Photo-Aligned Ferroelectric Liquid Crystal Devices with Novel Electro-Optic Characteristics

crystals

Review Photo-Aligned Ferroelectric Liquid Crystal Devices with Novel Electro-Optic Characteristics

Vladimir Chigrinov 1,2,* , Qi Guo 3 and Aleksey Kudreyko 4

1 School of Physics and Optoelectronic Engineering, Foshan University, Foshan 528325, China 2 Department of Theoretical Physics, Moscow Region State University, 141014 Mytishi, Russia 3 School of Instrumentation and Optoelectronic Engineering, Beihang University, Beijing 100191, China; [email protected] 4 Department of Medical Physics and Informatics, Bashkir State Medical University, 450008 Ufa, Russia; [email protected] * Correspondence: [email protected]

Received: 18 May 2020; Accepted: 19 June 2020; Published: 1 July 2020

Abstract: This paper examines diﬀerent applications of ferroelectric liquid crystal devices based on photo-alignment. Successful application of the photo-alignment technique is considered to be a critical breakthrough. A variety of display and photonic devices with azo dye aligned ferroelectric liquid crystals is presented: smart glasses, liquid crystal Pancharatnam–Berry phase optical elements, 2D/3D switchable lenses, and laser therapy devices. Comparison of electro-optical behavior of ferroelectric liquid crystals is described considering the performance of devices. This paper facilitates the optimization of device design, and broadens the possible applications in the display and photonic area.

Keywords: photo-alignment of ferroelectric liquid crystals; electro-optic modes; new devices; imaging technologies; displays; photonics

1. Introduction The appearance of portable and wearable devices has led to an ever-increasing demand for next-generation displays with ultra-high resolution, light weight, low power consumption, and high efficiency [1,2]. However, the response time of nematic liquid crystals (LCs) is too slow for fast-switching devices. Fast response time of LCs is required to reduce motion blur, to enable field sequential color system (FSC) [3], and to improve low-temperature performance. Versatile photonic devices, e.g., LC lenses, tunable focusers, and wave front correctors, must have 2π modulation. Therefore, much effort has been made to improve the response time of various electro-optic modes in liquid crystals [4]. One of the most interesting functional materials for future display devices is chiral smectic liquid crystals. Reduced symmetry of such liquid crystals can exhibit ferroelectric properties, thereby giving them high operational speed and resolution far superior to the usual nematic technology. Ferroelectricity in LCs does exist in smectic phase C* (SmC*). Interaction of anisometric molecules gives rise to a long-range order of long molecular axes. This leads to the anisotropy of various physical parameters. Centers of mass of the SmC* molecules are located in parallel plane layers. The SmC* molecules are confined to rotate within the smectic planes, while the director field n (i.e., the most probable orientation of long molecular axes and the optical axis) is constrained to tilt away from the layer normal by some angle θ, which depends on the FLC chemical structure and temperature. In order to gain some insight into the properties of SmC* phase, it is important to understand the concept of symmetry as it is applied to materials. Smectic layers of SmC* phase have a point symmetry

Crystals 2020, 10, 563; doi:10.3390/cryst10070563 www.mdpi.com/journal/crystals Crystals 2020, 10, 563 2 of 24 group C2. The secondary axis belongs to the smectic plane and is perpendicular to the director tilt plane. Absence of the symmetry planes is associated with the chirality of SmC* molecules. This means that if molecules have a dipole moment, the spontaneous polarization Ps will be directed along the C2 polar axis. The tilt angle θ value does not depend on the layer position, while the azimuthal angle ϕ (which speciﬁes the director orientation) does change. As a result, the director ﬁeld and the spontaneous polarization exhibit a helical structure. Such FLCs are known as helical liquid crystals. Although 35–45 years have passed since pioneering studies were undertaken [5] (in comparison, the fundamental discoveries in nematic LCs for display technology were made in the late 1960s), the temperature dependent behavior of ferroelectric liquid crystals (FLCs) and surface alignment problems have not been solved. Understanding the physics of FLCs for display systems and photonic devices has become crucial for modern devices. In this review we describe recent achievements in the application of FLCs for prototype devices and strategies to overcome the remaining obstacles for large-scale commercial application of smectic liquid crystals. The discussed applications also address the fundamental questions about the electrical and optical properties of soft condensed matter, unusual types of self-organization at the nanometer scale. Section2 describes the photo-alignment technique, which is the fundamental basis for the fabrication of the prototype devices. Synthesized sulfonic azo dye SD-1 is deemed preferable to obtain high alignment quality of FLCs, especially for large and curved surface areas. Section3 addresses the polymer stabilization of FLCs for electrically suppressed helix ferroelectric liquid crystal (ESHFLC) mode. This section is relevant to the inherent temperature dependent behavior of FLCs. The reported results indicate that the optical contrast ratio in polymer-stabilized FLCs is higher than 4,000:1 and the smectic cone angle θ remains fairly constant within the temperature range from 10 C to 40 C. Section4 − ◦ ◦ is a series of subsections dealing with the application of FLCs for smart glasses, Pancharatnam-Berry phase lens, 2D/3D switchable displays, high resolution displays, and photobiomodulation. Applications considered in Section4 mainly use the photo-alignment technique.

2. Photo-Induced Surface Alignment of FLCs The rubbing technique or mechanical buffing of polyimide layers remains the current method of choice for the large-scale production of liquid crystal (LC) displays. Meanwhile, there is a widespread interest in photo-alignment because rubbing produces many impurities, electrostatic charges on polyimide aligning films, and mechanical damage. These undesired effects can be completely avoided if the photo-alignment technique is used [6]. In particular, the photo-alignment technique is highly promising for FLCs because of its inherent sensitivity to non-uniformities and damage of aligning layers. A homogeneous, reproducible, and steady FLC alignment is essential for bistable switching. The bistability phenomenon and multiplex mode degradation of passively addressed FLC display cells, which were aligned by the ordinary rubbing technique, has been reported [7]. In order to improve the FLC alignment quality, various aligning surfaces have been studied. A hybrid between linearly photopolymerized polymers and liquid crystal polymer layers provides good alignment quality of deformed helix ferroelectric liquid crystal (DHFLC), but it requires complex technological methods. Photoanisotropic azo dye films combined with polyvinylalcohol can also be used for photo-alignment of FLCs, but this approach does not prevent the bistability degradation [8]. Defect-free FLC displays were also fabricated using UV-irradiated polyimide films by a two-step exposure method. A high contrast ratio and perfectly bistable switching were demonstrated. Murakami et al. fabricated a half-V-shaped photo-aligned FLC display with the electro-optic performance, which overcomes the FLC display, fabricated by rubbing with a high switching angle under low voltage [9]. Thus, the photo-alignment technique has demonstrated its success for fabrication of polymer-stabilized V-shape and half-V-shaped FLC displays. Sufficiently long UV-illumination of polyimide films shows that it promotes defect-free alignment of surface-stabilized FLC layers. Synthesized sulfonic azo dye SD-1 was successfully tested for the alignment of FLCs and nematic LCs. Pure reorientation of molecular absorption oscillators perpendicular to the polarization plane of the UV light without any photochemical transformations Crystals 2020, 10, x FOR PEER REVIEW 3 of 26

long UV-illumination of polyimide films shows that it promotes defect-free alignment of surface- stabilized FLC layers. Synthesized sulfonic azo dye SD-1 was successfully tested for the alignment of FLCs and nematic LCs. Pure reorientation of molecular absorption oscillators perpendicular to the polarization plane of the UV light without any photochemical transformations and a minimum of undesired charges make this azo dye suitable for experimental studies. Therefore, high photo- alignment quality of FLCs on azo dye SD-1 layers can be achieved. CrystalsBelow,2020, 10 in, 563 this section а method for FLC photo-alignment using SD-1 aligning layers with various3 of 24 thicknesses is described. Thе FLC alignment quality and the steadiness of bistable switching during multiplex operation of passively addressed FLC display cells are also considered. We also compare andthe surface a minimum free ofenergy undesired for different charges FLC make aligning this azo layers dye suitable (i.e., rubbed for experimental polyimides studies.and photo-aligned Therefore, highazo dyes). photo-alignment Thе approach quality proves of FLCs to beon appropriate azo dye SD-1 for layersthe comparative can be achieved. characterization оf different FLC Below,aligning in su thisrfaces section [10]. a method for FLC photo-alignment using SD-1 aligning layers with various thicknesses is described. The FLC alignment quality and the steadiness of bistable switching during multiplex2.1. Experimental operation of passively addressed FLC display cells are also considered. We also compare the surface free energy for different FLC aligning layers (i.e., rubbed polyimides and photo-aligned azo dyes).In The an effort approach to study proves azo to dye be SD-1 appropriate layers, variou for thes comparativefilms of different characterization azo dye layer of thicknesses different FLC on aligningITO surfaces surfaces were [10]. prepared using N,N-dimethylformamidе (DМF) solutions with azo dye concentrations ranging from 0.2% tо 1.3%. The azo dye solutions were spin-coated on ITO electrodes 2.1.at 3500 Experimental rpm and the surfaces were dried at 100 °C. UV light was used to irradiate the surface of the SD-1 layer. An interference filter at 365 nm and a polarizing filter were also used. The intensity of In an effort to study azo dye SD-1 layers, various films of different azo dye layer thicknesses on ITO polarized light was 6 mW/cm2. The thickness of the azo dye layer was measured by an atomic force surfaces were prepared using N,N-dimethylformamide (DMF) solutions with azo dye concentrations microscope (AFM). The SD-1 layer thickness was varied within 0 and 12 nm as shown in Figure 1a. ranging from 0.2% to 1.3%. The azo dye solutions were spin-coated on ITO electrodes at 3500 rpm The best alignment quality was achieved under asymmetric boundary conditions, i.e., only one and the surfaces were dried at 100 C. UV light was used to irradiate the surface of the SD-1 layer. ITO surface was treated by the SD-1◦ layer, while another ITO surface was simply washed in the DMF. An interference filter at 365 nm and a polarizing filter were also used. The intensity of polarized light This is essential to avoid the competition between the alignment effects produced by the substrates. was 6 mW/cm2. The thickness of the azo dye layer was measured by an atomic force microscope (AFM). The distаnces between the surfaces were 1.5 μm аnd 4.85 μm. Thus, after assembling the FLС cells, The SD-1 layer thickness was varied within 0 and 12 nm as shown in Figure1a. asymmetric boundary conditions of the FLС layer were arranged (Figure 1b).

(a) (b)

FigureFigure 1.1. (a()a )Dependence Dependence of ofSD-1 SD-1 layer layer thickness thickness versus versus the theSD-1 SD-1 solution solution concentration concentration in N,N- in N,N-dimethylformamidedimethylformamidе (DMF) (DMF) spin-coated spin-coated on the ITO on surface the ITO at surface3500 rрm; at (b 3500) schematic rpm; (representationb) schematic representationof the ferroelectric of the liquid ferroelectric crystal (FL liquidС) cell crystal with (FLC) asymmetric cell with boundaries. asymmetric boundaries.

TheTraditional best alignment polyimide quality PMDA-ODA, was achieved a polymeric under asymmetric diаnhidrid boundaryе and 4,4’-oxydianiline conditions, i.e., layer only with one ITOuniform surface FLC was alignment treated by on the its SD-1rubbed layer, surface, while was another used. ITO Next, surface the wasquality simply of the washed cells, which in the DMF.were Thisprepared is essential by using to avoid the rubbed the competition polyimide between layer an thed alignmentthe photo-aligned eﬀects produced azo dye by layer the substrates.SD-1 was µ µ Thecompared. distances Isotropic between phase the surfaces of the FLC were mixtures 1.5 m and (FL 4.85С-408Аm., FL Thus,С-451 afterА and assembling FLС-445, the which FLC cells,were asymmetricsynthesized boundaryby the P. conditionsN. Lebedev of Physical the FLC Instit layerute were of arrangedRussian Academy (Figure1b). of Sciences) was injected into Traditionalthe cell by capillary polyimide action. PMDA-ODA, Electro optical a polymeric measurements dianhidride were andcarried 4,4’-oxydianiline оut for the wavelength layer with λ uniform= 632.8 nm. FLC alignment on its rubbed surface, was used. Next, the quality of the cells, which were prepared by using the rubbed polyimide layer and the photo-aligned azo dye layer SD-1 was compared. Isotropic2.2. Fundamental phase of Results the FLC mixtures (FLC-408A, FLC-451A and FLC-445, which were synthesized by the P. N. Lebedev Physical Institute of Russian Academy of Sciences) was injected into the cell by capillary action. Electro optical measurements were carried out for the wavelength λ = 632.8 nm.

2.2. Fundamental Results

Sufficiently large exposure time texp (exposure energy) and decrease of dislocation density improve the contrast ratio (Figure2a). This is confirmed by FLC textures images, which are depicted in Figure2b,c. Bright stripes in Figure2b show the dislocation lines in the dark state. The dislocations vanish when the exposure time is sufficiently long (see Figure2c). Crystals 2020, 10, x FOR PEER REVIEW 4 of 26

Sufficiently large exposure time texp (exposure energy) and decrease of dislocation density improve the contrast ratio (Figure 2a). This is confirmed by FLC textures images, which are depicted in Figure 2b,c. Bright stripes in Figure 2b show the dislocation lines in the dark state. The dislocations vanish when the exposure time is sufficiently long (see Figure 2c). Asymmetric boundary conditions play a crucial role even if the rubbed polyimide layer is used instead of the azo dye layer (Figure 1b, Figure 2d,e). In this case, the defects can be suppressed (Figure 2e). Asymmetric fabrication of the substrates was essential to avoid the competition in the alignment, which is governed by the surfaces. The ITO electrode itself favors focal-conic or planar degenerated alignments, i.e., all directions on the substrate plane are energetically identical. In contrast to this, the rubbed polyimide layer (or the photo-aligned SD-1 layer) gives the preferred direction of the Crystals 2020,molecular10, 563 alignment, which dominates within the entire FLC cell (Figure 2b–e). Here, the SD-1 layer 4 of 24 thickness was determined by the rotation speed and concentration of SD-1 solution during the spin- coating fabrication process. (a) (b) (c)

(d) (e) (f) (g)

Figure 2. (Figurea) Contrast 2. (a) Contrast ratio dependence ratio dependence on on the the exposure exposure time time for for FLC FLC cells, cells, which which were filled were with filled the with the compoundcompound FLC-451A. FLC-451A. The SD-1 The SD-1 layer layer was was illuminated illuminated by by linearly linearly polarized polarized UV light UV (λ = light 365 nm) (λ at= 365 nm) the normal incidence with the power density of 6 mW/cm2. The FLC layer thickness is 4.85 μm. at the normal incidence with the power density of 6 mW/cm2. The FLC layer thickness is 4.85 µm. Polarized optical microscopy images: dark state of two photo-aligned cells filled with the FLC-451A, Polarized opticalthe cell gap microscopy is 1.5 μm; the images: irradiation dark time state of the of SD-1 two surface photo-aligned is (b) 10 min cells and ( filledc) 60 min. with Here, the the FLC-451A, the cell gapSD-1 is 1.5 layerµm; thickness the irradiation is 13 nm. ( timed) Polarized of the optical SD-1surface microscopy is ( bimages) 10 min of the and dark (c) state 60 min. of two Here, cells the SD-1 layer thicknessfilled with is 13 the nm. FLC-445 (d) Polarized with different optical boundary microscopy conditions: imagessymmetric of (both the darkITO layers state are of covered two cells filled with the FLC-445with 20 nm with rubbed diff PMDA-ODAerent boundary layers) and conditions: (e) asymmetric, symmetric i.e., one ITO (both layer ITO is covered layers with are the covered 20 with nm rubbed PMDA-ODA layer. The FLC layer thickness is 1.5 μm for both cells (d,e). (f) Polarized 20 nm rubbed PMDA-ODA layers) and (e) asymmetric, i.e., one ITO layer is covered with the 20 nm optical microscopy images of the dark state of two photo-aligned cells, which were filled with the rubbed PMDA-ODAhelix-free FLC-445: layer. the The irradiation FLC layer time thicknessof the SD-1 issurface 1.5 µ mis 10 for min; both (g) cellsthe irradiation (d,e). (f )time Polarized of the optical microscopySD-1 images surface of is the60 min. dark The state SD-1 oflayer two thickne photo-alignedss is 13 nm, and cells, the FLC which layer were thickness filled is with1.5 μm. the The helix-free FLC-445: theinsets irradiation in e–g show time the scale of the of images. SD-1 surfaceThe striped is 10structure min; (ofg )the the insets irradiation has 8 μm timeperiodicity. of the SD-1 surface is 60 min. The SD-1 layer thickness is 13 nm, and the FLC layer thickness is 1.5 µm. The insets in (e–g) Helix-free FLCs are very sensitive to the alignment quality of the dye layer. Therefore, in Figure µ show2f,g the scalewe observe of images. more Theuniform striped alignment structure than of th thee deformed insets has helix 8 FLCm periodicity. mixtures. The contrast ratio of the cell filled with the helix-free FLC-445 was superior at 1500:1 for texp = 60 min and 400:1 for texp = Asymmetric10 min. This boundary fact allows conditions us to conclude play that a crucialthe helix-free role structure even if theof FLCs rubbed is preferable polyimide for photo- layer is used instead of thealignment. azo dye layer (Figures1b and2d,e). In this case, the defects can be suppressed (Figure2e). Asymmetric fabrication of the substrates was essential to avoid the competition in the alignment, 2.3. Prototype which is governed by the surfaces. The ITO electrode itself favors focal-conic or planar degenerated alignments, i.e., all directions on the substrate plane are energetically identical. In contrast to this, the rubbed polyimide layer (or the photo-aligned SD-1 layer) gives the preferred direction of the molecular alignment, which dominates within the entire FLC cell (Figure2b–e). Here, the SD-1 layer thickness was determined by the rotation speed and concentration of SD-1 solution during the spin-coating fabrication process. Helix-free FLCs are very sensitive to the alignment quality of the dye layer. Therefore, in Figure2f,g we observe more uniform alignment than the deformed helix FLC mixtures. The contrast ratio of the cell filled with the helix-free FLC-445 was superior at 1500:1 for texp = 60 min and 400:1 for texp = 10 min. This fact allows us to conclude that the helix-free structure of FLCs is preferable for photo-alignment.

