanostructures of cholesteric LC (ChLC), LC blue phase, and holographic polymer-dispersed LC.

Cholesteric LC
Dissolving a chiral compound in a nematic LC causes helix formation, resulting in the selective reflection of circular light corresponding to the helical pitch and helical sense. The pitch, the distance over which the director field rotates by 2π radians, is sensitive to external stimuli such as temperature, light, electric field, and mechanical stress [20][21][22][23][24][25]. Kurihara et al. have reported on the photochemical change in transparency as well as the selective reflection of a cholesteric LC (ChLC) resulting from the photoisomerization of a chiral azobenzene compound doped in a host nematic LC [26][27][28][29][30][31]. It is based on the photochemical change in helical twisting power (HTP) of the chiral azobenzene compounds under irradiation of UV and visible light. A combination of a rigid chiral diol core and azobenzene group led to larger photochemical changes in HTP, resulting in a reversible change of selective reflection over whole range of visible region with faster photochemical switching as shown in Figure 1. HTPs of the chiral azobenzene compounds increased or decreased by UV light, and the color change of reflection light was seen even with naked eyes.
On the other hand, Muševič et al. developed ChLC microdroplets, in which the arrangement of ChLC molecules was with parallel anchoring of the LC molecules at the surface. The helical structure of the LC originates from the center of the droplet and gives rise to concentric shells of constant refractive index. The introduction of chirality in ChLCs in the spherical shape resulted in the formation of a multilayered spherical Bragg resonator with an alternation periodic refractive index [32]. This dielectric structure is optically equivalent to the wellknown Bragg-onion optical microcavity. In addition, Li et al. reported photoresponsive ChLC droplets with large helical twisting power [33].

LC blue phase
Blue phases (BPs) are among the most fascinating photonic nanostructures in the area of LCs [34]. They are known to self-organization into the 3D frustrated nanostructures, which originate from the competition between the packing topology and chiral forces. The periodic nanostructures are self-organized from the so-called double-twisted cylinders that are usually stabilized by the formation of defects or disclinations. As the 3D PCs, BPs feature remarkable bandgap tunability over an extensive spectrum in response to various external stimuli such as temperature [35,36], mechanical strength [37], electric field [38], and light [39][40][41]. The spectral band gap position of selective reflection can be readily predetermined by varying the chiral fraction of BP mixtures. However, BPs have received increasing attentions especially in recent years, but the reflection wavelength tuning is usually quite narrow. The ability to dynamically tune the photonic bandgap in cubic BPs across a wide wavelength range is highly desirable. Effort to improve the instability and irreversibility of BPs has been made with polymer-stabilized structure [18] and incorporation of unusual bent-core mesogen [42].

Holographic polymer-dispersed LC
Holographic polymer-dispersed liquid crystal (H-PDLC) materials belong to a phase separation material system where the LCs can form droplets, of controllable sizes, that are phase-separated from the polymer-rich regions during the photopolymerization process. Interestingly, it has enabled the in situ fast one-step fabrication of self-organized PCs. The control of the bandwidth of the reflection resonance, related to the light intensity and spatial porosity distributions, was investigated to optimize the optical performance.
For example, Cartwright et al. reported a graded reflection grating based on the H-PDLC material system [43,44]. An obvious rainbow-colored reflection was observed from the same viewing angle by a modification to this standard configuration where the triangular prism is replaced with a cylindrical lens. Modified optical setup is based on a cylindrical lens to generate a graded holographic interference pattern along the x-axis from the incident and reflected beams. It is expected to compact graded PBG structures for linear variable optical filters. The marriage of holography and self-organized LC materials is highly expected to produce a diverse range of novel 1D, 2D, and 3D nanostructures for various applications in electrooptical filters, wavelength division multiplexer, free-space optical switches, information display devices, and so on [45][46][47][48][49].

