Self-assembled liquid crystal architectures for soft matter photonics

Self-assembled architectures of soft matter have fascinated scientists for centuries due to their unique physical properties originated from controllable orientational and/or positional orders, and diverse optic and photonic applications. If one could know how to design, fabricate, and manipulate these optical microstructures in soft matter systems, such as liquid crystals (LCs), that would open new opportunities in both scientific research and practical applications, such as the interaction between light and soft matter, the intrinsic assembly of the topological patterns, and the multidimensional control of the light (polarization, phase, spatial distribution, propagation direction). Here, we summarize recent progresses in self-assembled optical architectures in typical thermotropic LCs and bio-based lyotropic LCs. After briefly introducing the basic definitions and properties of the materials, we present the manipulation schemes of various LC microstructures, especially the topological and topographic configurations. This work further illustrates external-stimuli-enabled dynamic controllability of self-assembled optical structures of these soft materials, and demonstrates several emerging applications. Lastly, we discuss the challenges and opportunities of these materials towards soft matter photonics, and envision future perspectives in this field.


Introduction
"Soft matter" is firstly proposed by Pierre-Gilles de Gennes in his Nobel acceptance speech in 1991, which describes materials between aqueous substances and ideal solids, such as colloids, foams, liquid crystals (LCs), gels, polymers, and active matter 1,2 . Soft matter materials lay the advantageous foundations of living systems in nature due to their spontaneous self-assembly of functional organizations and the superior ability to sense, function, and response to various environmental stimuli [3][4][5][6] . The weak interaction among soft building blocks triggers a fragile balance between entropic and enthalpic contributions to the free energy [7][8][9] , which facilitates the self-assembly of multiple length-scale microstructures 8,10,11 with phenomena closely related to both the inherent characteristics of nanomaterials and the structural engineering of building blocks throughout a spatial region 3,12,13 . For instance, chameleon exhibits a rapid and reverse shift of color patterns when interacting with the outside by actively structuring non-close-packed guanine nanocrystals within the skin [14][15][16] . Till now, remarkable soft materials with a wide variety of complex configurations 17 , colorful patterns 18 , metastable states 11 , and macroscopic softness 19,20 have provided valuable inspirations for addressing modern challenges in wide ranges of areas [21][22][23] , especially in advanced optical and photonic technologies 24 , driving the development of soft matter photonics.
LC represents one of the most attractive soft matter systems [25][26][27][28][29][30][31] . In living organisms, biocomponents including proteins, deoxyribonucleic acids (DNAs), polysaccharides, and lipids are kept in LC states through well-defined self-assembly processes, which play important roles in plentiful life activities, including metabolism, information delivery, and interoceptive awareness 32,33 . As the name indicates, LCs possess both the fluidic property of conventional liquids and the ordering nature of crystals 1,6,[34][35][36] . They can achieve intriguing and programmable hierarchical superstructures with a high sensitivity to external stimuli, such as electric field 37,38 , light exposure 39,40 , magnetic field 41 , mechanical action 42 , and interface conditions 43,44 . The anisotropic molecular structures, combined with the long-range orientational order and adaptive stimulus-responsiveness, endow LCs with desirable birefringent optical performance 45,46 . This feature makes LCs an unfailing paradigm for display industries 45,47 , with the annual value of production reaching hundreds of billions of dollars. From this point of view, it is believed that, as a crucial material to life itself and displays manufacture, LCs show great potentials to promote the thriving topic of soft matter optics [48][49][50][51][52] . On the other hand, LCs derived from biomass (such as cellulose, DNA, tobacco mosaic virus, chiral polypeptide LC solvent, and silk) are appealing candidates for developing soft and sustainable optical platforms 53,54 .
Naturally derived bio-based LCs provide new opportunities towards the replacement of existing non-renewable optical platforms with renewable, biocompatible, and biodegradable systems that match the high performance of their synthetic counterparts, while minimizing waste, environmental degradation, and energy-intensive input 55 . Additionally, the features of hierarchical and tailorable structures, stimuli-responsiveness, functionalized capabilities, and facilitation of formation of different material formats make them ideally suitable for soft and smart photonic materials.
Over the past years, optical systems based on LCs (typical thermotropic and bio-based lyotropic LCs) have experienced a booming development, promoting the emergence of new phenomena, functions, and applications. In this article, we present recent advances in the fabrications, manipulations, and applications of selfassembled optical LC architectures, Fig. 1. We first summarize the basic properties of typical thermotropic LCs and bio-based lyotropic LCs, i.e., nematic phase LCs, smectic phase LCs, cholesteric phase LCs, blue phase LCs, and celluloses. Next, we analyze the manipulation schemes of LC architectures, especially the topological defects and topographic configurations, with an emphasis on the dynamic control of these self-assembled optical structures. Subsequently, we describe several burgeoning optical and photonic applications, such as smart displays, optical imaging, and light field modulation devices. In the last part, we discuss the challenges and opportunities of these systems towards soft matter photonics, and provide visions for the future perspectives in this field.

Typical thermotropic liquid crystal architectures
Microstructures bridge the inherent properties of nanomaterial and the important functionalities of devices. To develop ideal LC-based devices, the priority is to tailor long-range ordered LC microstructures. Many efforts have been devoted into this field to creating on-demand LC superstructures with wide tunability. In this section, we focus on different LC phases, including nematic, smectic, cholesteric, and blue phase LCs, and describe the judicious control and dynamic modulation of LC microstructures, especially topological defects. The central idea relies on the stimulus-controlled self-organization of LC building blocks.