2.3. Prototype Uniform and continuous 3–5 nm thick photo-aligning layers provide high spatial optical uniformity within a large surface area (Figure3a). Therefore, we believe that photo-aligned FLC displays must have remarkable properties, which can be considered for technological background for passively addressed display devices. Consider a prototype of a passively addressed 64 64 pixel display, which was fabricated using × the photo-alignment technique under asymmetric boundary conditions. The display matrix has the dimensions of 33 33 mm and the FLC layer thickness is 5 µm thick. The fabricated display was × capable of memorizing the generated images for a period of several days after the driving voltage Crystals 2020, 10, x FOR PEER REVIEW 5 of 26

Uniform аnd continuous 3–5 nm thick photo-aligning layers provide high spatial optical uniformity within a large surface area (Figure 3a). Therefore, we believe that photo-aligned FLC displays must have remarkable properties, which can be considered for technological background for passively addressed display devices. Consider а prototype оf a passively addressed 64 × 64 pixel display, which was fabricated using the photo-alignment technique under asymmetric boundary conditions. The display matrix has the dimensionsCrystals 2020, 10 of, 56333 × 33 mm and the FLC layer thickness is 5 μm thick. The fabricated display 5was of 24 capable of memorizing the generated images for a period of several days after thе driving voltage was switched off (Figure 3b). The chosen 5 μm cell gap hаs better bistablity and multiplex operation µ steadiness,was switched and o isﬀ (Figureeasier to3b). assemble The chosen than 5traditionalm cell gap 1.5 hasμm bettercells. bistablity and multiplex operation steadiness, and is easier to assemble than traditional 1.5 µm cells.

(a) (b)

FigureFigure 3. 3. (a(a) )FL FLC-408A-basedС-408А-based 1.5 1.5 μµmm cell cell between between two two crossed crossed polarizers, polarizers, shown shown by by arrows. arrows. The The light light apertureaperture of of the cell is 33 × 33 μµm. The The dark and bright statesstates areare memorizedmemorized after after the the driving driving voltage voltage is × isswitched switched oﬀ off..(b ()b The) The generated generated image image by by passively passively addressed addresse photo-alignedd photo-aligned 64 6464 × display,64 display, which which was × wasmemorized memorized after after the drivingthe driving voltage voltage was was turned turned oﬀ. Theoff. The FLC-408A FLC-408A layer layer thickness thickness is 5 µism. 5 μm.

2.4.2.4. Summary Summary TheThe difference difference between between thе the surface surface energy energy оf the of aligning the aligning substrates substrates and the and FLC the surface FLC energy surface mainlenergyу mainlycharacterizes characterizes the overall the overall alignment alignment quality quality of FL ofС FLC cells. cells. However, However, both both structure structure and and thicknessthickness of of FLC FLC layers layers are are important. important. Perfect Perfect al alignmentignment quality quality can can be be achieved achieved only only by by means means of of asymmetricasymmetric boundary boundary conditions. conditions. The The exposure exposure energy energy о off th theе SD-1 SD-1 layer layer should should b beе sufficiently sufficiently high. high. AnotherAnother essential essential condition condition for for uniform uniform FLC FLC alignm alignmentеnt is is high high anisotropy anisotropy of of the the azo azo d dyeуе surface surface structure:structure: parallel parallel and and perpendicular perpendicular to to the the polari polarizationzation plane plane of of the the UV UV light. light. The The optimal optimal azo azo dye dye layerlayer thickness thickness (3 (3–5–5 nm) nm) provides provides the the highest highest multiplex multiplex oper operationation steadiness steadiness and and the the high high contrast contrast ratio.ratio. The The optimal optimal photo-aligning photo-aligning layer layer thickness thickness is is the the compromise compromise between between the the two two types types of of FLC FLC layerlayer alignment alignment instabilities: instabilities: focal-con focal-conicic domains domains and and ferroelectric ferroelectric domains. domains. 3. Polymer Stabilization of FLCs 3. Polymer Stabilization of FLCs Electrically suppressed helix ferroelectric liquid crystal is the diffraction-free electro-optical mode, Electrically suppressed helix ferroelectric liquid crystal is the diffraction-free electro-opticаl where high contrast ratio and fast response time are achieved with low driving voltage [11]. Two main mоdе, where high contrast ratio and fast response time are achieved with low driving voltage [11]. constraints are imposed on this electro-optical mode: alignment layer and the cell gap. In addition Two main constraints are imposed on this electro-optical mode: alignment layer and the cell gap. In to this, temperature dependence of cone angle adds new constraints to real applications of ESHFLC. addition to this, temperature dependence of cone angle adds new constraints to real applications of If the key problems are solved, then the ESHFLC mode can be promising for field sequential color ESHFLC. If the key problems are solved, then the ESHFLC mode can be promising for field sequential display, high-resolution displays (e.g., pico-projectors, 3D displays, micro-displays), and fast response color display, high-resolution displays (e.g., pico-projectors, 3D displays, micro-displays), and fast photonics devices (e.g., modulators, filters, and attenuators). The detailed characteristics of the ESHFLC response photonics devices (e.g., modulators, filters, and attenuators). The detailed characteristics of mode are given in ref. [12]. In this section, we outline the experimental issues of polymer stabilization, the ESHFLC mode are given in ref. [12]. In this section, we outline the experimental issues of polymer which does not change key parameters outside of the range of constraints for ESHFLCs. stabilization, which does not change key parameters outside of the range of constraints for ESHFLCs. The main problem hindering commercial exploitation of ESHFLC mode is the temperature The main problem hindering commercial exploitation of ESНFLС mode is the temperature dependence of the cone angle and the response time. It is known that the smectic C* cone angle dependence of the cone angle and the response time. It is known that the smectic C* conе anglе strongly decreases when temperature increases, in particular, near the ferroelectric to paraelectric phase stronglу decreases when temperature increases, in particular, near the ferroelectric to pаraelectric transition temperature. Consequently, the optical contrast decreases. On the other hand, high viscosity phase transition temperature. Consequently, the optical contrast decreases. On the other hand, high at lower temperatures is the reason for increased response time. Other FLC modes, e.g., DHFLC and anti-FLCs, demonstrate better temperature dependence, however, the optical quality of these modes needs further research. Strong chemical bonds of polymers can affect the alignment of LC molecules to a great extent [13]. Two types of polymer confinements in LCs are popular: 1) polymer-dispersed LCs, (the polymer concentration overcomes LC concentration), and 2) polymer-stabilized LCs (LC concentration is much higher than polymer concentration). When a polymer network is added into the FLC structure, the interaction between the polymer molecule and FLC molecule appears. Of course, this interaction Crystals 2020, 10, 563 6 of 24 imposes new constraints on the dynamics of the FLC molecules. Next, we consider some experimental details with polymer-stabilized FLCs.

3.1. Experimental The main effects of the addition of polymer in the FLC structure are the change of the phase transition temperature and the appearance of light scattering. The latter is due to the different refractive indexes of polymer and FLC. For the experiment we used monomer RMM257 (Merck) as the guest and pure FLC FD4004N (Dianippon Ink and Chemical Ltd.) as the host components, respectively. The chosen FLC exhibits the phase sequence: Cr Smectic C* (SmC*) Smectic A (SmA) N I at temperatures 20, 72, 95, and → → → → − 105 ◦C, respectively [14]. The FLC monomer composite was prepared by mixing the monomer in pure FLC. The monomer concentrations ranged from 2 to 15% wt/wt. The photoinitiator concentration ranged from 2 to 10% wt/wt of the monomer concentration. Higher values of the photoinitiator concentrations could accelerate the polymerization rate. After several optimization experiments, we found that 8% wt/wt of the monomer gives sufficiently fast polymerization rate without undesired effects. By heating the mixture to the isotropic transition temperature (~120 ◦C) and agitating the vial containing the guest–host mixture, uniform distribution of the monomer within the FLC structure was achieved. To fabricate the cell, we used ITO-coated substrates as the electrodes, which were coated with the azo dye SD-1. Then, the substrates were irradiated by UV light to enable us to achieve planar alignment. The thickness of the cells was maintained at 1.5 µm. The mixture at the temperature slightly lower than the N-I transition temperature was filled into the cells by capillary action. If, after the polymerization process, the composites show optical parameters (at 30 ◦C) that satisfy the ESHFLC constraints, then the cell can be considered successful. The polymerization at lower temperatures immediately after the cell filling is important to avoid the near surface localization of the polymer network.Crystals 2020 An, 10, illustration x FOR PEER REVIEW of the ESHFLC cell is shown in Figure4a. 7 of 26

(a) (b)

FigureFigure 4.4. (a) Schematic representationrepresentation of of the the ESHFLC ESHFLC cell, cell, where wherep pis is the the helix helix pitch pitch of of the the FLC FLС and аndd is d theis the cell cell gap. gap. (b )( Temperatureb) Temperature dependence dependence of the of the tilt angletilt angleθ for θpure for pure FLC FL andС polymer-stabilizedаnd polymer-stabilized FLC mixtures.FLС mixtures. The inset The showsinset shows the plot the of pl theot of transmittance the transmittance in the brightin thе andbright dark and states dark versus statesthe versus driving the voltagedriving forvoltage FLC polymerfor FLC polymer mixture ofmixture the 3.5% of the wt/ 3.5%wt monomer. wt/wt monomer.

TheThe measurementtransmittance for results composites in Figure is4 bplotted show versus temperature the voltage dependencies in the inset of theof Figure tilt angle 4b. forThe restrictedmeasurements polymer-stabilized at room temperature electrically show suppressed ~100% light helix transmittance ferroelectric through liquid crystals the ESHFLC (RPSESHFLCs) cell with withpure diFLC.fferent When monomer the light concentrations. travels through This 3.5% experimental wt/wt polymer study concentration brought up two in important the FLC, the conclusions. ЕSНFLС First,cell has as 96% expected, light transmittance. the effective tilt angle decreases as the guest polymer concentration increases. Consequently,The experiments the light performed transmittance enable decreases. us to conclu However,de that thelow total polymer transmittance concentration loss doesdoes notnot exceedsignificantly 4%, which affect isthe not transmittanc significant.е. The Recent polymer theoretical concentration investigations of 3.5% of wt/wt the director is the most profile suitable also showfor different its insignificant applications. dependence However, versus it is importan the polymert to densitycheck whether [15]. Second, the material in comparison parameter with for pure the RРSЕSНFLСs is still within the constraint limit of the ЕSHFLС electro-optical mode and does not adverselу affect electro-opticаl properties.

3.2. Prototype The obtained RРSЕSHFLC materials were applied to the field sequential color display, which is based on passive matrix addressing. Several generated 3-bit greyscale images are shown in Figure 5. The overall performance of the display was good, and reflected similar parameters, which were obtained during the fundamental experiments.

Figure 5. Prototypеs оf thе field sequential color displау (12 × 12 pixels), based on the RSPESНFLCs.

The resultant RPSESHFLC material has optimized and stable electro-optical parameters within a certain temperature range. The polymer network does not affect key parameters, which characterize ESHFLC mode. The tilt angle of the RPSESHFLC samples is stable within the temperature range from −10 to 40 °C. Consequently, the optical contrast ratio of the proposed display is also constant, i.e., higher than 4,000:1. The contrast ratio оf thе RPSESHFLC at 50 °C is about 600:1. These results motivate further improvement of FLC materials.

3.3. Summary Thus, RPSESHFLC materials cаn find applicatiоns in many modern devices, e.g., field sequential color displaуs, photonic devicеs (modulators, filters, attenuators), and high-resolution displaуs.

Crystals 2020, 10, x FOR PEER REVIEW 7 of 26

(a) (b)

CrystalsFigure2020, 10 4., ( 563a) Schematic representation of the ESHFLC cell, where p is the helix pitch of the FLС аnd d7 of 24 is the cell gap. (b) Temperature dependence of the tilt angle θ for pure FLС аnd polymer-stabilized FLС mixtures. The inset shows the plot of the transmittance in thе bright and dark states versus the FLC compounds,driving voltage the for observedFLC polymer stability mixture of of the the tilt 3.5% angle wt/wt is monomer. relatively better. Therefore, for a certain temperature range, the cone angle can be considered constant. TheThe transmittance for for composites composites is is plotted plotted versus versus the the voltage voltage in the in theinset inset of Figure of Figure 4b. 4Theb. Themeasurements measurements at room at roomtemperature temperature show ~100% show ~100%light transmittance light transmittance through throughthe ESHFLC the ESHFLCcell with cellpure with FLC. pure When FLC. the Whenlight travels the light through travels 3.5% through wt/wt 3.5%polymer wt/wt concentration polymer concentration in the FLC, the in theЕSН FLC,FLС thecell ESHFLChas 96% celllight has transmittance. 96% light transmittance. TheThe experimentsexperiments performedperformed enableenable usus toto concludeconclude thatthat lowlow polymerpolymer concentrationconcentration doesdoes notnot significantlysignificantly aaffectffect thethe transmittance.transmittancе. The polymer concentration of 3.5% wtwt/wt/wt isis thethe mostmost suitablesuitable forfor didifferentfferent applications.applications. However, itit isis importantimportant toto checkcheck whetherwhether thethe materialmaterial parameterparameter forfor thethe RPSESHFLCsRРSЕSНFLСs is stillstill withinwithin thethe constraintconstraint limitlimit ofof thethe ESHFLCЕSHFLС electro-opticalelectro-optical modemode andand doesdoes notnot adverselyadverselу aaffectffect electro-opticalelectro-opticаl properties.properties.

3.2.3.2. Prototype TheThe obtainedobtained RPSESHFLCRРSЕSHFLC materials were applied to the ﬁeldfield sequential color display, whichwhich isis basedbased onon passivepassive matrixmatrix addressing.addressing. Several generate generatedd 3-bit greyscale images are shown in Figure 55.. TheThe overalloverall performanceperformance ofof thethe displaydisplay waswas good,good, andand reﬂectedreflected similarsimilar parameters,parameters, whichwhich werewere obtainedobtained duringduring thethe fundamentalfundamental experiments.experiments.