PBG tuning of 3D photonic crystals
Three-dimensional PCs such as opal and inverse opal structure can be easily prepared by colloidal self-assembly approach. The fabrication of inverse opal structure infiltrated by azobenzene LC molecules for 3D PCs and the mechanism of photoswitching of PBG are described.
For 3D PCs, the reflection wavelength (λ) under normal incident condition is expected by equation of Bragg diffraction as follows [50] where d is the diameter of the sphere, n sphere and n void are the refractive indices of the sphere and the medium in the voids of opal, respectively, and f is the volume fraction of the sphere. In this way, the peak wavelength can be roughly tuned by two approaches: either by control of the diameter of the sphere or by control of the refractive index. If one can control photochemically phase transition or birefringence of LCs in 3D PCs, they will be candidates for applications to optical devices such as display and memory.
On the other hand, light among various promising stimuli shows distinct and significant advantages due to its spatial, remote, and temporal controllability. It is also noteworthy that the light sources with different wavelength, intensity, and polarization are readily accessible. Photocontrol of chemical and physical functions of various molecular systems has been studied vigorously using photochemical configurational change of azobenzene derivatives [51][52][53][54]. The photochromism of azobenzene and its derivatives due to cis-trans isomerization has been widely investigated. For PC containing azobenzene molecules, 'photochemical phase transition' between the alignment phase and isotropic phase, or 'photoinduced optical anisotropy' based on photoselective reorientation behavior is very useful parameter for selforganization and optical photonic crystal switching. Here, fabrication of three-dimensional (3D) PC containing azobenzene molecules and photoswitching of PBG are discussed.

PBG tuning by phase transition change
Three-dimensional PCs such as opal and inverse opal structure can be infiltrated with stimuliresponsive materials, thus enabling PBG tuning. Kubo et al. reported on the drastic change in optical properties for the inverse opal structure containing low molar mass LCs, by taking advantage of the nematic-isotropic phase transition [50]. It is based on change in the light scattering intensity between LC phase and isotropic phase.
SiO 2 inverse opal template could be fabricated by infiltration of SiO 2 spheres into the voids of polystyrene (PS) opal film and calcination of PS as shown in Figure 2. When SiO 2 inverse opal film infiltrated with 4-cyano-4′-pentylbiphenyl (5CB), the reflection spectra of the LC-infiltrated inverse opal film were changed with increase in the temperature as shown in Figure 3. In the initial state at 25°C, 5CB is in the nematic phase and the film is white due to light scattering. The reflection has two weak peaks. With the gradual temperature rise, the positions of the peaks shifted slowly and a distinct peak at around 610 nm, which is derived  from PBG, appeared rapidly at the phase transition temperature into the isotropic phase at 40°C. The color of film becomes red. This result shows that switching of the optical properties can be realized due to a phase transition of the LCs, leading to thermos-tunable PCs.
Kubo et al. also reported the photoswitching of reflection for SiO 2 inverse opal film infiltrated with photo-response azobenzene containing LC mixture [55]. By UV light, a reflection peak increased rapidly and then saturated, indicating appearance of PBG. The trigger for this drastic change was the trans-cis photoisomerization of the azobenzene derivatives in the films. The excitation of the π-π* transition of azobenzene molecules at around 360 nm results in a transformation from trans to cis isomers. The photoinduced cis form of azobenzene group has a bent shape and hence tends to disorganize the phase structure of nematic LCs.
The observed change could be reversed to the original state, decrease of reflectance, by irradiating with visible light. The recovery was induced by cis-trans photoisomerization, followed by a phase transition from the isotropic to the nematic phase under visible light (436 nm). Such a switching effect could also be observed in transmission spectra. It is supposed due to the fact that the LC molecules in the nematic phase in the spheroidal voids are aligned parallel to the void surfaces, but the orientation is completely random among spheres. It is similar to the phenomena observed for polymer-dispersed liquid crystal (PDLC). On the other hand, when the LC was transformed into an isotropic phase by UV light, the anisotropy in the dielectric constant disappeared. That is, the dielectric constant of the LC in all of the spheroidal voids became the same, and hence, a reflection peak due to Bragg diffraction appears [56][57][58]. On the basis of this scenario, dynamic change in the optical properties was observed repeatedly by alternating irradiation with UV and visible light. It was completely reversible, exhibiting good stability and reproducibility during the light irradiation cycle as shown in Figure 4.