Patterned structures in nematic phase LCs
Nematic LCs (NLCs), as the simplest state among various LC phases, are famous for the prevalent applications in displays 45,56,57 , due to their self-assembled long-range orientational ordering and fast response capability under electric fields 6,58-61 . Distinct from ordinary liquids, NLCs are optically uniaxial materials with anisotropic structural and physical properties (e.g., birefringence and dielectric anisotropy) 1,6 , Fig. 1, and possess the ability to form intriguing microstructures sensitive to external conditions 26,62,63 . All these unique features provide potential opportunities for NLCs in areas beyond displays, especially in optics and photonics 64 . The creation of elaborate patterns in NLCs is challenging. To achieve this, Kim et al. 65 employed a Si substrate with a periodic lattice of square and round air pockets to generate ordered topological defect arrays, Fig. 2a. By this means, stable pinwheel-like birefringent textures with vibrant birefringent colors are induced and their positions were well imposed by the three-dimensional (3D) surface topography of substrates. Xia et al. 66 reported a selforganized subtle saddle-splay arrangement of NLCs by introducing 3D topographic substrates with chemical patterns, Fig. 2b. The elastic constant K 24 was identified, which enters the Frank-Oseen free energy density for both chiral and achiral materials, and carries important insights into the nature of soft materials 2 .
To add more degrees of freedom of NLC arrangement, photoalignment is further proposed based on photoresponsive alignment materials, the sulphonic azo-dye SD1 for instance 67,68 , which can reorient their absorption oscillators perpendicular to the polarization of ultraviolet (UV) light due to the isomerization of azo groups and dichroic absorption of chromophores. A digitalmicromirror-device (DMD) based photopatterning system was further developed to generate arbitrary photoalignments for programmable multi-domain LC microstructures 67,69-71 , Fig. 2c. By utilizing a projection system with engineered plasmonic meta-masks, Guo et al. 72 achieved the high-throughput and high-resolution complex LC orientations, e.g. a flower pattern with a cluster of four topological defects as shown in Fig. 2d Moreover, Martineza et al. 73 realized dynamic highresolution patterning and re-patterning by adopting an azobenzene-containing surface monolayer, Fig. 2e. Desired dynamic microstructures can also be achieved by taking advantages of the stimuli responsiveness of NLCs [74][75][76][77] . For instance, Li et al. 78 electrically produced a series of intriguing microstructures forming 3D particlelike propagating solitary waves, Fig. 3a, which are also called director bullets, in analogy with light bullets (3D optical solitons). These bullets can survive collisions with restored shapes and velocities. Director bullet represents the distortion of LC directors, which periodically oscillates following the same frequency of electric fields. The underlying mechanism is the flexoelectric effect where the director distortion produces flexoelectric polarization. The coupling of the polarization and the electric field generates a Coulomb force which balances the viscosity dragging force. By changing the amplitude and/or frequency of the electric field, these solitons with tunable steering directions were reported 79 , Fig. 3b, enabling promising applications such as information delivery and micro-cargo manipulations 75 .

Topological defect structures in smectic phase LCs
Smectic phase is another important mesophase among thermotropic LCs 80,81 . Distinguished from NLCs, lamellar smectic LCs (SLCs) present both the long-range orientational order and positional order 82 , i.e., the long axes of molecules perpendicular or slightly tilted to the layer plane, Fig. 1. There are abundant topological superstructures with distinct morphologies discovered in SLCs under different external conditions [83][84][85] . One of the most eminent topological microstructures is called the focal conic domain (FCD) 2,82,86-90 , in which smectic layers are curved with a constant interlamellar spacing typically in molecular scales and wrapped around a pair of conjugate defect lines (an ellipse and one of the hyperbolae) [91][92][93] .
Much progress has been made to manipulate the geometric parameters of FCD arrays, including the size, shape, eccentricity, and lattice symmetry 94,95 , which brings diversified optical functionalities, e.g., microlensing [96][97][98] , vortex beam generating 99 , and optically selective masks 96 . Lavrentovich et al. 86 studied the magnetic and surface anchoring effects on the nucleation and growth of smectic FCDs in 1994. By utilizing 3D topographic substrates with periodic pillars and undulated surfaces, absorbing kaleidoscopic textures and hierarchical topological FCDs were self-assembled 65,100 , Fig.  4a, b. For simultaneously dictating the geometry and clustering characteristics of FCDs, Ma et al. 95 demonstrated the "smectic layer origami", where 3D construction of anisotropic defects is realized by two-dimensional (2D) preprogrammable photopatterning. These microstructures break the rotational symmetry while maintaining the radially gradient director field of LCs, enabling a metasurface-like polarization-selective diffraction, Fig. 4c. In addition, defect walls of smectic oily streaks were arbitrary manipulated and flexibly modulated by combining the photoalignment and electric field 101 ,  Temperature is a crucial parameter in the fine control of hierarchical smectic topological defects. Gim et al. 102 reported the dynamic defect morphogenesis during the in situ nematic-smectic phase transition by accurately controlling the temperature of LC droplets on the water, Fig. 5a. It was found that the director field geometry of nematics would strongly affect the geometry of subsequent smectic topological defects during the phase transition, which is rationalized by illustrating the similarity of ordering between nematics and smectics. Zappone et al. 103 examined the smectic pattern conversion from one-dimensional (1D) streaks to 2D fan-shaped FCD arrays when cooling from nematics to smectics, Fig. 5b. The long-standing prediction of an "intermediate" LC state based on an analogy with superconductors was further confirmed, which was first pointed by de Gennes in 1972 104 . By leveraging the sublimation and condensation of SLCs, Kim et al. 105 reported an intriguing morphology transformation of FCDs with nontrivial LC curvatures, Fig. 5c. Defect structures with significant positive Gaussian curvature of smectic layers were obtained, implying a lucrative thermal control of both the mean and Gaussian curvature of SLCs. The study of SLC sublimation was recently explicated by Vitral et al. 106 on smectic-isotropic interfaces. In addition, Boniello et al. 107 proposed electrically reversible and dynamically tunable defect patterns in polymer-stabilized SLCs, which overcomes the long-term intractable challenge, i.e., the highly ordered SLC is essentially irreversible under electric fields,  Fig. 4 Tailoring of smectic topological defects by geometric confinement and photoalignment. a Topographically patterned kaleidoscopic textures 65 . Reproduced from ref. 65 , with permission from AAAS. b Hierarchical assembly of FCDs by undulated surfaces 100 . Reproduced from ref. 100 , with permission from Royal Society of Chemistry. c Photopatterned topological FCDs 95 . Reproduced from ref. 95 , with permission from Wiley-VCH. d Photopatterned oily streaks 101 . Reproduced from ref. 101 , with permission from Wiley-VCH abundant topological defects in SLCs may blaze a trail in the fields of topology, self-assembly, structural patterning, and so on.