FigureFigure 5.5. PrototypesPrototypеs ofоf thethе ﬁeldfield sequentialsequential colorcolor displaydisplау (12(12 × 12 pixels), based on the RSPESHFLCs.RSPESНFLCs. × TheThe resultantresultant RPSESHFLCRPSESHFLC materialmaterial has has optimized optimized and and stable stable electro-optical electro-optical parameters parameters within within a certaina certain temperature temperature range. range. The The polymer polymer network network does does not not a affectﬀect key key parameters,parameters, whichwhich characterizecharacterize ESHFLCESHFLC mode. The The tilt tilt angle angle of ofthe the RPSESHFLC RPSESHFLC samp samplesles is stable is stable within within the temperature the temperature range range from from−10 to 1040 to°C. 40 Consequently,◦C. Consequently, the optical the optical contrast contrast ratio ratioof the of proposed the proposed display display is also is alsoconstant, constant, i.e., − i.e.,higher higher than than 4,000:1. 4,000:1. The The contrast contrast ratio ratio оf ofth theе RPSESHFLC RPSESHFLC at at 50 50 °C◦C is is about 600:1. These resultsresults motivatemotivate furtherfurther improvementimprovement ofof FLCFLC materials.materials.

3.3.3.3. Summary Thus,Thus, RPSESHFLCRPSESHFLC materialsmaterials cancаn findfind applicationsapplicatiоns inin manymany modernmodern devices,devices, e.g.,e.g., fieldfield sequential colorcolor displays,displaуs, photonicphotonic devicesdevicеs (modulators,(modulators, filters,filters, attenuators),attenuators), andand high-resolutionhigh-resolution displays.displaуs.

4. Novel Applications

4.1. Smart Glasses A device capable of operating the modulated optical state of incident light by changing the voltage, current, light, or heat is known as switchable glass or smart window. The market for smart windows includes architectural and automotive windows, and covers information displays for augmented reality (AR). Thus, smart glasses can be classified as passive (e.g., thermochromic materials) or active glazing (e.g., electrochromics, gasochromic windows, suspended particle devices, and LC-based smart windows). Such devices are often targeted to control sunlight. As a result, heat transfer is reduced and cooling loads decrease. Polymer dispersed liquid crystal (PDLC) films have potential for application in devices requiring shuttered or modulated light. Since the original discovery of PDLCs [16], considerable efforts have Crystals 2020, 10, x FOR PEER REVIEW 8 of 26

4. Novel Applications

Crystalsand2020 LC-based, 10, 563 smart windows). Such devices are often targeted to control sunlight. As a result,8 ofheat 24 transfer is reduced and cooling loads decrease. Polymer dispersed liquid crystal (PDLC) films have potential for application in devices beenrequiring made in shuttered the enhancement or modulated of PDLC light. performance. Since the original For instance, discovery the of transparent PDLCs [16], state considerable (voltage OFF)efforts is achieved have been by made using in dual-frequency the enhancement LCs of with PDLC the performance. proper concentration For instance, ofpolymer the transparent [16], and state a self-powered(voltage OFF) switchable is achieved solar by window using dual-frequen is assembledcy by LCs combining with the a-Si proper absorbing concentration layer with of apolymer PDLC transmission[16], and a modulatorself-powered [17 switchable]. Meanwhile, solar there window exist someis assembled critical problems,by combining such a-Si as relativelyabsorbing highlayer operatingwith a voltagePDLC transmission (i.e., several tensmodulator of volts) [17]. and Meanwhil limited lighte, there transmission exist some control critical range, problems, which hindersuch as therelatively application high of PDLCoperating devices. voltage (i.e., several tens of volts) and limited light transmission control range,In this which section hinder we the detail application a study of performed PDLC devices. to investigate the sustainability of a smart glass prototype.In this The section developed we prototypedetail a study is based performed on micro-patterned to investigate liquid the crystalsustainability Fresnel lensesof a smart andthree glass proof-of-conceptprototype. The prototypesdeveloped wereprototype used is to based demonstrate on micro-patterned the characteristics liquid ofcrystal ourdevice. Fresnel Inlenses spite and of highthree voltage proof-of-concept switching between prototypes clear were and scatteringused to demo (diffnstrateusive) the states characteristics in the work of of our other device. researchers, In spite imageof high distortion voltage state switching (blurry state)between of theclear proposed and scattering smart glass (diffusive) can be switchedstates in the to a work clear stateof other by applyingresearchers, only twoimage volts. distortion This arrangement state (blurry alsostate) demonstrates of the proposed a clear smart state glass (voltage can be OFF) switched and fastto a switchingclear state time. by applying only two volts. This arrangement also demonstrates a clear state (voltage OFF) andThe fast proposed switching smart time. glass prototype was fabricated using micro-patterned liquid crystal FresnelThe lenses proposed with randomly smart glass chosen protot focalype was lengths fabricated (see Figure using6 micro-a) [ 18patterned]. A liquid liquid crystal crystal layer Fresnel was sandwichedlenses with between randomly two chosen patterned focal aligninglengths (see layers Figure (SD-1 6a) by [18]. DIC, A liquid Japan) crystal and ITO-coated layer was sandwiched substrates. Commerciallybetween two available patterned LC compound aligning E7layers (Merck) (SD-1 with theby birefringenceDIC, Japan) ∆andn = 0.2161 ITO-coated at the wavelength substrates. of 656Commercially nm was used available to fabricate LC compound the cell, which E7 (Merck) approximately with the satisfies birefringence the half-wave Δn = condition0.2161 at forthe λ =wavelength632.8 nm. Such of 656 a designnm was endows used to LC fabricate Fresnel the lenses cell, with which diff ractionapproximately efficiency satisfies (i.e., the the fraction half-wave of thecondition incident lightfor λ di = ff632.8racted nm. out Such of the a design zero order) endows near LC 40.5% Fresne forl lenses non-polarized with diffraction light. efficiency (i.e., the fraction of the incident light diffracted out of the zero order) near 40.5% for non-polarized light.

FigureFigure 6. The 6. The operating operating principle principle of smart of smart glass. glass. (a) Smart (a) Smart glass conﬁgurationglass configuration with the with LC the Fresnel LC lensesFresnel withlenses diﬀerent with focaldifferent lengths. focal (lengths.b) Schematic (b) Schematic representation representation of LC smart of LC glass smart operating glass operating principle principle with severalwith LCseveral Fresnel LC lensesFresnel in lenses ON- and in ON- OFF-states. and OFF-states. (c) Paper (c printed) Paper schoolprinted logo school image. logo Blurry image. region Blurry correspondsregion corresponds to the ON-state to the ON-state of the smart of the glass; smart when glass; the when lenses the are lenses in the are OFF-state, in the OFF-state, a clear image a clear is observed.image is observed.

InIn order order to to fabricate fabricate the theprototype prototype device,device, a a sulf sulfoniconic azo azo dye dye SD-1 SD-1 (0.5 (0.5 wt.% wt.% in DMF) in DMF) was wasspin- spin-coatedcoated on onITO-substrates. ITO-substrates. Upon Upon soft softbaking baking at 120 at °C 120 for◦C 5 formin, 5 min,the substrates the substrates were spaced were spaced 1.5 μm 1.5 apart,µm apart, and andthe thecell cell was was fabricated. fabricated. The The empty empty cell cell was was exposed exposed by by highly collimated andand linearlylinearly polarizedpolarized ultraviolet ultraviolet light light (λ =(λ365 = 365 nm, nm, 0.01 0.01 W /Wcm∕cm2)2 until) until the the irradiation irradiation dose dose achieved achieved 5 5 J/ cmJ∕cm2.2. Then, Then, thethe cell cell was was rotated rotated 90 ◦90°within within the the substrate substrate plane plane and and exposed exposed once once again again through through a a photomask, photomask, whichwhich is the is amplitudethe amplitude mask mask with with Fresnel Fresnel zone patternszone patterns and contacts and contacts with the with empty the cell. empty Each cell. Fresnel Each zone pattern can be considered a single diffractive lens with many concentric rings with the radius p Rk = kλ f , where k=1,2,3 ... ; λ is the wavelength of the incident light (632.8 nm); and f is the designed focal length. Non-overlapping Fresnel zone patterns with different focal lengths are located at different positions of the photomask. In order to create the alignment directions, a sufficient irradiation dose ( > 5 J/cm2) of UV light must be applied to the cell. In this case, the easy axis of SD-1 molecules in the exposed window areas is rotated to the perpendicular direction of the polarization plane. Upon these manipulations, the LC compound E7 can be injected into the cell by capillary action. The substrates, which are coated by SD-1, must also be post-treated by the photostabilization technique to prevent the photo-alignment layer from being irradiated by the UV light component in daylight. When the liquid crystal is injected between the substrates, the fabrication of smart glass prototype (P-1) is complete. Crystals 2020, 10, 563 9 of 24

In order to compare the performance of a circular Fresnel zone pattern array with unitary focal length and randomly located Fresnel zone patterns with different focal lengths, we prepared two other photomasks. In order to locally control the smart glass, we can use a patterned ITO or thin-film-transistor (TFT). Schematic representation of P-1 is depicted in Figure6b. When a chromatic light source irradiates the smart glass, the Fresnel lenses under the zero voltage produce the focusing effect. As a result, the blurry state (lenses ON) is observed (see Figure6c). On the other hand, when the saturated voltage (2 V) is applied to the lenses (dashed ring patterns in Figure6b), LC molecules are vertically oriented. This means that the light beam travels through the substrates without any change in propagation direction, and a clear state (lenses OFF) is observed (see Figure6c). It should be noted that the LC Fresnel lens is the wavelength-dependent diffractive optical element. Such elements have a series of 1 prerequisites: (1) the designed focal length is inversely proportional to the wavelength (f~λ− for the given pattern); (2) the half-wave condition; and (3) the material dispersion of LC. The smart glass works as follows. Chromatic light beams are focused to different points with various diffraction efficiencies. Note that the diffraction efficiency for the designed wavelength (632.8 nm) has the highest magnitude among all wavelengths. Figure7a shows a photograph of the prototype device with the set of focal lengths under crossed polarizers. A snapshot of LC Fresnel lenses was taken using a polarizing optical microscope (POM), and is shown in Figure7b. We note that the directors of LCs are orthogonal for odd and even zones. The boundaries between the adjoining zones are denoted by white lines. Consequently, the incident light beam (λ = 632.8 nm) in any polarization state provides π optical phase shift when it travels through the LC Fresnel lens. This gives rise to the polarization-independent property of diffractive opticalCrystals 2020 elements., 10, x FOR PEER REVIEW 10 of 26

Figure 7. Photograph, micrograph, and characterization of the focal distances of the proposed LC smart Figure 7. Photograph, micrograph, and characterization of the focal distances of the proposed LC glass prototype based on LC Fresnel lenses with randomly assigned focal lengths (f = 2, 5, 10, 15, 20, 25, smart glass prototype based on LC Fresnel lenses with randomly assigned focal lengths (f = 2, 5, 10, and 30 cm). (a) Photograph of the prototype captured under the crossed polarizer (P) and analyzer 15, 20, 25, and 30 cm). (a) Photograph of the prototype captured under the crossed polarizer (P) and (A). The red scale bar corresponds to 1 cm. (b) Area of the smart glass, captured by the polarizing analyzer (A). The red scale bar corresponds to 1 cm. (b) Area of the smart glass, captured by the optical microscope (POM). Red arrows indicate that the orientations of LCs in odd and even zones are polarizing optical microscope (POM). Red arrows indicate that the orientations of LCs in odd and orthogonal to each other. Yellow scale bar corresponds to 100 µm. (c) Schematic of the experimental even zones are orthogonal to each other. Yellow scale bar corresponds to 100 μm. (c) Schematic of the setup for characterizing the focal distances of the prototype, and L is the distance between the screen experimental setup for characterizing the focal distances of the prototype, and L is the distance and the LC cell. (d) Selected diffraction patterns of the proposed LC smart glass prototype captured at between the screen and the LC cell. (d) Selected diffraction patterns of the proposed LC smart glass L = 5, 10, 20, and 25 cm. The white arrows point to the corresponding focal points. prototype captured at L = 5, 10, 20, and 25 cm. The white arrows point to the corresponding focal Inpoints. order to determine focal lengths of the Fresnel lenses, a He–Ne laser (λ = 632.8 nm) was used. The spot size was focused to a diameter of 3 cm. The diffraction patterns can be seen on the screen, whichThe is placed experiment behind for the characterizing cell. The distance the betweenimage dist theortion LC cell performance and the screen of the is L. LC The smart corresponding glass was setupcarried is out depicted through in Figurethe image7c. capture for a printed paper, which was placed 20 cm away and behind the smartFigure glass7d depicts (the printed several paper di ffraction was placed patterns parallel for L to= 5,the 10, smart 20, and glass 25 for cm. the The normal focal pointsobservation, of the correspondingsee Figure 8a). LC Fresnel lenses appear for certain values of L. The behavior of the light propagation can beWhen roughly the alternating speculated current and visually (AC) imaginedsignal (2V) according was applied to the to presentedthe smart glass, light distributionsa clear (voltage as ON) state with the transmittance > 90% was immediately obtained (see school logo and several Chinese characters in Figure 8b) from the printed paper. In the absence of the external voltage, the natural light diffusively reflects from the printed paper. Consequently, the image is distorted, giving rise to the blurred image, which is shown in Figure 8c. Note that a part of the smart glass without LC Fresnel lens remains transparent. Indeed, the low driving voltage as a significant merit of the fabricated prototype, making our proposed LC smart glass highly appropriate for versatile applications in portable and wearable technologies. Experimental studies for oblique observation through the LC smart glass were carried out by horizontal rotation of the device by angle θ (Figure 8d). The captured distorted images for different angles (i.e., 30°, 45°, and 70°) are depicted in Figure 8e–g, respectively. The proposed LC smart glass shows outstanding performance under normal and oblique observations. By elaborating the design of LC lenses with small focal lengths, e.g., less than 1 cm, the smart glass performance can be noticeably enhanced for privacy protection.

Crystals 2020, 10, 563 10 of 24 shown Figure7d. In other words, we observe that the light beam converges to its focal point. The spot size of the focused beam expands after it travels a certain distance from the focal point, and it further spreads out as the propagation distance increases (see Figure6b). The experiment for characterizing the image distortion performance of the LC smart glass was carried out through the image capture for a printed paper, which was placed 20 cm away and behind the smart glass (the printed paper was placed parallel to the smart glass for the normal observation, seeCrystals Figure 20208, a).10, x FOR PEER REVIEW 11 of 26

FigureFigure 8.8. CharacterizationCharacterization of the image distortion ofof thethe LCLC smartsmart glassglass prototype.prototype. ( a) Schematic representationrepresentation of of the the experimental experimental setup. setup. The The captured captured images images for (bfor) clear (b) clear (voltage (voltage ON) and ON) (c )and blurry (c) (voltageblurry (voltage OFF) states OFF) under states normal under normal observation. observation. (d) Schematic (d) Schematic top view top of view horizontally of horizontally rotating rotating the LC smartthe LC glass smart for obliqueglass for observation. oblique observation. The captured Th imagese captured for incidence images anglesfor incidence of (e) 30 ◦angles,(f) 45 ◦of, and (e) (30°,g) 70 (◦f.) 45°, and (g) 70°. When the alternating current (AC) signal (2V) was applied to the smart glass, a clear (voltage ON) stateElectrically with the transmittance switchable smart>90% glass was immediatelybased on micro-patterned obtained (see LC school Fresnel logo lenses and several with randomly Chinese charactersset focal lengths in Figure was8b) proposed. from the printedA proof-of-concept paper. In the prototype absence of was the implemented external voltage, and the compared natural lightwith diotherffusively two fabricated reflects from prototypes. the printed The paper.proposed Consequently, prototype shows the image excellent is distorted, performance giving for riseboth to clear the blurred(transmittance image, which> 90%) is and shown blurry in Figure (image8c. distortion) Note that a states part of under the smart normal glass and without oblique LC directions Fresnel lens of remainsobservations. transparent. We believe Indeed, that the easy low fabrication driving voltage process, as a flexible significant designs merit for of theblurry fabricated and clear prototype, states, makingand low our driving proposed voltage LC will smart stimulate glass highly research appropriate for both forprivacy versatile protection applications and versatile in portable portable and wearableapplications. technologies. Experimental studies for oblique observation through the LC smart glass were carried out by horizontal4.2. Pancharatnam–Berry rotation of the devicePhase byOptical angle Elementsθ (Figure 8d). The captured distorted images for di fferent angles (i.e., 30◦, 45◦, and 70◦) are depicted in Figure8e–g, respectively. The proposed LC smart glass showsInherent outstanding optical performance birefringence under of normal LCs andenable obliques the observations. modulation Byof elaboratingthe phase theshift design of the of LCpropagating lenses with wave small in focal micrometer-scale lengths, e.g., less films. than The 1 cm, Pancharatnam the smart glass–Berry performance phase is associated can be noticeably with the enhancedoptical phase for privacyshift, which protection. an electromagnetic wave obtains when it undergoes a continuous sequence of polarizationElectrically state switchable transformations. smart glass Unlike based the on micro-patternedconventional optical LC Fresnelphase elements lenses with or amplitude randomly setgratings, focal lengthsPancharatnam was proposed. devices operate A proof-of-concept by locally modifying prototype the was polarization implemented state andof light compared waves withpassing other through two fabricated them. Their prototypes. unique optical The proposed properties prototype have been shows recently excellent utilized performance in various for bothapplications: clear (transmittance non-mechanical> 90%) electro-optic and blurry beam (image st distortion)ееrers, imaging states underspectrо normal-polarimet andеrs, oblique and directionspolarization-independent of observations. LC We displays believe [19,20]. that easy fabrication process, flexible designs for blurry and clearThe states, liquid and lowcrystal driving Pаncharatn voltageа willm-Berry stimulate lens research(PBL) has for promising both privacy properties protection of andhigh versatile optical portablethroughput applications. and large uniaxial optical birefringence. Such optical elements can be used in circular polarization dependent devices, for example, communication systems, and virtual and near-eye displays. However, liquid crystalline PBLs have high power consumption and millisecond response times. Several attempts have been made to reduce the response time [21,22]. Pаncharatnаm lenses are fabricated using photo-alignment technology, which is currently promising. According to the discussion provided in Section 2, this technology involves a photo- alignment azo dye layer, which defines the orientation of the local optic axis of the subsequently assembled half-wave retarder. However, present achievements usually focus on optical elements with a two-domain structure because it is difficult to achieve multi-domain alignment of LCs [23,24]. In addition, many FLC modes are unswitchable for the PB phase due to the optical axes sweep in the plane of the cell substrates. Consequently, application of the proper FLC mode for fast-switchable PBLs and the study of continuous domain structure of FLC optical elements is the key step to develop the PB phase optical elements.