PBG tuning by molecular reorientation
For low-molar-mass LC-infiltrated inverse opal film as mentioned in the previous section, the stability of the switched state was not enough although the photoswitching was reversible. It is due to the thermal back reaction of the azobenzene compound and fluidity of the low-molar-mass LC. In contrast with low-molar-mass LCs, polymeric LCs are well known to show excellent storage stability. Kurihara et al. reported the stable and reversible shift of the Bragg diffraction band for SiO 2 inverse opal structure infiltrated with 4,4′-methoxy hexyloxy azobenzene polymethacrylate, PAz (schematic molecular structure is shown in Figure 5), and reflection change were investigated when irradiation of linearly polarized light (LPL) and circularly polarize light (CPL) of Ar + later (λ = 488 nm) [59].
Photoisomerization behavior is demonstrated by absorption band change as shown in Figure 6. By UV light, trans-cis isomerization is induced, the absorption peak at 360 nm decreases, and the peak at 450 nm increases (Figure 5(a) and (c)). By irradiation of visible light, cis-trans photoisomerization is induced, and reverse changes in absorption spectra are induced (Figure 6(b) and (d)).
Anisotropic state of azobenzene compounds based on their molecular orientation can be controlled via reversible isomerization behavior by light and thermal stimuli [60][61][62]. Namely, alignment state of trans-form is perpendicular to the direction of polarized light (known as Weigert effect [20,[63][64]), and the alignment is destroyed by formation of cis-form.
The orientation state is evaluated by polarized absorption spectra and by birefringence, Δn, measurement. From angle dependence of absorbance at the peak of 360 nm to the polarized axis, orientation direction perpendicular to the polarized direction of induced LPL was demonstrated. When Δn is estimated by change in transmitted light of the azobenzene film  placed between two crossed polarizers, Δn of PAz was estimated to be 0.128, as shown in Figure 7 [65].
Azobenzene polymer can be infiltrated into the SiO 2 inverse opal film by heating to melt. The pore diameter and volume fraction of SiO 2 inverse opal, d and f, were estimated by comparing wavelength of the reflection peaks of the SiO 2 inverse opal films infiltrated with various solvents with different refractive indices. For SiO 2 inverse opal film, d = 278 nm and f = 0.118, reflection spectra before and after injection of PAz are shown in Figure 8. The injection of PAz caused a shift of the reflection peak from 480 nm to 710 nm, because of an increase in the refractive index from 1.0 of air to 1.6 of PAz.
By LPL irradiation of light, reflection measured with polarizer parallel and perpendicular to the polarization direction of LPL is plotted in Figure 9. The reflection peaks depend on the irradiation time and polarization direction. By irradiation of a linearly polarized light at 488 nm (Ar + laser), azobenzene groups were aligned in the direction perpendicular to the polarization direction of the laser light.
After irradiation of LPL, reflection band shifted to longer wavelength with little shoulder at shorter wavelength. By observation with parallel and perpendicular polarizers, different two bands were respectively observed. The maximum wavelengths of the reflection bands at shorter and longer wavelength regions were 706 nm and 725 nm, respectively. Therefore, the refractive indices of PAz in the pores after the LPL irradiation, ordinary refractive index (n o ) and extraordinary refractive index (n e ), were estimated to be, respectively, 1.57 and 1.62 by using Bragg diffraction (Eq. (1)) with the parameters d (278 nm), f (0.118), and nSiO 2 (1.45). Namely, the photoinduced birefringence of PAz was 0.05 in the pores of SiO 2 inverse opal. It is less optical anisotropy comparing to solid film of PAz (0.13), indicating insufficient alignment

Conclusion
Optical properties of LCs represent the responsibility of external stimuli such as temperature, light, mechanical force, and electric and magnetic fields as well as chemical and electrochemical reactions, which are a driving force leading to dynamic change of materials from molecular level to macroscopic level. Dynamic system of LCs makes it a quite advantageous approach toward bottom-up nanofabrication of photonic materials with novel tuning mechanism. In addition, the responsiveness to various stimuli can attribute to fascinating tunability of PBG, which provides numerous opportunities in all-optical integrated circuits and nextgeneration communication system. Figure 9. Reflection spectra of Paz-infiltrated SiO 2 film with parallel and perpendicular LPL irradiation of Ar + laser and plot of the wavelength of reflection peak [59].