1D helical structures in cholesteric phase LCs
Chirality is pervasive in nature, ranging from neutrinos to nucleic acid 108 , seashells, and galaxies 109 , which brings nontrivial phenomena and attracts scientists across various domains to explore artificial chiral nanoarchitectonics 34,[110][111][112] . Cholesteric LCs (CLCs), also called chiral nematic LCs, exist ubiquitously in organisms 36 . They can self-organize into various elegant helical structures, Fig. 1, including fascinating fingerprints, planar textures, distinctive microshells as well as microgrid chiral structures. The helical pitch p, defined as the distance of a full turn rotation of anisotropic LC molecules 113 , is a decisive parameter for the helical microstructures, which can be flexibly tuned by different external stimuli such as temperature 114 , light 115 , electric field 116 , magnetic field 117 , mechanical stress 118 , and chemical conditions 119 . Intriguingly, when the periodicity of the helical structure of CLC is comparable with visible wavelengths, Bragg reflection occurs with a strong wavelength/polarization selectivity. The central where n is the average refractive index of LCs 1,120 . Thus, CLCs are also known as soft photonic crystals, i.e., periodic dielectric materials with photonic band gaps (PBGs).
Sustaining efforts have been devoted to dynamically and multi-dimensionally controlling the CLC helical configurations. For example, the photoalignment technique was adopted to arbitrarily control the helical axis orientation of CLCs, allowing the creation of large-area, high-quality, and more complex fingerprint patterns, such as Achimedean spiral and wave-like grating 121 125 . Adapted with permission from ref. 125 . Copyright 2021 American Chemical Society molecular switch and achieved a 3D manipulation of the helical axis of CLC together with the inversion of the handedness solely by light, Fig. 6b. The tractable generation, modulation, and termination of zigzag patterns were also demonstrated 123 . Moreover, Chen et al. 124 reported a photoresponsive microshell system with the tunable helical pitch enabled by a visible-light-driven chiral molecular switch, Fig. 6c. Jiang et al. 125 reported 2D ordered microgrid chiral structures in a CLC reactive mixture due to the photopolymerization-induced periodic deformation, which looks like the Helfrich-Hurault undulation when the electric or magnetic field exceeds the threshold value 41,126 , Fig. 6d. Besides conventional cholesterics, CLCs with oblique helicoidal state were also disclosed 127 . The pitch of this state has been reported to be able to continuously tuned by external electric 38 , magnetic 128 and light fields 129 . For instance, Xiang et al. 38 produced electrically tunable selective reflection of light ranging from UV to visible and infrared regions by using the oblique helicoid CLC structure with low driving electric fields, Fig. 7a. Thanks to the preservation of the simple sinusoidal modulation of LC ordering at different fields, the oblique helicoidal CLC could achieve the maximum intensity of Bragg reflection, scattering, and resonances. The dual stimulation of light and electric field permits reversible and dynamic transformations between helicoidal and oblique helicoidal states along with the handedness inversion and dynamic PBG control 130 , Fig. 7b. These works broaden the scientific content of microscopic molecular self-assembly in soft chiral materials, which may inspire potential applications based on CLC superstructures.