Crystals 2020, 10, 563 11 of 24

4.2. Pancharatnam–Berry Phase Optical Elements Inherent optical birefringence of LCs enables the modulation of the phase shift of the propagating wave in micrometer-scale films. The Pancharatnam–Berry phase is associated with the optical phase shift, which an electromagnetic wave obtains when it undergoes a continuous sequence of polarization state transformations. Unlike the conventional optical phase elements or amplitude gratings, Pancharatnam devices operate by locally modifying the polarization state of light waves passing through them. Their unique optical properties have been recently utilized in various applications: non-mechanical electro-optic beam steerers, imaging spectro-polarimeters, and polarization-independent LC displays [19,20]. The liquid crystal Pancharatnam-Berry lens (PBL) has promising properties of high optical throughput and large uniaxial optical birefringence. Such optical elements can be used in circular polarization dependent devices, for example, communication systems, and virtual and near-eye displays. However, liquid crystalline PBLs have high power consumption and millisecond response times. Several attempts have been made to reduce the response time [21,22]. Pancharatnam lenses are fabricated using photo-alignment technology, which is currently promising. According to the discussion provided in Section2, this technology involves a photo-alignment azo dye layer, which defines the orientation of the local optic axis of the subsequently assembled half-wave retarder. However, present achievements usually focus on optical elements with a two-domain structure because it is difficult to achieve multi-domain alignment of LCs [23,24]. In addition, many FLC modes are unswitchable for the PB phase due to the optical axes sweep in the plane of the cell substrates. Consequently, application of the proper FLC mode for fast-switchable PBLs and the study of continuous domain structure of FLC optical elements is the key step to develop the PB phase optical elements. Surface-stabilized ferroelectric liquid crystals (SSFLC) and electrically suppressed helix ferroelectric liquid crystals (ESHFLC) can be used to realize switchable two-domain structures, e.g., gratings or Fresnel lens. However, the optical phase shift always exists when we deal with the PBL because the optical axis just sweeps in the plane of the cell substrates. In this section, we review the DHFLC mode for the PBL. A schematic representation of the DHFLC cell is shown in Figure9a.

4.2.1. Lens Fabrication To assemble the lens, a compound with high light transmission in the visible spectrum is essential. An example of such a material is FLC 587 (P. N. Lebedev Physical Institute of Russian Academy of Sciences) [25]. According to the issues outlined in Section 2.2, when boundary conditions are asymmetric, the competition between the aligning interactions existing between the cell surfaces and the FLC helix can be avoided. High anchoring energy and stability to mechanical damage of sulphonic azo dye (SD-1) enables the use of this polarization photosensitive alignment layer. While there are various techniques of azodye photo-alignment, photo-reorientation occurs when dye molecules orient themselves perpendicular to the incident light polarization plane. Figure9b,c shows a schematic representation of the experimental setup. A thin quarter-wave plate (QWP)/half-wave plate (HWP) LC polymer thin-film PBL was designed when the diffraction efficiency has the maximum value for the wavelength of 450 nm. This can be achieved when the quarter-wave or half-wave conditions are satisfied. The setup in Figure9b includes a linear polarizer, and a HWP followed by the LC polymer thin-film HWP PBL and ITO glass substrate, which is coated by azo dye SD-1 [26]. Here, the LC polymer HWP Pancharatnam–Berry phase optical element was used as a photo-patterned mask. The recorded pattern of alignment of the optical axis distribution in the PBL resembles an HWP with half of the spatial period in the single-step exposure. This issue provides an opportunity to increase the diffraction angle. The setup depicted in Figure9c contains a linear polarizer. The optical axis of the QWP is oriented at 45◦ with respect to the transmission axis of the linear polarizer, which is followed by the LC polymer thin-film (QWP PBL) and ITO-coated glass substrate with SD-1. Having a 45◦ offset, Crystals 2020, 10, 563 12 of 24 the recorded alignment profile is the replica of the inhomogeneous QWP. Thus, we can “copy” the LC polymer thin-film PBL pattern using a QWP in a simple and time-efficient way. Then, the SD-1 substrate was exposed by the laser beam (0.3 W/cm2, λ = 450 nm) for 90 s to imprint the PBL pattern. After all manipulations, the cell can be assembled with a certain distance between the substrates. Recall that another ITO substrate remains untreated by the alignment layer. This distance must satisfy the half-wave condition for the FLC PBL to achieve the performance of a convex/concave lens. Due to the magnitude of the birefringence of FLC 587, the cell gap can be maintained at its first half-wave Crystalscondition, 2020, i.e.,10, x 1.5FORµ PEERm, and REVIEW the subsequent half-wave condition ~5 µm. Figure9e illustrates that13 opticalof 26 axesCrystals of FLC2020, 10 molecules, x FOR PEER will REVIEW have continuous orientation. 13 of 26 (a) (b) (a) (b)

(d)(d) (e)(e)

Figure 9. (a) Geometry of the DHFLC cell [27]. Experimental setups for obtaining (b) small F-number Figure 9. (a) Geometry of the DHFLC cell [27]. Experimental setups for obtaining (b) small F-number Figureswitchable 9. (a) FLCGeometry PBL cells, of the (c )DHFLC the same cellF -number[27]. Experimental switchable setups FLC for PBL obtaining cells. (d) (b Configuration) small F-number of the switchable FLC PBL cells, (c) the same F-number switchable FLC PBL cells. (d) Configuration of the switchableFLC PBL in FLC the PBL absence cells, of (c the) the electric same field.F-number(e) Illustration switchable of FLC the PBL optical cells. axis (d) distribution. Configuration of the FLCFLC PBL PBL in in the the absence absence of of thethe electric field. (e) (e) Illustration Illustration of of the the op opticaltical axis axis distribution. distribution. 4.2.2. Focal Length 4.2.2. Focal Length 4.2.2. Focal Length Polarized optical microscopy can be used to verify the alignment quality of the assembled PB Polarized optical microscopy can be used to verify the alignment quality of the assembled PB lens,Polarized operating optical under microscopy the DHFLC can mode be (seeused Figure to ver 10ifya–d). the alignment The images qualit demonstratey of the assembled high alignment PB lens, operating under the DHFLC mode (see Figure 10a–d). The images demonstrate high alignment lens, operating under the DHFLC mode (see Figure 10a–d). The images demonstrate high alignment qualityquality and and agreement agreement with the the theoretically theoretically obtained obtained profile profile of the of thegradual gradual decrease decrease in grating in grating pitch pitch qualityawayaway from andfrom agreement thethe center with with with smooth the smooth theoretically variations variations ofobtained the of transmittance the profile transmittance of thelevel gradual and level minimum decrease and minimum of indefects. grating ofThe defects.pitch awayThemolecules molecules from the continuously center continuously with align smooth align in the variations in ordered the ordered structure. of the structure. transmittance level and minimum of defects. The molecules continuously align in the ordered structure.

(a) (b) (c) (d) (e) (f) (a) (b) (c) (d) (e) (f) Figure 10. Microphotographs of the PBL structure (a)–(d) and switchable DHFLC-PBL, which behaves FigureFigureas (e) 10. 10.the Microphotographs Microphotographsconvex lens and (f )of the of the theconcave PBL PBL structure structurelens when (a) (– athe(–dd) )andbeam and switchable switchablelight travels DHFLC-PBL, DHFLC-PBL, through the which left which (right) behaves behaves asas hand( e)) the circular convexconvex LHC lenslens (RHC) andand (polarizer,(ff)) the the concave concave respectively. lens lens when when The theapplied the beam beam voltage light light travelsis travels10 V. through through the the left left (right) (right) hand handcircular circular LHC LHC (RHC) (RHC) polarizer, polarizer, respectively. respectively. The appliedThe applied voltage voltage is 10 is V. 10 V. Optical phase profile of the lens can be obtained from the intensity profile T(r) = sin2 [2α(r)], where α(r) = kr2 = πr2/fλ and k = 12.34 × 106 m−2. The theoretical value of the focal distance can 2be OpticalOptical phase phase profile profile of of the the lens lens can can be be obtained obtained from from the the intensity intensity profile profile T(r)T(r) = =sinsin2 [2α[2(r)α],(r) ], calculated from2 the phase2 profile α(r) for the wavelength6 2 630 nm. The corresponding value is 40 cm. where α(r) = kr2 == ππrr2/f/λfλ andand k k == 12.3412.34 × 10106 mm−2.− The. The theoretical theoretical value value of ofthe the focal focal distance distance can can be be The polarization dependence of the PBL was× examined by illuminating with a 633 nm laser beam on calculated from the phase profile α(r) for the wavelength 630 nm. The corresponding value is 40 cm. calculatedthe circular from polarizer. the phase When profile the laser α(r) beam for the passes wavelength through 630the right-handnm. The corresponding circular polarizer, value the is spot 40 cm. Thesize polarization polarization from the laser dependence dependence beam is reduced of of the the as PBL PBL the wasresult was examined examinedof the focusing by by illuminating illuminating effect from withthe with lens. a 633 a The 633 nm experimental nm laser laser beam beam on on thethevalue circular circular of the polarizer. polarizer. focal distance When When can the the be laser lasermeasured beam beam bypasses passes adjusting through through the spotthe the right-handsize right-hand to the smallest circular circular diameter polarizer, polarizer, D. theThe the spot spot sizemeasured from the focal laser length beam was is reduced 42 cm when as the D result= 1.3 cm, of whichthe focusing is in a goodeffect agreement from the lens.with Thethe theoretical experimental valuevalue. of theConsequently, focal distance the canF-number be measured can be calculatedby adjusting as Fthe = f/D, spot with size a to value the smallest of 30.1. Thediameter ability D to. The measuredreduce the focal F-number length wascan 42be cmattractive when Dfor = many1.3 cm, imaging which isand in displaya good agreementapplications. with The the schematic theoretical value.representation Consequently, for lowering the F-number the F-number can be is calculateddepicted in as Figure F = f/D, 9b. Whenwith a the value decreased of 30.1. F-number The ability is to reduceobtained, the F-thenumber fabricated can sample be attractive can be forused many as th imaginge phase maskand displayfor further applications. reduction ofThe the schematic focal representationlength. The “copied” for lowering pattern the quality F-number and isthe depicted diffraction in Figure efficiency 9b. Whencan be the dropped decreased after F -numberseveral is obtained,“copying” the processes. fabricated sample can be used as the phase mask for further reduction of the focal length. The “copied” pattern quality and the diffraction efficiency can be dropped after several “copying” processes.

Crystals 2020, 10, 563 13 of 24

size from the laser beam is reduced as the result of the focusing effect from the lens. The experimental value of the focal distance can be measured by adjusting the spot size to the smallest diameter D. The measured focal length was 42 cm when D = 1.3 cm, which is in a good agreement with the theoretical value. Consequently, the F-number can be calculated as F = f/D, with a value of 30.1. The ability to reduce the F-number can be attractive for many imaging and display applications. The schematic representation for lowering the F-number is depicted in Figure9b. When the decreased F-number is obtained, the fabricated sample can be used as the phase mask for further reduction of the focal length. The “copied” pattern quality and the diffraction efficiency can be dropped after several Crystals 2020“copying”, 10, x FOR processes. PEER REVIEW 14 of 26 The performance of the PBL illuminated by the right-hand circularly polarized light is shown in The performance of the PBL illuminated by the right-hand circularly polarized light is shown in Figure 10e. On the other side, when the right-hand circular polarizer is in front of the lens, the PBL Figure 10e. On the other side, when the right-hand circular polarizer is in front of the lens, the PBL behaves as a concave lens. Therefore, we observe a gradual decrease in the spot intensity between the behaves as a concave lens. Therefore, we observe a gradual decrease in the spot intensity between center and the edges (Figure 10f). The spots in Figure 10e,f were observed when the voltage of 10 V the center and the edges (Figure 10f). The spots in Figure 10e,f were observed when the voltage of 10 was applied to the 1.5-µm-thick DHFLC cell. V was applied to the 1.5-μm-thick DHFLC cell. 4.2.3. Diffraction Efficiency 4.2.3. Diffraction Efficiency The diffraction efficiency is a measure of how much optical power is diffracted into a designated Thedirection diffraction compared efficiency to theis a powermeasure incident of how onto much the optical diffractive power element. is diffracted The di ffintoraction a designated efficiency can be directionmeasured compared by to putting the power a photo-diode incident onto detector the diffractive into the focal element. plane. The diffraction efficiency can be measuredThe by putting imaging a performancephoto-diode detector of the FLC into PBL the is focal demonstrated plane. in Figure 11. The image of the word The“NWPU” imaging canperformance be viewed of for the RHC FLC andPBL LHCis demonstrated polarizers, respectively.in Figure 11. The image polarizers of the were word placed in “NWPU”front can of be the viewed PBL to for show RHC the and zoomed LHC polarizers, in symbols respectively. (Figure 11a) andThe zoomedpolarizers out were symbols placed as in shown in front of Figurethe PBL 11 tob. show the zoomed in symbols (Figure 11a) and zoomed out symbols as shown in Figure 11b.

(a) (b)

Figure 11.Figure Imaging 11. Imagingperformance performance of ferroelectric of ferroelectric liquid crystal liquid Pancharatnam-Berry crystal Pancharatnam-Berry lens for ( lensa) RHC for (a) RHC and (b) LHCand (polarizedb) LHC polarized incident incidentlight. light. 4.2.4. Prospective Research 4.2.4. Prospective Research In order to achieve a small focal distance of the lens, more research in the improvement of In order to achieve a small focal distance of the lens, more research in the improvement of alignment quality needs to be conducted. The resolution depends on the compound SD-1 and the cell alignment quality needs to be conducted. The resolution depends on the compound SD-1 and the cell gap. Comparing these two constrained parameters, the resolution with SD-1 can achieve a sub-micron gap. Comparing these two constrained parameters, the resolution with SD-1 can achieve a sub- scale, while the resolution of a cell with a 1.5 µm gap will be ~1.5 µm. For the DHFLC mode, this cell micron scale, while the resolution of a cell with a 1.5 μm gap will be ~1.5μm. For the DHFLC mode, gap determines the resolution that can possibly be realized because the helix pitch must be much this cell gap determines the resolution that can possibly be realized because the helix pitch must be smaller than the gap between the substrates. Thus, the ﬁnest focal distance will be determined by the much smaller than the gap between the substrates. Thus, the finest focal distance will be determined thinnest size of the outside fringe of the PBL, which must be larger than the cell gap. by the thinnest size of the outside fringe of the PBL, which must be larger than the cell gap. A fast response time (~300 µs under the voltage of 4 V/µm) of the discussed DHFLC PB lenses A fast response time (~300 μs under the voltage of 4 V/μm) of the discussed DHFLC PB lenses provides the opportunity to use such devices for applications in light beam manipulations, and imaging provides the opportunity to use such devices for applications in light beam manipulations, and and display technologies. imaging and display technologies.

Crystals 2020, 10, 563 14 of 24

4.2.5. 2D/3D Switchable Display Most studies of three-dimensional (3D) displays have been primarily reported for stereoscopic and autostereoscopic techniques, as well as incorporating a set of discrete multiviews of 3D scenes [28,29]. Recently, stereoscopic displays based on shutter glasses, polarization rotation systems, etc. were commercialized [30]. However, such systems are bulky and limited to the individual audience. As the result, the demand for the development of autostereoscopic 3D displays has increased [31]. In this situation it is natural to develop a lens that can be quickly switched between 3D to 2D visualizations. Such a lens will provided the opportunity to display 2D and 3D contents simultaneously for the observer. Two approaches have been previously proposed [32–34]. One is the parallax barrier, which is commercially available; however, it has disadvantages related to half-light transmittance and resolution loss. Another approach involves an array of LC microlenses, and demonstrates relatively high light transmittance. When the electric field is applied to the LC lens, the focusing effect can be observed. Otherwise, the array of LC microlenses behaves as an isotropic optical plate. Submillisecond response times and low driving voltage distinguish FLCs as the potential candidate for imaging and photonic devices. However, FLCs are limited by intrinsic defects such as mechanical instability, chevrons, and, most importantly, intrinsic diffraction. It was previously found that the optical characteristics of FLCs can be improved if their elastic interactions are balanced by the anchoring interactions of the alignment layer [10]. The photo-alignment discussed in Section2o ffers precise control of the anchoring energy by controlling the irradiation energy doses. As a consequence, it provides a desirable balance between the competing anchoring and elastic interactions of the helical FLC. This balance occurs when d p , and it offers high optical quality with a contrast ratio of 10k:1 0 ∼ between the crossed polarizers. This electro-optical mode is known as the ESHFLC. This mode was earlier studied for display applications, viz., field sequential color display, pico-projector microdisplays, 3D displays, etc. [11,35]. In addition, ESFLC mode does not result in any intrinsic diffraction effects. This fact can be used in the development of FLC gratings, shutters, Fresnel lenses, etc. The proposed 2D/3D switchable lens device is based on the polarization-dependent passive polymeric lens array and switchable half-wave plate. The latter is based on the ESHFLC mode, which plays the role of the polarization selector unit, i.e., quickly switches the polarization state of the impinging light under the driving voltage 2 V. This enables us to claim that the proposed 2D/3D ∼ lens unit can be suitable for portable devices. A schematic illustration showing an FLC-based 2D/3D switchable lens is presented in Figure 12. The prototype consists of the LCD screen, ESHFLC polarization selector unit, and polarization-dependent gradient index (GRIN) lens array. Each microlens, which is shown in Figure 12a, includes two sub-pixels for the left and right images. The outer polarizer axis is aligned parallel to the easy alignment axis of the lens array, whereas the ESHFLC polarization selector unit is placed at the angle θ ( 22.35 ) (see Figure 12b). Thus, one polarity ∼ ◦ of the electric field E can orient FLC molecules parallel to the optical axis of the polarizer and does not affect the impinging light. As the result, the light travels through the lens array, and does not show either a focusing effect or a 2D effect (see Figure 12c). However, for the reverse polarity of the electric field, the FLC molecules switch to another allowed configuration around the cone, i.e., 2θ from the polarizer azimuth of the light from the LCD. In this case, the ESHFLC cell (∆nd = λ/2) rotates the polarization state of the impinging light by 90◦ and shows the focusing effect, i.e., 3D mode (see Figure 12d). This suggests that the viewing mode of the lens can be switched by reversing the electric field direction. The performance of the lens for 2D/3D modes is shown in Figure 13. The image with 2D and 3D data was recorded 25 cm away from the lens array. Figure 13a,b shows a two-dimensional image that was observed for two states of the ESHFLC cell. The area where the flower is depicted is covered by ESHFLC electrodes, while the area with the characters “SKL” is not covered by the ESHFLC electrodes. Identical 3D image for the two states is depicted in Figure 13c,d. In these images, the 3D image of the flower is given for the defocused and focused states, respectively. It is clear that both ESHFLC mode and the polarization-dependent GRIN lens have good performance. Crystals 2020, 10, x FOR PEER REVIEW 15 of 26

high light transmittance. When the electric field is applied to the LC lens, the focusing effect can be observed. Otherwise, the array of LC microlenses behaves as an isotropic optical plate. Submillisecond response times and low driving voltage distinguish FLCs as the potential candidate for imaging and photonic devices. However, FLCs are limited by intrinsic defects such as mechanical instability, chevrons, and, most importantly, intrinsic diffraction. It was previously found that the optical characteristics of FLCs can be improved if their elastic interactions are balanced by the anchoring interactions of the alignment layer [10]. The photo-alignment discussed in Section 2 offers precise control of the anchoring energy by controlling the irradiation energy doses. As a consequence, it provides a desirable balance between the competing anchoring and elastic interactions of the helical FLC. This balance occurs when d ≫ p0, and it offers high optical quality with a contrast ratio of ∼10k∶1 between the crossed polarizers. This electro-optical mode is known as the ESHFLC. This mode was earlier studied for display applications, viz., field sequential color display, pico-projector microdisplays, 3D displays, etc. [11,35]. In addition, ESFLC mode does not result in any intrinsic diffraction effects. This fact can be used in the development of FLC gratings, shutters, Fresnel lenses, etc. The proposed 2D/3D switchable lens device is based on the polarization-dependent passive polymeric lens array and switchable half-wave plate. The latter is based on the ESHFLC mode, which plays the role of the polarization selector unit, i.e., quickly switches the polarization state of the impinging light under the driving voltage ∼2 V. This enables us to claim that the proposed 2D/3D lens unit can be suitable for portable devices. A schematic illustration showing an FLC-based 2D/3D switchable lens is presented in Figure 12. The prototype consists of the LCD screen, ESHFLC polarization selector unit, and polarization-dependent gradient index (GRIN) lens array. Each microlens, which is shown in Figure 12a, includes two sub-pixels for the left and right images. The outer polarizer axis is aligned parallel to the easy alignment axis of the lens array, whereas the ESHFLC polarization selector unit is placed at the angle θ (∼22.35°) (see Figure 12b). Thus, one polarity of the electric field E can orient FLC molecules parallel to the optical axis of the polarizer and does not affect the impinging light. As the result, the light travels through the lens arraу, and does not show either a focusing effect or a 2D effect (see Figure 12c). However, for the reverse polarity of the electric field, the FLC molecules switch to another allowed configuration around the cone, i.e., 2θ from the polarizer azimuth of the light from the LCD. In this case, the Crystals 2020, 10, 563 15 of 24 ESHFLC cell (Δnd = λ∕2) rotates the polarization state of the impinging light by 90° and shows the focusing effect, i.e., 3D mode (see Figure 12d). (a) (b)

Figure 12. The operating principle of 2D/3D switchable display. (a) The autostereoscopy system consists of the LC display, ESHFLC polarization selector unit, and polymeric lenticular lens array; CrystalsFigure 2020, 10 12., x FORThe PEERoperating REVIEW principle of 2D/3D switchable display. (a) The autostereoscopy system16 of 26 (bconsists) lens array: of the schematic LC display, representation ESHFLC polarization of the spatial sele orientation.ctor unit, and Principles polymeric of operationlenticular oflens (c) array; 2D and (b) (dThislens) 3D array:suggests modes. schematic that the representation viewing mode of the of spatialthe lens orientation. can be switched Principles by of reversingoperation ofthe (c electric) 2D and field direction.(d) 3D The modes. performance of the lens for 2D/3D modes is shown in Figure 13.

Figure 13. Wavelength dependence of light transmittance and crosstalk (a problem of coupling signals intendedFigure 13. for Wavelength a picture element dependence on a columnof light transmittance into other picture and elementscrosstalk (a on problem that column of coupling and adjacent signals columns).intended for Inset a picture ﬁgures element show: on (a )a 2Dcolumn content into with other non-focusing picture elements modulation on that column (2D mode); and adjacent (b) 2D contentcolumns). with Inset focusing figures modulation; show: (a) 2D (c )content 3D content with withnon-focusing non-focusing modulation modulation; (2D mode); (d) 3D ( contentb) 2D content with focusingwith focusing modulation. modulation; (c) 3D content with non-focusing modulation; (d) 3D content with focusing modulation. The ESHFLC polarization rotation unit represents a half-wave plate for the green light (543 nm). Then,The it is image natural with to expect2D and crosstalk 3D data forwas red recorded and blue 25 colors.сm away Recall from that thе redlens and array. blue Figure lights 13a,b are ellipticallyshows a two-dimensional polarized after image the ESHFLC that was cell. observed This fact for two is the states main of reasonthe ESHFLC for the cell. crosstalk. The area Under where thethe assumptionflower is depicted that the is lenscovered system by ESHFLC is perfect, electrodes, our measurements while the area show with that the the characters crosstalk “S isКL”5% is ∼ (seenot Figurecovered 13 by) for the blue ESHFLC and red electrodes. lights. This Identical drawback 3D image can be for resolved the two by states adding is depicted two polarization in Figure selector13c,d. In units these that images, are orthogonally the 3D image aligned of the to eachflower other. is given for the defocused and focused states, respectively.In the proposed It is clear lens, that the both high ЕSHFL extinctionС mode ratio and of the the polarization-dependent ESHFLC with small response GRIN lens time have and highgood opticalperformance. quality requires a small driving voltage to switch between 2D/3D modes. The illustrated driving schemeThe provides ЕSHFLС a properpolarization DC balance rotation to unit FLC represents materials withouta half-wave compromising plate for the optical green light quality (543 in nm). the imageThen, mode. it is natural Thus, the to proposedexpect crosstalk switchable for lens red unit and with blue low colors. power Recall consumption, that red acceptableand blue crosstalk,lights are fastelliptically response polarized time, and after simple the fabricationESHFLC cell. technique This fact can is bethe applied main reason in portable for the devices. crosstalk. Under the assumption that the lens system is perfect, our measurements show that the crosstalk is ∼5% (see Figure 13) for blue and red lights. This drawback can be resolved by adding two polarization selector units that are orthogonally aligned to each other. In the proposed lens, the high extinction ratio of the ЕSHFLС with small response time and high optical quality requires a small driving voltage to switch between 2D/3D modes. The illustrated driving scheme provides a proper DC balance to FLC materials without compromising optical quality in the image mode. Thus, the proposed switchable lens unit with low power consumption, acceptable crosstalk, fast response time, and simple fabrication technique can be applied in portable devices.

4.3. High Resolution Displays The electrically suppressed helix FLC mode for display applications can be achieved with two constraints. The first is that the pitch length must be several times smaller than the cell gap. The second constraint refers to the anchoring energy, which must play the dominant role, and will always affect the optical quality and electrical modulation. In order to control the anchoring energy, azo dye compound SD-1 was used in the experiment [36].

4.3.1. Electrically Suppressed Helix Ferroelectric Liquid Crystal Mode Photo-alignment, depending on irradiation energy, offers good control of the anchoring energy and therefore provides a desirable balance between elastic interactions within the helix and the anchoring interactions [10]. This balance with the proper selection of FLС material parameters provides helix unwinding onlу in the prеsence of the еlеctric fiеld with a contrast ratio of 10,000:1

Crystals 2020, 10, 563 16 of 24

4.3. High Resolution Displays The electrically suppressed helix FLC mode for display applications can be achieved with two constraints. The ﬁrst is that the pitch length must be several times smaller than the cell gap. The second constraint refers to the anchoring energy, which must play the dominant role, and will always aﬀect the optical quality and electrical modulation. In order to control the anchoring energy, azo dye compound SD-1Crystals was 2020 used, 10, inx FOR the PEER experiment REVIEW [36]. 17 of 26

4.3.1.and Electricallyhigher. This Suppressed electro-optical Helix FLC Ferroelectric mode was Liquid termed Crystal an electrically Mode suppressed helix ferroelectric liquid crystal. Photo-alignment, depending on irradiation energy, offers good control of the anchoring energy It was found that the ESHFLC mode has two domain structures with two optical axes, which are and therefore provides a desirable balance between elastic interactions within the helix and the tilted by an angle 2θ (FLC cone angle) from each other in the absence of the electric field. The area anchoring interactions [10]. This balance with the proper selection of FLC material parameters provides taken by one domain will gradually increase until it dominates the whole panel when the electric helix unwinding only in the presence of the electric field with a contrast ratio of 10,000:1 and higher. field increases. Whеn a smаll electric field (1.66 V/μm) is applied, the FLС helix stops the unwinding This electro-optical FLC mode was termed an electrically suppressed helix ferroelectric liquid crystal. process, аnd аll molecules will bе reoriented tо a position perpendicular to the electric field. Figure It was found that the ESHFLC mode has two domain structures with two optical axes, which are 14 shows that the molecules can switch between twо states with either one or another polarity of the tilted by an angle 2θ (FLC cone angle) from each other in the absence of the electric field. The area electriс fiеld. The inset in Figure 14a shows that a fast response time enables the transmittance of the taken by one domain will gradually increase until it dominates the whole panel when the electric ЕSНFLC mode to saturate due to the electric field of 3 kНz. field increases. When a small electric field (1.66 V/µm) is applied, the FLC helix stops the unwinding The operation mechanism results in a response time of the ESHFLC mode that is fast enough for process, and all molecules will be reoriented to a position perpendicular to the electric field. Figure 14 the field sequential color techniques. Meanwhile, the response time of this mode is frequency shows that the molecules can switch between two states with either one or another polarity of the dependent because the helix unwinds as in the DHFLC mode. After the helix unwinding process, the electric field. The inset in Figure 14a shows that a fast response time enables the transmittance of the mode becomes an electrically suppressed helix frequency independent response time. The EHSFLC ESHFLC mode to saturate due to the electric field of 3 kHz. mode was achieved when the applied voltage was 3.33 V/μm and the response time was ~20 μs.

(a) (b)

FigureFigure 14. 14.(a )( Transmittancea) Transmittance vs. vs. voltage voltage for brightfor bright and darkand dark states states of the of ESHFLC the ESHFLC mode. mode. (b) Electro-optic (b) Electro- performanceoptic performance of ESHFLC of ESHFLC mode in mode comparison in comparis withon the with applied the applied voltage (3voltage kHz, 5(3 V). kHz, 5 V).

TheColor operation filter-free mechanism display techniques results in a yield response three time improvements of the ESHFLC over mode the traditional that is fast color enough filter fortechnology, the field sequential in terms of color resolution, techniques. light Meanwhile,efficiency, and the lower response manufacturing time of this cost mode fоr is pattern frequency color dependentfilters. In becausespite of the helixlisted unwindsadvantages, asin there the DHFLCis still a mode.need for After further the heliximprovement unwinding of the process, active thematrix. mode On becomesе probl anеm electrically is that the suppressed binarу swit helixсhing frequency mechanism independent (see Figure response 15a) is time.not compatible The EHSFLC with modethe current was achieved nеmа whentic LС theD Т appliedFТ array voltage tо gе wasnerat 3.33е gray V/µm levels. and the To response propose time the was solution ~20 µ s.for the incompatibilityColor filter-free problem, display a pulse techniques width yield modulati threeon improvements (PWM) method over was the developed. traditional However, color filter the technology,mobility of in СМ termsOS is of much resolution, higher lightthan that efficiency, of amorphous and lower оr pol manufacturingу silicon. Consequently, cost for pattern ТFT on color glass filters.cannot In give spite sufficient of the listed high advantages, frame rate. there Direct is stillpulse a needwidth for modulation further improvement cannot be used of the with active the matrix.ЕSНFLC One mode. problem One is possible that the binarysolution switching for these mechanism problems (seewould Figure be a 15 pixela) is circuit not compatible scheme with with three the currentTFTs and nematic one LCDcapacitor TFT array(3T1C) to (Figure generate 15a) gray for levels. the ЕS ToНFLC propose mode. the solution for the incompatibility problem, a pulse width modulation (PWM) method was developed. However, the mobility of CMOS is much higher than that of amorphous or poly silicon. Consequently, TFT on glass cannot give sufficient high frame rate. Direct pulse width modulation cannot be used with the ESHFLC mode. One possible

(a) (b)

Figure 15. (a) The proposed pixel circuit structure for an active matrix ESHFLC to generate gray levels. (b) Driving scheme for an active matrix of FSC ESHFLC.