3D cubic structures in blue phase LCs
Self-assembled blue phase LCs (BPLCs) are highly chiral states with unique complex 3D cubic lattices that cannot be artificially fabricated by micromachining 131,132 , Fig. 1. Three typical categories of BPLCs (BPI, BPII and BPIII), each of which has its unique structural characteristics, are determined by the strength of chiral interactions. BPI and BPII are composed of double-twist cylinders packed in body-centered and simple cubic lattices, respectively, and BPIII (foggy phase) possesses random and flexible structures, similar to the isotropic phase [133][134][135] . As the name indicates, one of the most noticeable properties of BPLCs is their selective reflection (like CLCs) with the reflected color largely depending on the helical pitch. In addition, the soft nature of LCs renders them highly responsive to external stimuli, such as electric field and light, resulting in tunable characteristics of BP-based optical performances 136 . The narrow temperature range of BPLCs is considered as an Achilles' heel for practical applications. To overcome this shortcoming, polymer-stabilized BPLCs 131,137 , microstructure-stabilized BPLCs 138 , and new BPLC materials/composites [139][140][141][142] were developed, following the pioneering work by Kikuchi 131 . Xiang and Lavrentovich 143 further demonstrated a BP-templated soft material system to expand the temperature range.
Recently, the monocrystalline alignment of BPLCs has received broad attention, because it avoids multicolored mosaic polycrystalline textures consisting of randomly distributed small platelet domains. Chen et al. 144 reported large single photonic crystals in BPLCs based on a gradient-temperature scanning technique, Fig. 8a. These giant single crystals exhibited substantially sharp PBGs, long-range periodicity in all dimensions, and well-defined lattice orientation. The nucleation and growth of large uniform BPII single crystals with the domain size larger than 10 μm were developed on a chemically patterned substrate with alternative regions of different LC anchoring 43 . Moreover, Bukusoglu et al. 145 studied the confinement and surface anchoring effects on the orientation of BPs, which provides a new tool to tailor the structure and optical properties of BP films.
By introducing a molecular switch functionalized nanocage, BPLCs are endowed with a reversible photoresponsive characteristic, which can be light-switched between the BPII and the cholesteric phases 146 . Consequently, well-defined biphasic micropatterns with both single soft cubic lattice and helical superstructures are disclosed, Fig. 8b. In addition, mechanical stimulation is another important manner to tune functional BPLC structures. Schlafmann et al. 147 synthesized a fully solid BPLC elastomer that retains 3D nanostructures with dynamic reconfiguration upon photopolymerization. The remarkable tunabilities of the lattice constants and related optical performances of BPLC elastomers are validated through mechanical deformations as well as thermal and chemical stimulations, Fig. 8c. The electric field, environmental temperature, and humidity can alter the BP lattice orientation and transform the BP nanostructures as well, providing controllable on-demand optical properties [148][149][150][151][152] . The continuous progress of BPLC manipulation will advance the development of stimuli-responsive intelligent optical devices, such as high-performance 3D tunable lasers.

Bio-based lyotropic LC architectures
Liquid-crystalline phase exists widely in biosystems through precisely controlled self-assembly, such as cell membranes, nucleic acids, proteins, polysaccharides and lipids. These bio-based LCs are also promising for soft matter photonics, which benefit the emergence of sustainable and biocompatible optical systems. Cellulose is among the most studied bio-LCs in this context owing to its availability in large quantities and outstanding optical/ photonic material characteristics. It is a polysaccharide mainly derived from plants, fungi, and bacteria in the form of cellulose fibers, microfibers, and nanofibers. Over the past decades, great achievements have been made in the design and fabrication of optical micro-and nanostructures through the hierarchical manufacturing of celluloses to develop soft and smart optical devices for multiple high-tech applications, including smart displays, information processes, soft actuators, and smart windows [153][154][155][156] . In this section, we summarize the state-of-art photonic structure designs constructed from cellulose and their dynamic behaviors in response to environmental stimuli.
Cellulose is skillfully employed by plant kingdom to create unusual optical functions by assembling CLC nanostructures 153,157 . These photonic architectures have been an inspiration for the construction of artificial photonic materials based on cellulose that can mimic Fig. 7 Dynamic tuning of oblique helicoidal architectures in CLCs. a Field-controlled oblique helicoidal CLC structure 38 . Reproduced from ref. 38 , with permission from Wiley-VCH. b Electrically stimulated transformation of CLC superstructures 130 . Adapted from ref. 130 , with permission from AAAS natural designs, properties, and functions 153,157 . Pure cellulose and cellulose derivatives, such as cellulose nanocrystals (CNCs), hydroxypropyl cellulose (HPC), and ethyl cellulose, can spontaneously self-assemble to generate CLCs [158][159][160][161][162][163] . CNC is a highly crystalline nanorod with a high aspect ratio that can spontaneously exhibit lyotropic LC behavior in a water suspension [164][165][166][167] . Such CLC structure can be preserved in solid films 164,168 . Because of the chiral interaction between nanorods, the CNC suspensions and films always self-assemble into left-handed CLCs to selectively reflect left-circularly polarized light [164][165][166][167] . The CLC structures of CNC films represent 1D photonic crystals that give iridescent colors, Fig. 9a. HPC is produced by the etherification of cellulose, which introduces hydroxypropyl groups onto the polymer chain. HPC has the same self-assembly performance as CNC, except that HPC has a right-handed chiral nematic structure, which is opposite to CNC 169 .
In addition to harnessing intrinsic 1D CLC structure for coloration, constructing nanoarchitectures on the surface of nanocellulose film is another effective approach. For example, 2D cellulose photonic crystals were fabricated by applying hot embossing or replica molding technique 170 , Fig. 9a. Both methods could produce highly-ordered periodic photonic structures to display characteristic iridescence. Hierarchical CLC structures were developed by shaping either surface topography or bulk periodicity on the microscale 171,172 . For instance, floral-mimetic hierarchically ordered photonic cellulose films that combine nanoscale CLC organization and microscale wrinkly surface topography were designed by leveraging soft nanoimprinting lithography 171 . The CNC nanorods close to the air-water interface can be freely assembled into CLC organization, while the CNC orientation near the template-CNC interface is anchored along the surface plane of its waved surface with the direction of the spiral axis remaining perpendicular to the undulating surface, Fig. 9a. In another scenario, CNC hydrogel containing vertically aligned uniform periodic structures was developed 172 . Such a thin hydrogel sheet contains CLC structures with helical axes parallel to the surface on the nanoscale and grating structures on the microscale, giving hierarchical signatures, Fig. 9a.
Cellulose-derived photonic structures are sensitive to various stimuli, such as humidity, solvents, gases, mechanical strain, and temperature, because of its large amount of hydroxyl groups and good compatibility with other materials 154,155 , enabling the establishment of dynamically responsive optical systems.
Cellulose is a hygroscopic material. Water molecules can strongly combine with hydroxyl groups in the amorphous region of CNCs or HPC, resulting in overall expansion and an increase in pitch 173,174 . Various compounds, such as polyethylene glycol (PEG) 175  as humidity sensors 175 , Fig. 9b. The addition of PEG allows an obvious improvement in the flexibility and the sensitivity of the photonic film to humidity. The composite film showed excellent cyclic stability and reversibility by constantly regulating humidity. Moreover, photonic cellulose films can also be adjusted to respond to organic solvents and gases [180][181][182] .
Mechanical stress, such as compression, shear, and stretching, is an effective method to manipulate optical properties of the cellulose films due to the rapid and reversible responsiveness, ease of handling, and controllable features. So far, several strategies such as coassembly with weakly interacting additives 178,183-187 , post processing 188 , or laminations 189,190 have been pursued to generate flexible and mechanically responsive photonic films. Using a coassembly method, a uniform and stretchable CNC/elastomer composite was synthesized 183 , Fig. 9c. The resulting composite film can be stretched by over 900% and show reversible and rapid structural color changes. Mechanochromic HPC laminates were prepared by using large-scale, low-cost continuous coating and encapsulation 190 . The pressure response of HPC films can be quantified by optical analysis of pressure-induced color changes, enabling the   Fig. 9 Cellulose-based representative photonic structures and the corresponding stimulus responsiveness. a Schematics (top rows) and images (bottom rows) of representative photonic architectures 163,[170][171][172] . Images reproduced with permissions: 1D-helicoidal structure 163  Polymers with thermal responsiveness are usually introduced into CNC matrix to enable thermally responsive chiral optical materials. Poly(N-isopropylacrylamide) (PNIPAM), known for its unique thermal and wet fracturing effects 191 , has been used to produce patchy PNIPAM-grafted CNCs with the aid of a surface-initiated atom transfer radical polymerization method 192 , Fig. 9d. The unique topological morphology design allows for an increase in translational and rotational degrees of freedom with the collapse of the PNIPAM chains. As such, its suspension exhibited optical anisotropic at 25°C, but disappeared at 40°C, Fig. 9d. This behavior is different from "brush" PNIPAM-modified CNCs, whose suspension exhibited birefringence at both 25°C and 40°C, Fig. 9d.