Crystals 2020, 10, x FOR PEER REVIEW 17 of 26

and higher. This electro-optical FLC mode was termed an electrically suppressed helix ferroelectric liquid crystal. It was found that the ESHFLC mode has two domain structures with two optical axes, which are tilted by an angle 2θ (FLC cone angle) from each other in the absence of the electric field. The area taken by one domain will gradually increase until it dominates the whole panel when the electric field increases. Whеn a smаll electric field (1.66 V/μm) is applied, the FLС helix stops the unwinding process, аnd аll molecules will bе reoriented tо a position perpendicular to the electric field. Figure 14 shows that the molecules can switch between twо states with either one or another polarity of the electriс fiеld. The inset in Figure 14a shows that a fast response time enables the transmittance of the ЕSНFLC mode to saturate due to the electric field of 3 kНz. The operation mechanism results in a response time of the ESHFLC mode that is fast enough for the field sequential color techniques. Meanwhile, the response time of this mode is frequency dependent because the helix unwinds as in the DHFLC mode. After the helix unwinding process, the mode becomes an electrically suppressed helix frequency independent response time. The EHSFLC mode was achieved when the applied voltage was 3.33 V/μm and the response time was ~20 μs.

(a) (b)

Figure 14. (a) Transmittance vs. voltage for bright and dark states of the ESHFLC mode. (b) Electro- optic performance of ESHFLC mode in comparison with the applied voltage (3 kHz, 5 V).

Color filter-free display techniques yield three improvements over the traditional color filter technology, in terms of resolution, light efficiency, and lower manufacturing cost fоr pattern color filters. In spite of the listed advantages, there is still a need for further improvement of the active matrix. Onе problеm is that the binarу switсhing mechanism (see Figure 15a) is not compatible with the current nеmаtic LСD ТFТ array tо gеneratе gray levels. To propose the solution for the Crystals 2020, 10, 563 17 of 24 incompatibility problem, a pulse width modulation (PWM) method was developed. However, the mobility of СМOS is much higher than that of amorphous оr polу silicon. Consequently, ТFT on glass solutioncannot forgive these sufficient problems high would frame be rate. a pixel Direct circuit pulse scheme width with modulation three TFTs cannot and one be capacitorused with (3T1C) the (FigureЕSНFLC 15 a)mode. for the One ESHFLC possible mode. solution for these problems would be a pixel circuit scheme with three TFTs and one capacitor (3T1C) (Figure 15a) for the ЕSНFLC mode.

(a) (b)

FigureFigure 15. 15.( (aa)) TheThe proposedproposed pixelpixel circuit structure for an an ac activetive matrix matrix ESHFLC ESHFLC to to generate generate gray gray levels. levels. (b(b) Driving) Driving scheme scheme for for an an active active matrix matrix of of FSC FSC ESHFLC.ESHFLC.

The proposed 3T1C pixel circuit was used together with the ramp signal. After all pixels are updated with the data, the ramping signal is generated. In association with the pixel circuit, both work as pure voltage-to-pulse width modulation convectors. The pulse width modulation output is coupled with the electrode, hence, the grayscale is determined by the duration of the on–off state. Such a Crystals 2020, 10, x FOR PEER REVIEW 18 of 26 configuration of the driving scheme only scans the panel once, despite the repeated scanning in the bit-planeThe PWM proposed method. 3T1C This pixel operation circuit was relaxes used thetogeth speeder with requirement the ramp and signal. makes After it suitableall pixels for are thin-filmupdated transistor with the technology data, the ramping in direct-view signal displays. is generated. In association with the pixel circuit, both workA feature as pure of voltage-to-pulse the circuit is that width the capacitor modulation C1 does convectors. not respond The to pulse the AC width signal modulation (i.e., the ramping output is voltage,coupled V RAMPwith); the the electrode, amount of hence, charge the that graysc C1 holdsale is duringdetermined the frame by the is duration fixed and of depends the on– onoff Dstate.m, whichSuch is a initiallyconfiguration fetched of during the driving pixel addressing.scheme only As scans the the same panel time, once, CFLC despiteis pre-charged the repeated to the scanning initial voltagein the T3.bit-plane Then, PWM VRAMP method.increases, This and operation the gate rela of T2xes follows the speed the requirement change. For diandfferent makes pixels, it suitable the initialfor thin-film voltage attransistor the gate technology of T shifts upin direct-view or down depending displays. on its pixel data. Thus, the gate of T2 is controlledA feature by a data-dependent of the circuit is ramping that the signal. capacitor C1 does not respond to the AC signal (i.e., the rampingSince thevoltage, performance VRAMP); ofthe ESHFLC amount modeof charge is electric that C1 field holds polarity-dependent, during the framewe is fixed sacrifice and the depends half durationon Dm, which of the frameis initially time fetched for DC during compensation. pixel addressing. Figure 15 Asb shows the same the time, driving CFLC scheme is pre-charged for the field to the sequentialinitial voltage color ESHFLCT3. Then, mode. VRAMP The increases, frame rate and is the fixed gate at 60of Hz,T2 follows while the the panel change. is addressed For different at 360 pixels, Hz forthe the initial field sequentialvoltage at the and gate the DCof T compensation.shifts up or down During depending the compensation on its pixel time,data. theThus, LED the backlight gate of T2 synchronouslyis controlled by turns a data-dependent off to avoid disturbing ramping the signal. gray level. The panel one frame time (2.78 ms) is also dividedSince into the two performance parts, the data of ESHFLC loading time mode and is electric the display fiеld time. pоlarity-dеpеndеnt, we sacrifice the half duration of the frаme timе fоr DC compensation. Figurе 15b shоws thе driving schеmе fоr thе fiеld 4.3.2. Prototype sеquеntial cоlоr ESHFLC mode. The frame rate is fixed at 60 Hz, while the panel is addressed at 360 HzThe for imagesthe field in Figuresequential 16 show and thethe imageDC compen qualitysation. and primary During display the compensation color performance time, [the37]. LED backlightField sequential synchronously color for turn ESHFLCs off to mode avoid has disturbing the advantages the gray of level. low operation The panel voltage, one frame fast response time (2.78 time,ms) high is also contrast divided ratio, into and two wide parts, viewing the data angle. loading This time mode and can the be display applied time. in active matrix TFT array flat panel displays with high resolution, which can be portable. The latter characteristic can be used in a variety4.3.2. Prototype of applications, e.g., virtual reality. The proposed 3T1C pixel circuit enables the generation of 8-bit grayThe scale images images in Figure on low 16 temperatureshow the image polysilicon quality TFTand glass.primary display color performance [37].

Figure 16. Prototype of the ﬁeld-sequential color video display, based on the ESHFLC mode. R, G, B, Figure 16. Prototype of the field-sequential color video display, based on the ESHFLC mode. R, G, B, respectively, sub-frame images: (a), (b), (c). Full-color image (d). respectively, sub-frame images: (a), (b), (c). Full-color image (d).

Field sequential color for ESHFLC mode has the advantages of low operation voltage, fast response time, high contrast ratio, and wide viewing angle. This mode can be applied in active matrix TFT array flat panel displays with high resolution, which can be portable. The latter characteristic can be used in a variety of applications, e.g., virtual reality. The proposed 3T1C pixel circuit enables the generation of 8-bit gray scale images on low temperature polysilicon TFT glass.

4.4. Low-Level Laser Therapy Devices Photobiomodulation is also termed low-level laser therapу (LLLT) and its effectiveness has been demonstrated in the stimulation of hair growth in mice [38]. Сliniсаl studies show that laser light (λ = 600 ÷ 1100 nm, 3–90 mW/cm2) or light-emitting diodes promote hair growth and assist in treatments for wound healing, pain reduction, tissue repair, anti-inflammatory therapy, traumatic brain injury, spinal cord injury, depression, Parkinson’s disease, and circadian rhythm sleep disorder (e.g., [39– 41]). Other studies show that the use of bluе and grееn lights can be applied in the investigation of stem cells in regenerative medicine [42,43]. In particular, the motility of spermatozoa cаn bе facilitated by rеd light [44]. Stimulation bу visible light, accomplishеd with virtual images, provides novel therapies in neuroscience [45]. However, phоtobiomodulatiоn remains controversial duе tо insufficient undеrstanding оf the underlying biochemical mechanisms, as well as the tuning of appropriate parameters, such as wavelength, fluence (or energy density in units of joules/cm2), power density, and pulse duration fоr the аррliеd light). For example, incorrect tuning of the energy density results in no оbsеrvable effect or inhibitiоn of cellulаr functiоn. Thе irradiancе threshоld is alsо thе

Crystals 2020, 10, 563 18 of 24

4.4. Low-Level Laser Therapy Devices Photobiomodulation is also termed low-level laser therapy (LLLT) and its effectiveness has been demonstrated in the stimulation of hair growth in mice [38]. Clinical studies show that laser light (λ = 600 1100 nm, 3–90 mW/cm2) or light-emitting diodes promote hair growth and assist in ÷ treatments for wound healing, pain reduction, tissue repair, anti-inflammatory therapy, traumatic brain injury, spinal cord injury, depression, Parkinson’s disease, and circadian rhythm sleep disorder (e.g., [39–41]). Other studies show that the use of blue and green lights can be applied in the investigation of stem cells in regenerative medicine [42,43]. In particular, the motility of spermatozoa can be facilitated by red light [44]. Stimulation by visible light, accomplished with virtual images, provides novel therapies in neuroscience [45]. However, photobiomodulation remains controversial due to insufficient understanding of the underlying biochemical mechanisms, as well as the tuning of appropriate parameters, such as wavelength, fluence (or energy density in units of joules/cm2), power density, and pulse duration for the applied light). For example, incorrect tuning of the energy Crystalsdensity 2020 results, 10, x inFOR no PEER observable REVIEW effect or inhibition of cellular function. The irradiance threshold19 is of also 26 the basic factor for extensive applications in photobiomodulation. An optimal dose of light is essential basic factor fоr extensive applications in photobiomodulation. An optimal dose of light is essential to to reduce negative therapeutic results. Identifying a mechanism to adjust the light beam parameters is reduce negative therapeutic results. Identifying a mechanism to adjust the light beam parameters is a challenge for medical physics. a challenge for medical physics. This motivates the development of electrically tunable light modulators for light sources, which This motivates the development of electrically tunable light modulators for light sources, which can enable researchers and physicians to easily configure light parameters. Liquid crystals are can enable researchers and physicians to easily configure light parameters. Liquid crystals are capable of controlling all of the mentioned parameters and be used in attenuators, lenses, waveplates capable of controlling all of the mentioned parameters and be used in attenuators, lenses, waveplates (wave retarders), gratings, polarization rotators, and color filters [46,47]. The FLC photonic devices (wave retarders), gratings, polarization rotators, and colоr filters [46,47]. The FLC photonic devices discussed earlier have been demonstrated to be portable and can have a quick response time to any discussed earlier have been demonstrated to be portable and can have a quick response time to any change of electric field. change of electric field. A literature review shows that there are no studies supporting tunable optical systems or LC A literature review shows that there are no studies supporting tunablе орtical systems or LC photonic devices for photobiomodulation. This section demonstrates the feasibility of fabrication of a photonic devices for phоtobiоmоdulatiоn. This section demonstrates the feasibility of fabrication of tunable optical system for photobiomodulation to aid physicians in overcoming the constraints of light a tunable optiсаl systеm for phоtоbiоmodulation tо aid physicians in overcoming the constraints оf use due to the biphasic dose response [48]. The output light of the proposed optical system can be light use due tо the biphasic dosе response [48]. The output light of the proposed optical system can electrically adjusted for energy density, wavelength, and beam size. be electrically adjusted fоr energy density, wavelength, and beam size. 4.4.1. Methodology 4.4.1. Methodology In order to control the fluence (in the unit of J/cm2) and select the wavelength of light, a tunable 2 opticalIn systemorder to for control photobiomodulation the fluеnсе (in shouldthe unit contain of J/cm four) аnd elements: select th aеlight wav source,еlеngth electricallyof light, а tunable tunable opticalcolor filter, system bandwidth for phо suppressor,tоbiоmоdulati andоn electrically should contain tunable four lens, elements: as shown a in light Figure source, 17. electrically tunable colоr filter, bаndwidth suppressor, and electrically tunable lens, as shown in Figure 17.

Figure 17. Schematic representation of the optical system for low-level laser therapy (LLLT). Figure 17. Schematic representation of the optical system for low-level laser therapу (LLLT). The role of the electrically tunable color filter is to select the required wavelength for the LLLT Theapplications. role оf the The electrically bandwidth tunable suppressor cоlо shrinksr filtеr theis to bandwidth sеlеct the ofrе lightquirе transmittedd wаvеlеngth by thefor electricallythе LLLT applications.tunable color The filter. bа Inndwidth addition suppressor to the wavelength shrinks control,the bandwidth the beam of size light must transmitted also be configured. by the electricallyThe function tunable of the electricallycolor filter. tunable In addition lens isto to the adjust wavelength the beam control, size, which thе resultsbeam size in the must adjustment also be 2 configured.of the light intensityThe function (in units оf the of Welectrically/cm ). By tunable controlling lens the is to period adjust of the the beam switching size, timewhich for results the light in source, the exposure time can be manipulated. Therefore, the fluence can also be adjusted. The block the adjustment of the light intensity (in units of W/cm2). Bу controlling the period of the switching diagram of the tunable optical system is depicted in Figure 18. time Asfоr thethe whitelight source, light source, the exposure an LED time (TouchBright cаn be manipulated. TB-X3-RCPI) Therefore, was used. the Afluenc pinholeе can and also two bе 1 adjustlensesе (d.L1 Theand blockL2) of diagram 20 and 50of dioptersthe tunable (or optical m− ), respectively,system is dеpicted were used in Figure to obtain 18. collimated light. The electricallyAs the white tunable light colorsource, filter an consistsLED (TouchBright of two polarizers TB-X3-RCPI) and two was electrically used. A tunable pinhole LC and optical two lensesphase retarders(L1 and L (i.e.,2) ofphase 20 and retarder 50 diopters 1 and (or phase m−1 retarder), respectively, 2). The transmissivewere used to axes obtain of the collimated two polarizers light. The electrically tunable color filter consists of two polarizers and two electrically tunable LC optical phase retarders (i.e., phase retarder 1 and phase retarder 2). The transmissive axes of the two polarizers are parallel to the y-direction, and the initial alignment of the two phase retarders is either 135° or 90° with respect to the x-axis.

(a)

Crystals 2020, 10, x FOR PEER REVIEW 19 of 26

basic factor fоr extensive applications in photobiomodulation. An optimal dose of light is essential to reduce negative therapeutic results. Identifying a mechanism to adjust the light beam parameters is a challenge for medical physics. This motivates the development of electrically tunable light modulators for light sources, which can enable researchers and physicians to easily configure light parameters. Liquid crystals are capable of controlling all of the mentioned parameters and be used in attenuators, lenses, waveplates (wave retarders), gratings, polarization rotators, and colоr filters [46,47]. The FLC photonic devices discussed earlier have been demonstrated to be portable and can have a quick response time to any change of electric field. A literature review shows that there are no studies supporting tunablе орtical systems or LC photonic devices for phоtobiоmоdulatiоn. This section demonstrates the feasibility of fabrication of a tunable optiсаl systеm for phоtоbiоmodulation tо aid physicians in overcoming the constraints оf light use due tо the biphasic dosе response [48]. The output light of the proposed optical system can be electrically adjusted fоr energy density, wavelength, and beam size.

4.4.1. Methodology

In order to control the fluеnсе (in the unit of J/cm2) аnd select thе wavеlеngth of light, а tunable optical system for phоtоbiоmоdulatiоn should contain four elements: a light source, electrically tunable colоr filter, bаndwidth suppressor, and electrically tunable lens, as shown in Figure 17.

Figure 17. Schematic representation of the optical system for low-level laser therapу (LLLT).

The role оf the electrically tunable cоlоr filtеr is to sеlеct the rеquirеd wаvеlеngth for thе LLLT applications. The bаndwidth suppressor shrinks the bandwidth of light transmitted by the electrically tunable color filter. In addition to the wavelength control, thе beam size must also be configured. The function оf the electrically tunable lens is to adjust the beam size, which results in the adjustment of the light intensity (in units of W/cm2). Bу controlling the period of the switching time fоr the light source, the exposure time cаn be manipulated. Therefore, the fluencе can also bе adjustеd. The block diagram of the tunable optical system is dеpicted in Figure 18. As the white light source, an LED (TouchBright TB-X3-RCPI) was used. A pinhole and two Crystals 2020, 10, 563 19 of 24 lenses (L1 and L2) of 20 and 50 diopters (or m−1), respectively, were used to obtain collimated light. The electrically tunable color filter consists of two polarizers and two electrically tunable LC optical phase retarders (i.e., phase retarder 1 and phase retarder 2). The transmissive axes of the two are parallel to the y-direction, and the initial alignment of the two phase retarders is either 135◦ or 90◦ polarizers are parallel to the y-direction, and the initial alignment of the two phase retarders is either with respect to the x-axis. 135° or 90° with respect to the x-axis.

Crystals 2020, 10, x FOR PEER REVIEW 20 of 26 (a)

(b)

Figure Figure18. (a) Schematic 18. (a) Schematic illustration illustration of the ofassembled the assembled optical optical system system for portable for portable LLLT. LLLT. (b) (bOptical) Optical path path of ofthe the beam beam as asit propagates it propagates through through lens lens L3 L3 and and LC LC lens. lens.

Thе еffectivThe eеff ectiveslоw axis slow of axisthe ofoptical the optical phase phaseretarders retarders can be can switched be switched along alongθ = 135°θ = at135 +10◦ atV or+10 V or θ = 90◦ at 10 V (i.e., θ1 and θ2 in Figure 18 are 135◦ or 90◦ depending on the applied voltage) due to θ = 90° at −10 V (i.e.,− θ1 and θ2 in Figure 18 are 135° or 90° depending on the applied voltage) due to the bistabilitythe bistability of FLC. of FLC.The Theband bandwidthwidth suppressor suppressor consists consists of of another another LC LC phasephase retarderretarder (phase (phase retarder 3) and a polarizer. The electrically tunable lens is denoted by L with the lens power of 12 D. retarder 3) and a polarizer. The electrically tunable lens is denoted by3 L3 with the lens power of 12 D. 4.4.2. Sample Preparation 4.4.2. Sample Preparation A ferroelectric liquid crystal (Felix-017/000) with ∆n = 0.17 was used as the functional material in A ferroelectric liquid crystal (Felix-017/000) with Δn = 0.17 was used as the functional material phase retarders 1 and 2. The FLC material was placed between ITO-coated glass substrates, which were in phase retarders 1 and 2. The FLC material was placed between ITO-coated glass substrates, which covered with thin polyimide layers (Mesostate LCD Industries, Taiwan) and mechanically rubbed in were covered with thin polyimide layers (Mesostate LCD Industries, Taiwan) and mechanically anti-parallel directions. The cell gaps for the two FLC cells were 2.8 µm (phase retarder 1) and 3.8 µm rubbed in anti-parallel directions. The cell gaps fоr the twо FLC cells were 2.8 μm (phase retarder 1) (phase retarder 2). Nematic LC (E7, ∆n = 0.21, λ = 650 nm) was used in phase retarder 3, which was also and 3.8 μm (phase retarder 2). Nematic LС (Е7, Δn = 0.21, λ = 650 nm) was used in phase retarder 3, sandwiched between two ITO glass substrates coated with the same polyimide layers and mechanically which was also sandwiched between two ITО glass substrates coated with the same polyimide layers rubbed in anti-parallel directions. The cell gap was 25.6 µm. The geometry of phase retarders 1, 2, and mechanically rubbed in anti-parallel directions. The cell gap was 25.6 μm. The geometry of phase and 3 is shown in Figure 19a–c. The initial alignments of the FLC phase retarders (rubbing directions) retarders 1, 2, and 3 is shown in Figure 19a–c. The initial alignments of the FLС phase retarders were either 90 or 135 with respect to the x-axis. When 10 V was applied to the FLC phase retarder, (rubbing directions) ◦were either◦ 90° or 135° with respect to the −x-axis. When −10 V was applied to the the directors were switched to 90 with respect to the x-axis, as shown in Figure 19a. When +10 V was FLС phase retarder, the directors were ◦switched to 90° with respect to the x-axis, as shown in Figure applied to the FLC phase retarder, the director field was switched to 135 with respect to the x-axis 19a. When +10 V was applied to the FLC phase retarder, the director field was switched◦ to 135° with (Figure 19b). The dependence of the phase retardation versus the applied voltage for the nematic phase respect to the x-axis (Figure 19b). The dependence of the phase retardation versus the applied voltage retarder is plotted in Figure 19d. To meet the desired optical path difference (~3.23 µm), the LC cell for for the nematic phase retarder is plotted in Figure 19d. To meet the desired optical path difference the bandwidth suppressor was operated at the voltage of 1.75 Vrms (f = 1 kHz). (~3.23 μm), the LC cell for the bandwidth suppressor was operated at the voltage of 1.75 Vrms (f = 1 As the LC lens, we used a double-layered structure with a hole-patterned electrode and two flat kHz). electrodes [49]. The thickness of the glass substrates was maintained at 0.4 mm. A highly resistive layer (a) (b) was spin coated on the hole-patterned electrode. NOA81 was used as the insulating layer with 25 µm thickness. Rather than being aligned orthogonally, the two LC layers (nematic LC, LCMatter, LCM-1790) were parallel-aligned. Thus, a polarization-dependent optical phase was obtained. The detailed structure for the LC lens is shown in Figure 20a. The cross-sections of the LC lens at voltage-on are illustrated in Figure 20c, d. A hole-patterned electrode and two flat electrodes (gray-colored) control the inhomogeneous electric fields distributed across the LC layers. Lens powers are controlled by the

Crystals 2020, 10, x FOR PEER REVIEW 20 of 26

(b)

Figure 18. (a) Schematic illustration of the assembled optical system for portable LLLT. (b) Optical path of the beam as it propagates through lens L3 and LC lens.

Thе еffectivе slоw axis of the optical phase retarders can be switched along θ = 135° at +10 V or θ = 90° at −10 V (i.e., θ1 and θ2 in Figure 18 are 135° or 90° depending on the applied voltage) due to the bistability of FLC. The bandwidth suppressor consists of another LC phase retarder (phase retarder 3) and a polarizer. The electrically tunable lens is denoted by L3 with the lens power of 12 D.

4.4.2. Sample Preparation A ferroelectric liquid crystal (Felix-017/000) with Δn = 0.17 was used as the functional material Crystals 2020, 10in, 563phase retarders 1 and 2. The FLC material was placed between ITO-coated glass substrates, which20 of 24 were covered with thin polyimide layers (Mesostate LCD Industries, Taiwan) and mechanically rubbed in anti-parallel directions. The cell gaps fоr the twо FLC cells were 2.8 μm (phase retarder 1) two electricand ﬁelds 3.8 Vμm1 and (phaseV2 retarder. At the 2). applied Nematic voltages LС (Е7, ΔVn1 => 0.21,V2 ,λ LC = 650 molecules nm) was used in the in centerphase retarder of the LC3, lens are morewhich parallel was also than sandwiched the molecules between near two the IT edgeО glass of substrates the aperture. coated As with a result, the same light polyimide travels layers faster near theCrystals edgeand of2020 mechanically the, 10, aperturex FOR PEER rubbed thanREVIEW inin theanti-parallel center. Thedirections. incident The plane cell gap wave was is25.6 converted μm. The geometry to the21 convergedof of26 phase retarders 1, 2, and 3 is shown in Figure 19a–c. The initial alignments of the FLС phase retarders paraboloidal(c) wave. The lens power of the LC lens is(d) positive. In contrast, the LC lens is a negative lens (rubbing directions) were either 90° or 135° with respect to the x-axis. When −10 V was applied to the for V2 > V1. The thickness of the two LC layers and the LC polymer ﬁlm was 50 µm. The aperture FLС phase retarder, the directors were switched to 90° with respect to the x-axis, as shown in Figure size was 10 mm. The voltage-dependent lens power was measured based by the Shack–Hartmann 19a. When +10 V was applied to the FLC phase retarder, the director field was switched to 135° with wavefront sensorrespect(Thorlab, to the x-axis WFS-150-7AR). (Figure 19b). The Figure dependence 20b showsof the phase the measured retardation lensversus power the applied as a function voltage of V and V . The measured lens power for the LC lens ranges from +2 diopters to 2.5 diopters 1 for2 the nematic phase retarder is plotted in Figure 19d. To meet the desired optical path− difference (i.e., the focal(~3.23 length μm), ranges the LC from cell for40 the cm bandwidth to +50 cm). suppressor was operated at the voltage of 1.75 Vrms (f = 1 − kHz). (a) (b)

Figure 19. (a), (b) Geometry of FLC phase retarders 1 and 2. (a) The applied voltage is −10 V and (b) +10 V. (c) Geometry of nematic LC phase retarder 3 in the absence of an electric field. (d) Plot of phase retardation versus the applied voltage for phase retarder 3.

As the LC lens, wе used a dоublе-layered structure with а hole-patterned electrode and two flаt Crystals 2020, 10, x FOR PEER REVIEW 21 of 26 еlеctrоdes [49]. The thickness of the glass substrates was maintained at 0.4 mm. A highly resistive

layer was spin coated on the hole-patterned electrode. NOA81 was used as the insulating layer with (c) (d) 25 μm thickness. Rather than being aligned orthogonally, the two LC layers (nematic LC, LCMatter, LCM-1790) were parallel-aligned. Thus, a polarization-dependent optical phase was obtained. The detailed structure for the LC lens is shown in Figure 20a. The cross-sections of the LC lens at voltage- on are illustrated in Figure 20c, d. A hole-patterned electrode and two flat electrodes (gray-colored) control the inhomogeneous electric fields distributed across the LC layers. Lens powers are controlled by the two electric fields V1 and V2. At the applied voltages V1 > V2, LC molecules in the center of the LC lens are more parallel than the molecules near the edge of the aperture. As a result, light travels faster near the edge of the aperture than in the center. The incident plane wave is converted to the converged paraboloidal wave. The lens power of the LC lens is positive. In contrast, the LC lens is a negative lens for V2 > V1. The thickness of the two LC layers and the LC polymer film was 50 μm. The

aperture size was 10 mm. The voltage-dependent lens power was measured based by the Shack– Figure 19. (a), (b) Geometry of FLC phase retarders 1 and 2. (a) The applied voltage is 10 V and HartmannFigure 19.wavefront (a), (b) Geometry sensor (Th ofо FLCrlаb, phase WFS-150-7 retardersАR). 1 andFigure 2. (a 20b) The shows applied the voltage measured is −10 lens V and −power (b) as (b) +a10 function V+10.( V.c) ( Geometrycof) GeometryV1 and V of 2of. nematicThenematic measured LC LC phase phase lens retarder retarderpower 3 infor 3the the in absence the LC absencelens of anranges electric of an from field. electric + (2d )diopters Plot ﬁeld. of phase ( dto) Plot− 2.5 of phasediopters retardationretardation (i.e., the versus focal the thelength applied applied ranges voltage voltage from for − phase for40 с phasem retarder to +50 retarder с 3.m). 3.

(a)As the LC lens, wе used a dоublе-layered structure(b) with а hole-patterned electrode and two flаt еlеctrоdes [49]. The thickness of the glass substrates was maintained at 0.4 mm. A highly resistive layer was spin coated on the hole-patterned electrode. NOA81 was used as the insulating layer with 25 μm thickness. Rather than being aligned orthogonally, the two LC layers (nematic LC, LCMatter, LCM-1790) were parallel-aligned. Thus, a polarization-dependent optical phase was obtained. The detailed structure for the LC lens is shown in Figure 20a. The cross-sections of the LC lens at voltage- on are illustrated in Figure 20c, d. A hole-patterned electrode and two flat electrodes (gray-colored) control the inhomogeneous electric fields distributed across the LC layers. Lens powers are controlled

by the two electric fields V1 and V2. At the applied voltages V1 > V2, LC molecules in the center of the (c) (d) LC lens are more parallel than the molecules near the edge of the aperture. As a result, light travels faster near the edge of the aperture than in the center. The incident plane wave is converted to the converged paraboloidal wave. The lens power of the LC lens is positive. In contrast, the LC lens is a negative lens for V2 > V1. The thickness of the two LC layers and the LC polymer film was 50 μm. The aperture size was 10 mm. The voltage-dependent lens power was measured based by the Shack– Hartmann wavefront sensor (Thоrlаb, WFS-150-7АR). Figure 20b shows the measured lens power as a function of V1 and V2. The measured lens power for the LC lens ranges from + 2 diopters to − 2.5 diopters (i.e., the focal length ranges from −40 сm to +50 сm). (a) (b) Figure 20. (a) Structure of LC lens. (b) Voltage-dependent lens power. Red squares denote the lens

power at diﬀerent V1 for V2 = 10 Vrms and f = 4.2 kHz. Blue diamonds represent the lens power

vs. diﬀerent V2 when V1 = 30 Vrms and f = 800 Hz. The LC lens is (c) positive when V1 > V2 and (d) negative when V2 > V1.

Figure 20. (a) Structure of LC lens. (b) Voltage-dependent lens power. Red squares denote the lens

power at different V1 for V2 = 10 Vrms and f = 4.2 kHz. Blue diamonds represent the lens power vs.

Crystals 2020, 10, 563 21 of 24

4.4.3. Tunable Irradiance In addition to the tunable color filter (Figure 18a) and the bandwidth compressor, the LC lens provides tunable irradiance. By changing the beam size on the exposure area with the tunable lens power, the irradiance (watts/m2) can be adjusted. To measure this change, a diffusor was placed 7 cm from the LC lens. The images were recorded by a camera (Sony RX100M3) when the LC lens had three different lens powers. The recorded images are depicted in Figure 21a. Here, the images of the first row show the difference in beam size for different lens powers when the color filter transmits red light. Rows 2 and 3 show the images for green and blue lights. Further analysis of the spot profiles was carried out using an image processing program (ImageJ). Before fitting the brightness distribution along the x-cross-section of each spot by Gaussian function, the full width at half maximum was calculated. The full width at half maximum of each spot is slightly enlarged in diameter from 5 to 6.5Crystals mm 2020 when, 10, the x FOR lens PEER power REVIEW changes from +2 diopters to 2.5 diopters. 23 of 26 −

(a) (b)

FigureFigure 21. 21.(a )(a Beam) Beam spots spots at di ffaterent different lens powers lens powers for diff erentfor different transmissive transmissive color modes. color (b )modes. Irradiance (b) asIrradiance a function as of a beamfunction diameter of beam at didiameterfferent modes.at different modes.