Smart displays
A primary goal of displays is to create smart devices which can adaptively respond to various external stimuli, such as light, electricity, and force, delivering real-time desired information. Structural colors, which rely on the meticulous design of microstructural architectures to obtain colorful characteristics without pigments or dyes, show promising potentials for applications in displays, decoration, and anti-counterfeiting 193,194 , due to their distinctive features of environmental friendliness and high stability, and capacity to produce brilliant, fading-resistant, tunable, and high-resolution colors. Here, we focus on the stimuli-adaptive color-tuning display systems based on structural colors.
The way to remotely, spatially, and temporally control the structural color from CLCs has attracted significant attention. Recently, Wang et al. 195 achieved reversibly light-activated structural colors across the whole visible spectrum through the photoisomerization of a halogenbonded axially chiral switch, which is an important step toward smart photodisplay devices, Fig. 10a. By employing a visible-light-driven chiral fluorescent molecular switch, rewritable multimodal CLC fluorescence/reflection display devices were realized by Li et al. 115 . Qin et al. 196 proposed a new strategy of reflective displays with lightdriven black ground and RGB structural colors. In addition, geminate security labels have been realized by patterning two-tone CLC microdroplets with both the structural and fluorescent colors, Fig. 10b, which opened a new avenue for anti-counterfeiting technologies 197 . In addition, unique optical materials of oblique helicoidal CLCs are also appealing candidates for smart displays 129,198 , which possess both twist and bend LC orientations, and are highly sensitive to the electric and light fields, resulting in a feasible tunability of the structural color consequently 129 .
CLC elastomers serve as a structural-color-changing platform as well, which not only preserve the selforganization of LCs but also enable a strain deformation [199][200][201] . Kim et al. 199 proposed twin-layer pneumatically inflating thin membranes composed of highly stretchable CLC elastomers and PDMS film to demonstrate compact pixelized structural colors with broadband spectral tunability, Fig. 10c. The different transverse deformations via pneumatic actuation lead to divergent colorations, which originates from the anisotropic elasticity induced large Poisson's ratios of LCs. Schmidtke et al. 202 found that the actuation of biaxial stress results in substantially enhanced photonic properties of freestanding CLC elastomer coatings, allowing potential applications in tunable optical filters.
The capacity to create unique chiral optical properties, display vivid structural colors, and to respond rapidly and reversibly to a variety of stimuli makes photonic cellulose films potentially viable for smart display, information encryption, and ant-counterfeiting applications 154 . As an example, a re-printable photonic paper was prepared by incorporating chiral nematic CNCs into a chemicallycrosslinked polycation 203 . The film showed controllable wettability via anion exchange, resulting in extremely low color contrast in the dry state but high contrast in the wet state, which enables reversible display and hiding of the encoded information, Fig. 10d. In another study, a series of structural color materials with multiple dynamic photonic responsiveness and high-resolution patterns were developed by mixing cellulose molecules with acrylamide monomers 173 . By synergistically utilizing dual-responsive behavior to humidity and UV light, a multicolor pattern and a quick response (QR) code were demonstrated, Fig. 10e. Flexible cellulose photonic materials that are capable of significantly simplifying device construction, allowing real-time stimulation visualization, and readily detecting mechanical and physical signals have shown the potential as "photonic skins" to replace bulky and rigid electronic devices.
By integrating the shape memory and self-assembly characteristics of HPC simultaneously into a multi-layer flexible film structure, an ultra-adaptive and stably wearable pixelated photonic skin that can be used for precise monitoring of human motion and structural health of buildings and bridges was developed 204 , Fig. 10f. Such pixelated photonic device allows for a vivid color response by applying different mechanical stimuli to it. A clear color transition from red to blue was observed when the applied pressure is increased, Fig. 10f.

Optical imaging
Optical lens is a common but indispensable optical element in diverse imaging areas, such as telescopes, binoculars, and cameras [205][206][207] . Traditional optical lens mainly depends on the regulation of dynamic phase based on the isotropic medium, which hinders the lightweight, miniaturization, and integration. The Pancharatnam-Berry (PB) phase is discovered by S. Pancharatnam (1956) 208 , and later generalized by M. Berry (1984) 209 , which is a geometric phase associated with the polarization of light 210 . Through rationally designing the director field of LCs, LC imaging devices can be developed based on PB phase, which show non-negligible merits of planar and ultra-thin configurations, tunable characteristics, and especially polarization slectivity [211][212][213][214] .
Recently, High-quality reflective polymeric CLC PB lenses were reported with a diameter D = 2.45 cm and low f-numbers (f/2, f/0.9, f/0.45, f/0.33) at 550 nm 215 , which can be converging or diverging, depending on the handedness and direction of the incident light. Zhan et al. 216 presented large-scale, cost-effective, and ultrabroadband PB lenses with structured LC polymers, which overcame the critical issue of chromatic aberration originating from the optical dispersions of materials, Fig. 11a Fig. 11b. They further demonstrated switchable chromatic aberration by applying a bias voltage for different purposes. CLC is also a kind of popular soft material in lens fabrications [218][219][220] . A PB phase lenticular microlens with a polarizationdependent focal length by immersing chiral gold nanoparticles doped CLCs in water was reported by Perera et al. 220 , Fig. 11c. Polarization imaging, especially four-dimensional visual imaging including 1D polarization and 3D space information of the target, is a special and promising technology for future optical imaging 98,[221][222][223][224] . Recently, Ma et al. 98 proposed an efficient approach based on a well-designed asymmetric topological microlens array for the fourdimensional visual imaging by a single snapshot, Fig. 11d. The demultiplexing of both the depth and polarization information carried by the targets was demonstrated. The above studies show the superiority of LCs in the accurate control and manipulation of the light propagation, which are expected to innovate existing imaging technologies for the needs of contemporary science and technology.

Light field modulation
Thanks to the high birefringence, self-assembled superstructure, huge sensitivity to external fields, and easier manipulation of optical axis distributions that differ from solid crystals 120 , LCs come into prominence in the field of light field modulation [225][226][227][228] , and present dynamic optical functions just like or even beyond metasurfaces ranging from visible to terahertz regions 217,229,230 . Vortex beam is one of the most popular and versatile beams 231,232 , with wide applications in optical tweezers 233 , optical communication 234 , quantum computation 235 , and coronagraph 236 . Till now, plentiful of optical schemes have been proposed to generate vortex beams [237][238][239] , such as fork gratings, qplates, spin-orbit coupling, spatial light modulators (SLMs), and deformable mirrors, etc. Among them, LC-based light field modulation devices occupy an important position. For instance, Wei et al. 67 demonstrated LC fork gratings for the vortex beam generation by using a DMD-based dynamic mask photopatterning system, Fig. 12a. Kobashi et al. 240 demonstrated the efficient and polychromatic generation of broadband optical vortices by creatively adopting CLC fork gratings, Fig. 12b. Moreover, LCs are also effective tools for the detection of orbital angular momentum (OAM) modes, especially in mode-division multiplexing. Chen et al. 241 further introduced a concept of digitalized chiral superstructures and produced a Dammann fork grating for simultaneous detection of multiplexed optical vortex beams without the mode crosstalk or distortion, Fig. 12c. By using rationally photoaligned CLCs, detections including vector beam and hybrid OAM modes were all successfully realized, which greatly expands the role of LCs in optical communications.
LC defects can also be exploited for the generation and modulation of the optical field 242,243 . Voloschenko et al. 244 firstly clarified vortex beams generated by dislocations in CLC finger structures. Son et al. 99 Fig. 12f. Different modes of vector laser beams were effectively emitted from a radial nematic droplet. By elaborately designing the topological LC superstructures, more structured light lasers were shown, enabling tunable and compact photonic devices. Besides, there are also lots of works dealing with spatiotemporal optical vortex 248 , spin-split photon mode 249 and 3D spiral optical fields 239 based on liquid crystalline materials, which may provide a satisfactory platform for tough requirements in cutting-edge photonics.