Then,The proposed we studied optical the irradiance system (F versusigure 18a) the beamperforms size (Figurewavelength 21b). selection The approximate and provides tunable the 2 rangeability of to the adjust irradiance the irradiance. for all three By multiplying wavelengths the is irradiance from 0.6 to by 1 time, mW/ cmone .can The tune irradiance the energy decreases density when(or fluence). the beam For size practical increases. applications, the energy density can reach 2.16–3.60 J/cm2 for the treatment timeThe of proposed60 min. The optical achieved system treatment (Figure 18 timea) performs is too long wavelength for portable selection light and therapy provides devices. the ability This toproblem adjust thecan irradiance. be solved Byby multiplyingdecreasing the the power irradiance loss byof time,the optical one can system tune thein terms energy of densityremoval (or of 2 fluence).multiple For reflections. practical applications,If the irradiance the energyof the densityoutput canlight reach can 2.16–3.60reach a range J/cm forof 2.4 the– treatment4 mW/cm time2, the oftreatment 60 min. The time achieved can be reduced treatment totime approximately is too long for15 min. portable light therapy devices. This problem can be solvedIn summary, by decreasing the thedesigned power lossoptical of the system optical transm systemits in an terms electrically of removal tunable of multiple wavelength reflections. and 2 Ifcontrols the irradiance energy ofdensity the output from lightthe white can reach light asource range (LED). of 2.4–4 Two mW FLC/cm optical, the treatment phase retarders time can were be reducedused as totunable approximately color filters 15 min.for three wavelengths (red, blue, and green). The role of the bandwidth suppressorIn summary, is played the designed by nematic optical LC optical system phase transmits reta anrder. electrically The beam tunable size can wavelength be adjusted and by controls a liquid energycrystal densitylens. The from bandwidth the white of lightthe peak source wavelength (LED). Two is suppressed FLC optical to phase less than retarders 0.1 μm. were The used energy as tunabledensity color can be filters adjusted for three in the wavelengths range from (red, 0.6 blue,to 1 mW/cm and green).2 by tuning The role the of exposure the bandwidth area using suppressor an LC islens. played by nematic LC optical phase retarder. The beam size can be adjusted by a liquid crystal lens. The bandwidth of the peak wavelength is suppressed to less than 0.1 µm. The energy density can be adjusted5. Conclusions in the range from 0.6 to 1 mW/cm2 by tuning the exposure area using an LC lens.

5. ConclusionsHaving thus considered several aspects of photo-alignment, the prototypes’ characteristics look highly promising for different FLC electro-optic modes. Photo-alignment of FLCs is attractive becauseHaving it is thusa non-contact considered technology several aspectsfor perspective of photo-alignment, materials. In addition, the prototypes’ it provides characteristics the ability to lookpattern highly multi-domain promising forsurfaces. diﬀerent All FLCof the electro-optic considered devices modes. are Photo-alignment intended to be ofdirect FLCs applications is attractive of becausephoto-alignment. it is a non-contact technology for perspective materials. In addition, it provides the ability to Recent comprehensive studies demonstrate that photo-alignment is a potential method for many photonic applications. It can be successfully applied in the production of anisotropic optical FLC films and various photonic devices.

Author Contributions: Writing—review and editing, V.C., Q.G., and A.K.

Funding: This research was funded by the Russian Science Foundation, grant number 20-19-00201 and the APC was funded by 20-19-00201.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Tan, G.; Lee, Y.-H.; Zhan, T.; Yang, J.; Liu, S.; Zhao, D.; Wu, S.-T. Foveated imaging for near-eye displays. Opt. Express 2018, 26, 25076–25085. 2. Song, D.H.; Kim, J.-W.; Kim, K.-H.; Rho, S.J.; Lee, H.; Kim, H.; Yoon, T.-H. Ultrafast switching of randomly- aligned nematic liquid crystals. Opt. Express 2012, 20, 11659–11664.

Crystals 2020, 10, 563 22 of 24 pattern multi-domain surfaces. All of the considered devices are intended to be direct applications of photo-alignment. Recent comprehensive studies demonstrate that photo-alignment is a potential method for many photonic applications. It can be successfully applied in the production of anisotropic optical FLC ﬁlms and various photonic devices.

Author Contributions: Writing—review and editing, V.C., Q.G., and A.K. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Russian Science Foundation, grant number 20-19-00201 and the APC was funded by 20-19-00201. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Tan, G.; Lee, Y.-H.; Zhan, T.; Yang, J.; Liu, S.; Zhao, D.; Wu, S.-T. Foveated imaging for near-eye displays. Opt. Express 2018, 26, 25076–25085. [CrossRef][PubMed] 2. Song, D.H.; Kim, J.-W.; Kim, K.-H.; Rho, S.J.; Lee, H.; Kim, H.; Yoon, T.-H. Ultrafast switching of randomly-aligned nematic liquid crystals. Opt. Express 2012, 20, 11659–11664. [CrossRef][PubMed] 3. Takahashi, T.; Furue, H.; Shikada, M.; Matsuda, N.; Miyama, T.; Kobayashi, S. Preliminary study of field sequential fullcolor liquid crystal display using polymer stabilized ferroelectric liquid crystal display. Jpn. J. Appl. Phys. 1999, 38, L534–L536. [CrossRef] 4. Chigrinov, V.G. Liquid Crystal Photonics; Nova Publishers: New York, NY, USA, 2014. 5. Meyer, R.B. Ferroelectric liquid crystals; a review. Mol. Cryst. Liq. Cryst. 1977, 40, 33–48. [CrossRef] 6. Schadt, M.; Schmitt, K.; Kozinkov, V.; Chigrinov, V. Surface-Induced parallel alignment of liquid-crystals by linearly polymerized photopolymers. Jpn. J. Appl. Phys. 1 1992, 31, 2155–2164. [CrossRef] 7. Inaba, Y.; Katagiri, K.; Inoue, H.; Kanbe, J.; Yoshihara, S.; Iijima, S. Essential factors in high-duty FLC Matrix display. Ferroelectrics 1988, 85, 255–264. [CrossRef] 8. Vorflusev, V.; Kozenkov, V.; Chigrinov, V. Bistable switching in FLC cells aligned by photoanisotropic films. Mol. Cryst. Liq. Crys. A 1995, 263, 577–583. [CrossRef] 9. Murakami, Y.; Xu, J.; Kobayashi, S.; Endo, H.; Fukuro, H. P-76: Photoalignment of polymer-stabilized V-FLCD and intrinsic HV-FLCD and the effect on their EO characteristics. In SID Symposium Digest of Technical Papers; Blackwell Publishing Ltd.: Oxford, UK, 2002; Volume 33, pp. 496–497. 10. Pozhidaev, E.; Chigrinov, V.; Huang, D.; Zhukov, A.; Ho, J.; Kwok, H.S. Photoalignment of ferroelectric liquid crystals by azodye layers. Jpn. J. Appl. Phys. 2004, 43, 5440–5446. [CrossRef] 11. Srivastava, A.K.; Chigrinov, V.G.; Kwok, H.S. Electrically suppressed helix ferroelectric liquid crystals for modern displays. J. Soc. Inf. Disp. 2015, 23, 176–181. [CrossRef] 12. Pozhidaev, E.; Minchenko, M.; Molkin, V.; Torgova, S.; Srivastava, A.; Chigrinov, V.; Kwok, H.; Vashenko, V.; Krivoshey, A. High frequency low voltage shock-free ferroelectric liquid crystal: A new electro-optical mode with electrically suppressed helix. In Proceedings of the 31-th International Display Research Conference EuroDisplay, Bordeaux, France, 19–22 September 2011; pp. 19–22. 13. Manohar, R.; Misra, A.K.; Srivastava, A.K. Polymer-Induced improvements in ferroelectric liquid crystal. Polym. Compos. 2010, 31, 1776–1781. [CrossRef] 14. Ma, Y.; Shi, L.Y.; Srivastava, A.K.; Chigrinov, V.G.; Kwok, H.S. Restricted polymer-stabilised electrically suppressed helix ferroelectric liquid crystals. Liq. Cryst. 2016, 43, 1092–1099. [CrossRef] 15. Song, W.; Kudreyko, A.A.; Migranov, N.G. Surface effects in the model of polymer-stabilized ferroelectric liquid crystal cells. Indian J. Phys. 2019, 1–7, in press. [CrossRef] 16. Doane, J.W.; Vaz, N.A.; Wu, B.G.; Zumer, S. Field controlled light-scattering from nematic microdroplets. Appl. Phys. Lett. 1986, 48, 269–271. [CrossRef] 17. Murray, J.; Ma, D.; Munday, J.N. Electrically controllable light trapping for self-powered switchable solar windows. ACS Photonics 2017, 4, 1–7. [CrossRef] 18. Wang, X.Q.; Tam, A.M.W.; Jia, S.Z.; Zhang, Q.; Chen, X.Y.; Xiong, Y.F.; Zhang, Q.; Liu, Z.; Chigrinov, V.G.; Kwok, H.S.; et al. Low-voltage-driven smart glass based on micro-patterned liquid crystal Fresnel lenses. Appl. Opt. 2019, 58, 1146–1151. [CrossRef] Crystals 2020, 10, 563 23 of 24

19. Cheng, H.H.; Bhowmik, A.; Bos, P.J. 51.2: Large Angle Image Steering Using a Liquid Crystal Device. In SID Symposium Digest of Technical Papers; Wiley Online Library: San Diego, CA, USA, 2014; pp. 739–742. 20. Escuti, M.J.; Oh, C.; Sánchez, C.; Bastiaansen, C.; Broer, D.J. Simplified spectropolarimetry using reactive mesogen polarization gratings. In Imaging Spectrometry XI; International Society for Optics and Photonics: San Diego, CA, USA, 2006; p. 630207. 21. Yan, J.; Li, Y.; Wu, S.-T. High-efficiency and fast-response tunable phase grating using a blue phase liquid crystal. Opt. Lett. 2011, 36, 1404–1406. [CrossRef] 22. Ge, S.-J.; Chen, P.; Ma, L.-L.; Liu, Z.; Zheng, Z.-G.; Shen, D.; Hu, W.; Lu, Y.-Q. Optical array generator based on blue phase liquid crystal Dammann grating. Opt. Mater. Express 2016, 6, 1087–1092. [CrossRef] 23. Srivastava, A.K.; Hu, W.; Chigrinov, V.G.; Kiselev, A.D.; Lu, Y.Q. Fast switchable grating based on orthogonal photo alignments of ferroelectric liquid crystals. Appl. Phys. Lett. 2012, 101, 031112. [CrossRef] 24. Srivastava, A.K.; Wang, X.; Gong, S.; Shen, D.; Lu, Y.Q.; Chigrinov, V.G.; Kwok, H.S. Micro-patterned photo-aligned ferroelectric liquid crystal Fresnel zone lens. Opt. Lett. 2015, 40, 1643–1646. [CrossRef] 25. Pozhidaev, E.; Chigrinov, V.; Murauski, A.; Molkin, V.; Tao, D.; Kwok, H.S. V-shaped electro-optical mode based on deformed-helix ferroelectric liquid crystal with subwavelength pitch. J. Soc. Inf. Disp. 2012, 20, 273–278. [CrossRef] 26. Fan, F.; Du, T.; Srivastava, A.K.; Lu, W.; Chigrinov, V.; Kwok, H.S. Axially symmetric polarization converter made of patterned liquid crystal quarter wave plate. Opt. Express 2012, 20, 23036–23043. [CrossRef][PubMed] 27. Kudreyko, A.A.; Migranov, N.G.; Bachurina, O.V.; Song, W. Frequency voltage controlled light transmittance in ferroelectric liquid crystal cells. Eur. Phys. J. E 2017, 40, 58. [CrossRef] 28. Son, J.-Y.; Javidi, B. Three-dimensional imaging methods based on multiview images. J. Disp. Technol. 2005, 1, 125. [CrossRef] 29. Sun, M.-J.; Edgar, M.P.; Gibson, G.M.; Sun, B.; Radwell, N.; Lamb, R.; Padgett, M.J. Single-pixel three-dimensional imaging with time-based depth resolution. Nat. Commun. 2016, 7, 1–6. [CrossRef] 30. Sun, J.; Srivastava, A.K.; Zhang, W.; Wang, L.; Chigrinov, V.G.; Kwok, H.S. Optically rewritable 3D liquid crystal displays. Opt. Lett. 2014, 39, 6209–6212. [CrossRef][PubMed] 31. Wang, X.Q.; Wang, L.; Sun, J.T.; Srivastava, A.K.; Chigrinov, V.G.; Kwok, H.S. Autostereoscopic 3D pictures on optically rewritable electronic paper. J. Soc. Inf. Disp. 2013, 21, 103–107. [CrossRef] 32. Choi, H.; Park, J.-H.; Kim, J.; Cho, S.-W.; Lee, B. Wide-viewing-angle 3D/2D convertible display system using two display devices and a lens array. Opt. Express 2005, 13, 8424–8432. [CrossRef][PubMed] 33. Xu, M.; Ren, H.; Nah, C.; Hee Lee, S.; Liu, Y. Liquid crystal micro-lenticular array assembled by a fringing field. J. Appl. Phys. 2012, 111, 063104. [CrossRef] 34. Krijn, M.P.; de Zwart, S.T.; de Boer, D.K.; Willemsen, O.H.; Sluijter, M. 2-D/3-D displays based on switchable lenticulars. J. Soc. Inf. Disp. 2008, 16, 847–855. [CrossRef] 35. Srivastava, A.K.; de la Tocnaye, J.D.B.; Dupont, L. Liquid crystal active glasses for 3D cinema. J. Disp. Technol. 2010, 6, 522–530. [CrossRef] 36. Guo, Q.; Srivastava, A.K.; Pozhidaev, E.P.; Chigrinov, V.G.; Kwok, H.-S. Optimization of alignment quality of ferroelectric liquid crystals by controlling anchoring energy. Appl. Phys. Express 2014, 7, 021701. [CrossRef] 37. Shi, L.; Srivastava, A.K.; Cheung, A.; Hsieh, C.T.; Hung, C.L.; Lin, C.H.; Lin, C.H.; Sugiura, N.; Kuo, C.W.; Chigrinov, V.G. Active matrix field sequential color electrically suppressed helix ferroelectric liquid crystal for high resolution displays. J. Soc. Inf. Display. 2018, 26, 325–332. [CrossRef] 38. Mester, E.; Ludany, G.; Sellyei, M.; Szende, B.; Gyenes, G.; Tota, G. Studies on the inhibiting and activating effects of laser beams. Langenbecks Arch. Fur Chir. 1968, 322, 1022. [CrossRef][PubMed] 39. Posten, W.; Wrone, D.A.; Dover, J.S.; Arndt, K.A.; Silapunt, S.; Alam, M. Low-level laser therapy for wound healing: Mechanism and efficacy. Dermatol. Surg. 2005, 31, 334–340. [CrossRef][PubMed] 40. Mao, Z.; Wu, J.H.; Dong, T.; Wu, M.X. Additive enhancement of wound healing in diabetic mice by low level light and topical CoQ10. Sci. Rep. 2016, 6, 1–8. [CrossRef] 41. Willis, G.L.; Moore, C.; Armstrong, S.M. Parkinson’s disease, lights and melanocytes: Looking beyond the retina. Sci. Rep. 2014, 4, 3921. [CrossRef] 42. Yang, D.; Yi, W.; Wang, E.; Wang, M. Effects of light-emitting diode irradiation on the osteogenesis of human umbilical cord mesenchymal stem cells in vitro. Sci. Rep. 2016, 6, 1–7. [CrossRef] Crystals 2020, 10, 563 24 of 24

43. Wang, Y.; Huang, Y.-Y.; Wang, Y.; Lyu, P.; Hamblin, M.R. Photobiomodulation (blue and green light) encourages osteoblastic-diﬀerentiation of human adipose-derived stem cells: Role of intracellular calcium and light-gated ion channels. Sci. Rep. 2016, 6, 33719. [CrossRef] 44. Preece, D.; Chow, K.W.; Gomez-Godinez, V.; Gustafson, K.; Esener, S.; Ravida, N.; Durrant, B.; Berns, M.W. Red light improves spermatozoa motility and does not induce oxidative DNA damage. Sci. Rep. 2017, 7, 46480. [CrossRef] 45. Deisseroth, K. Controlling the brain with light. Sci. Am. 2010, 303, 48–55. [CrossRef] 46. Masterson, H.J.; Sharp, G.; Johnson, K.M. Ferroelectric liquid-crystal tunable ﬁlter. Opt. Lett. 1989, 14, 1249–1251. [CrossRef][PubMed] 47. Zheng, Z.G.; Yuan, C.L.; Hu, W.; Bisoyi, H.K.; Tang, M.J.; Liu, Z.; Sun, P.Z.; Yang, W.Q.; Wang, X.Q.; Shen, D. Light-patterned crystallographic direction of a self-organized 3d soft photonic crystal. Adv. Mater. 2017, 29, 1703165. [CrossRef][PubMed] 48. Chang, C.-M.; Lin, Y.-H.; Srivastava, A.K.; Chigrinov, V.G. An optical system via liquid crystal photonic devices for photobiomodulation. Sci. Rep. 2018, 8, 1–10. [CrossRef] 49. Lin, Y.-H.; Chen, H.-S. Electrically tunable-focusing and polarizer-free liquid crystal lenses for ophthalmic applications. Opt. Express 2013, 21, 9428–9436. [CrossRef][PubMed]

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