Soft actuators
Soft actuators have been a hot research theme for decades due to their significant flexibility in operations. LC-based materials are particularly encouraging, which can macroscopically deform in response to various external stimuli (light [250][251][252][253] , heat 254,255 , electric 37,256,257 and magnetic 258 fields), instead of using mechanical force to generate dynamic change. In addition, the facile engineering of LC microstructures adds another degree of freedom to control the shape change and motion of soft actuator 40,253,259 , enabling a series of complex tasks, such as oscillation, rotating, rolling, turning, twisting as well as their combinations, which are expected to replace current machinery parts.
Photoresponsive LC elastomers with light-driven flexible actuations have gained significant interest [250][251][252]260 . To generate highly programmable soft actuators, Huang et al. 251 integrated tunable fluorophores into LC elastomers. The combination of strong fluorescent emission and reversibly photoisomerization-induced deformation was utilized to mimic multiple biological functionalities, such as the shape morphing and discoloration behaviors of frillneck lizards, Fig. 13a. Cheng et al. 261 demonstrated the light-controlled friction and locomotion of a centimeter-long polymer stripe under a constrained condition of a human hair, Fig. 13b. The friction conditions of both the hair surface and the asymmetric actuator geometry, together with the photo-actuated LC deformation, lead to versatile directional locomotion.
Moreover, Babakhanov et al. 262 Fig. 13c. The dynamically thermal control of the surface topographies allows for particle rearrangements 262,263 . Oscillating CLC fingerprints were also achieved to wipe away dust by electrically modulating the topographic corrugation, Fig. 13d. Feng et al. 257 .
Besides polymerized LC actuators, small-molecular LCs with fluidic elastic properties can be used as functional actuators as well 73,122,264,265 . In 2006, Eelkema et al. 264 reported a collectively rotational CLC fingerprint system that rotated microscopic-scale objects by introducing a light-driven rotary nanomachine, Fig. 14a. Ma et al. 30 achieved programmable self-propelling actuators that could massively transport microparticles in customized trajectories by elaborately designing the selforganized microstructure-engaged CLC system, Fig. 14b. Yuan et al. 266 demonstrated the reconfigurable colloidal assembly based on optically switchable signs and amplitudes of the interactions of elastic colloidal monopoles, Fig. 14c. They also developed self-assembled LC colloidal nanomotors, enabling the unidirectional particle to spin with light-controlled handedness and frequency 267 . In addition, the dynamic manipulation of soliton-dressed spherical particles is also accomplished by Li et al. 75 based on the alternating current electrophoresis in NLCs, Fig. 14d.
Soft actuators based on photonic nanocelluloses have drawn great attention because of their superior humidity response, mechanical flexibility, color sensing, and biocompatibility, opening a sustainable avenue in visual mechanical sensors, wearable photonics, smart bionic actuators, and intelligent robots 42,[268][269][270][271][272][273][274][275] . For example, by mimicking the shell structure of scarab beetles, a photonic actuator based on CNCs was developed 275 . The composite films were prepared by sandwiching a uniaxial orientation polymer layer between two flexible CNC layers. The increase of environment humidity leads to the bend of the layered photonic films away from the wet air and consequently the change of structural color, Fig. 15a. Such photonic actuators showed excellent dynamic reversibility upon cyclic humidity change. More recently, mechanically flexible, deformable, and optically tunable composites were fabricated by assembling CNC with polyethylene glycol dimethacrylate (PEGMA) monomer 42 . Such photonic films exhibited sensitive and reversible moisturedriven actuation behavior and a variety of complex 3D deformation modes, accompanied by the variation of color appearances, Fig. 15b

Smart windows
As one of the least energy-efficient components in buildings, there is burgeoning interest in developing smart windows with the capability to dynamically control the transmittance of sunlight, lending benefits including energy efficiency, architectural beauty, eye protection, and privacy protection [276][277][278] . Thanks to the multiple stimuli-responsiveness, LCs as well as  264 , with permission from Springer Nature: Nature. b Programmable self-propelling actuators enabled by a dynamic helical medium 30 . Adapted from ref. 30 , with permission from AAAS. c Self-assembled reconfigurable colloidal monopoles 266 . Adapted from ref. 266 , with permission from Springer Nature: Nature. d Particle manipulation based on soliton-induced electrophoresis 75 . Adapted with permission from ref. 75 281 , where LC molecules are in the form of droplets dispersed in polymers, with fast response ability. To maintain an energy-efficient window view, self-powered smart windows are highly pursued 279,282 . Murray et al. 282 developed a self-powered hybrid switchable solar window by combining the PDLC system with a semiconducting absorber, enabling electrically controllable light trapping and on-demand power generation throughout the day. Another fully selfpowered and ultra-stable smart window was proposed based on elaborately fabricated CLCs and the triboelectric nanogenerator 279 , Fig. 16a. Thus, the window can be facilely driven between transparency and haziness by instantaneous mechanical stimuli. Recently, Yoon et al. 283 generated a robust LC smart window by a single-step dual-stabilization of LCs mixed with photoresponsive dopants and monomers, Fig. 16b. This smart window based on physical gels and polymer chambers exhibits fast response and low voltage actuation. Harnessing the beneficial features of nanomaterials into host materials provides a facile path to endow smart windows with multi-responsive and multi-functional capabilities 284 . Wang et al. 285 achieved a homogeneous dispersion of 2D materials into chiral LC superstructures by elaborately synthesizing mesogen-functionalized graphene, which enables multi-responsive smart window systems, Fig. 16c. Energy-efficient smart window applications were also demonstrated by using a roll-to-roll process to fabricate LC composites containing tungsten bronze nanorods 286 . They can reversibly switch the transmittance of light by temperature, electric field, and near-infrared light, showing advantages of wide temperature range, high flexibility, robust mechanical strength, long-term stability, and largearea processability.
In addition to typical thermotropic LCs, cellulose-based LC films have also been utilized as tunable reflectors for thermal management 156 . For example, De La Cruz et al. 287 designed a photonic composite film consisting of two cholesteric CNC reflectors sandwiching a nematic-like CNC retarder, Fig. 16d. This sandwich structure possesses a polarization-independent reflection (i.e. both lefthanded and right-handed circularly light can be effectively reflected), much higher reflection than a single cholesteric-like film, and tunable reflection from the visible to near-infrared ranges of the optical spectrum.    Fig. 16 Smart window applications. a A self-powered hybrid switchable solar window based on CLCs and a sliding triboelectric nanogenerator 279 . Adapted from ref. 279 , with permission from Elsevier. b Robust LC smart windows with low voltage switching 283 . Reproduced from ref. 283 , with permission from Wiley-VCH. c Smart windows based on LC/graphene composites 285 . Reproduced from ref. 285 , with permission from Elsevier. d CNCbased tunable photonic reflectors 287 . Adapted with permission from ref. 287

. Copyright 2018 American Chemical Society
The composite could achieve high reflectivity in the nearinfrared range while maintaining high transmittance in the visible spectral regime, making it suitable for smart window applications. Besides, smart windows were also designed based on a biopolymer-stabilized LC system composed of a CNC-based network and a nematic LC 288 . This system shows rapid voltage-off response time, good voltage-driven contrast between the scattering and transparent states, and a high haze factor.

Conclusion and prospects
Soft matter photonics (we refer to it here as "Soft Mattonics") is a burgeoning area of research and has attracted much attention in recent years. By combining "top-down" manufacturing technique with "bottom-up" self-assembly process of LCs, one can design and fabricate hierarchical superstructures with multiple degrees of freedom, which makes them splendid candidates for soft and smart photonics. In this article, we have highlighted recent studies focusing on the creation, manipulation, and application of self-assembled optical architectures based on typical thermotropic LCs (NLCs, SLCs, CLCs, and BPLCs) and bio-based lyotropic LCs (CNCs and HPCs). These soft materials show distinguished optical and responsive properties that are employed for different applications. For examples, NLCs exhibit long-range orientational ordering and the LC director distribution can be collectively modified by external fields, resulting in a large modulation of the optical phase and consequently the light transmittance. SLCs with highly ordered structures present both the orientational order and the positional order. They can self-organize into various topological defects with functionalities of micro-imaging, particle manipulating, beam steering, etc. CLCs with 1D periodic helical structures possess a PBG caused by Bragg reflection, which endows them with promising applications related to the control of specific polarization/ wavelength dependent functions. Self-organized doubletwisted BPLCs are 3D photonic materials with high chirality and degree of freedom in developing optical and photonic devices. In addition, the capability to construct photonic structures with different topologies and topographies provides complex, tunable, and multiple functionalities to these soft-matter-based optical platforms. Corresponding applications have been demonstrated, for instance, smart display, optical imaging, and light field modulation devices.
Although continuous and great progress has been made, related research in Soft Mattonics is still at its preliminary stage. It remains strong challenges for large-scale production and processing of these soft optical devices, owing to the difficulties in achieving uniformity of soft matter films, precise patterning over large areas, and stability of the systems. Realizing optimized "structure-property-function" relationships through efficient manufacturing technologies is still difficult. In addition, the seamless integration of soft-matter optical materials (as we described here) with existing optical components is an open challenge. Therefore, much effort needs to be devoted to exploring new manufacturing technologies to mutually optimize the correlations of materials, structures, and functions.
It is anticipated that self-assembled LC architectures with merits of easy fabrication, fine tunability, high flexibility, and remarkable stimuli-responsiveness would play important roles in the prosperous development of optoelectronics, optics, and photonics. More appealing soft architectures can be further expected in newly discovered LC phases. For example, Noel Clark group recently discovered ferroelectric nematic liquid crystalline phase with huge ε and high fluidity, which may open a band-new door for the nontrivial architectures with an exciting future 289 . In addition, Soft Mattonics with new optical functions would be worth expecting by combining LCs with other soft materials. For instance, silk protein, which is a natural structural protein that is mainly spun by spiders and silkworms, is an excellent candidate for soft optical materials owing to its capacity to develop a wide variety of photonic architectures that generate an optical response as a result of interaction between light and the nanostructure in which silk is molded, and to create flexible, tunable, complex, and multifunctional optical platforms [290][291][292][293][294][295] . The development of LC-silk photonic composites would provide expanded optical utility by leveraging their functional interplay and further extend the applications of LC devices towards the interface between optical technologies and biological world. By integrating LCs with cutting-edge electronic and robotic systems, multifunctional devices and advanced optical systems with desirable adaptive and active performances can be energetically anticipated. Further exploration in such thriving topic would not only broaden the knowledge of Soft Mattonics but also encourage multidisciplinary research from specialists across different disciplines and promote diverse soft and smart photonic applications.