Open Access
Add to BibSonomy
Abstract
We demonstrate a liquid crystal (LC)-based optical device with the polarization switching capability, which can store two different chiral images to be selected according to the polarization state of the viewing polarizer. The chiral dual-image device consists of chiral surface patterns for image storage and the LC layer as a tunable phase retarder. Each chiral surface pattern behaves as a helical photonic crystal that reflects circularly polarized light at a specific wavelength. Depending on the applied voltage across the LC layer, either a right-handed or a left-handed circular polarization image appears, and thus one of the two stored images can be selectively read by the polarization state. Our concept of the LC-based chiral image storage and selection provides simplicity in fabrication, flexibility in design, and high optical efficiency. It will be directly applicable for reflective-type 3D displays, color filters, and anti-counterfeiting devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, the optical image selection has been paid much attention for various optical applications such as optical data storage [1–5], color filters [6–11], three-dimensional (3D) displays [12–17], and anti-counterfeiting [18–21]. The image selection can be realized in several different ways, for example, the polarization state of light [1,4,13], the viewing position [8,14], the wavelength of light [5,22], or the selective chemical reaction [20]. Among them, the use of the polarization state of light for storing and retrieving different images has been extensively studied due to the structural simplicity of the optical system and the orthogonal characteristics of the polarization. For instance, asymmetric plasmonic nanostructure pixels representing color changes with the polarization direction have been utilized to construct dual images [7,9,11]. However, the existing schemes for the image selection based on the polarization of light encounter a few problems to be solved. For example, the image resolution is limited by the fixed polarization state of each pixel. Fixed phase retarders or nanostructures result in the polarization state of light to be unchangeable for each pixel [16,17], meaning that only a half of total pixels should be utilized for each image. Note that fixed structures cause the images to be permanently locked so that a particular polarization state of a reading beam is necessarily used for the selection of a specific image. In other words, once the images are permanently stored in the fixed structures, it is barely possible to retrieve them by other reading conditions. Moreover, the permanently locked images restrict their real-time modification and limit the range of the device application. Another point is that the image selection by the linear polarization suffers from the precise alignment for viewing. More specifically, the selection scheme of using the linear polarization requires the precise alignment between the polarization state of light and a viewing polarizer since the small amount of the misalignment inevitably involves unwanted crosstalk deteriorating the quality of image selection. Thus, it is very important to devise a new scheme of the image selection with the switching capability by the polarization state.
In this work, we demonstrate a liquid crystal (LC)-based optical device with the polarization switching capability which can store two different chiral images to be selected according to the polarization state of light through a viewing polarizer. In principle, our concept provides more degrees of freedom for the image selection by the combination of the optical and electrical retrieval schemes, which has not been realized previously. The chiral image device is composed of chiral surface patterns for image storage and the LC layer acting as a tunable phase retarder. The chiral surface patterns were constructed through the photo-polymerization on the bottom substrate. Note that the thickness of the LC layer was designed to satisfy the condition that under no applied voltage, the LC layer should act as a half-wave retarder. By the application of the applied voltage, on passing through the LC layer, the polarization state of light is switched from the right-handed circular (RHC) polarization to the left-handed circular (LHC) polarization, and vice versa. Based on the polarization switching capability between two orthogonal states of the circular polarization of light, one of the two chiral images can be independently selected in our device architecture.
2. Device configuration and operation principle
The schematic diagram of the device configuration illustrating our concept of the LC-based chiral image selection is depicted in Fig. 1. Suppose that two chiral surface patterns (‘A’ and ‘B’) with the right-handed chirality, reflecting the light of the RHC polarization within the photonic stop band, are prepared on the bottom substrate as shown in Figs. 1(a) and 1(b). An electrically tunable wave-retarder made of the LC layer is sandwiched between the top substrate and the bottom substrate with the two chiral surface patterns. The use of the LC as an electro-optical (EO) medium allows the high EO anisotropy, the low voltage operation, and the flexibility in design and fabrication of the EO device. The LC molecules are homogeneously aligned on the inner surfaces of the two substrates. The thickness of the LC layer is adjusted to produce the phase retardation of a half-wave. Under no applied voltage (off-state), the RHC polarization of light through the LC layer is transformed into the LHC polarization or vice versa. On the other hand, in the presence of an applied voltage (on-state), since the LC is reoriented vertically to the substrate plane and thus no phase retardation is produced. Accordingly, either the RHC or LHC polarization remains unchanged.
Fig. 1 Schematic diagram of the device configuration showing our concept of the image section of chiral surface images by the polarization state: (a) the image selected by the RHC filter under an applied voltage and (b) the image selected by the LHC filter under no voltage.
In Fig. 1(a), the chiral image selection through a RHC polarization filter is shown according to the applied voltage. In this case, the on-state image (‘A’) is selected whereas the off-state image (‘B’) is blocked by the filter. In the other case that a LHC polarization filter is placed, only the off-state image (‘B’) is selected by the filter as shown in Fig. 1(b). This means that one of two different images stored in the chiral surface patterns can be selected or blocked by the application of the applied voltage under a RHC or LHC filter.
3. Experimental
We describe the fabrication processes of our device of the image storage and selection in Fig. 2. A homogeneous alignment layer of polyimide (SE-6514H; Nissan Chemical Industries, Ltd.) was spin-coated on the inner surface of each patterned indium-tin-oxide (ITO) substrate at the rate of 2000 rpm for 30 s. The alignment layer was annealed at 180 °C for 1 hr. For the molecular alignment, the inner surfaces of both the top and bottom substrates were rubbed homogeneously. On the rubbed bottom substrate, the chiral surface patterns were fabricated using chiral reactive mesogen (CRM) solutions. Two CRM solutions of RMS11-067 and RMS11-068 (Merck) in toluene were blended at the weight ratio of 6.5:3.5 to achieve the reflection peak (λp) at 550 nm. The CRM mixture was spin-coated at the rate of 2000 rpm for 30 s on the bottom substrate. The substrate was then baked at 75 °C for 1 min and cooled down to room temperature for 1 min. The CRM patterns were polymerized by the exposure of ultraviolet (UV) light at the intensity of 20 mW/cm2 for 1 min through a photomask. The un-polymerized CRM region was washed out in toluene (Sigma Aldrich Korea) and acetone in sequence. The substrate was subsequently dried at 75 °C for 1 min to eliminate any residual solvent. A cell was assembled with the bottom substrate with the CRM patterns and the top substrate only with a homogenously rubbed alignment layer. The top and bottom substrates were assembled in an anti-parallel direction. Note that the alignment direction on the bottom substrate dictates both the orientation of the liquid crystals (LCs) outside the CRM pattern and the initial alignment of the CRM. The cell gap (d) was maintained using glass fiber spacers of 3 μm thick. A nematic LC (E7, no = 1.5216, ne = 1.7462, Δn = 0.2246; Merck) was injected into the empty cell by capillary action. The effective thickness (d1) of the LC layer and the height (d2) of the chiral surface pattern were d1 = 1.2 μm and d2 = 1.8 μm, measured using with a surface profiler (Alpha-step 200; KLA-TENCOR).
Fig. 2 Fabrication process of our LC-based chiral image device: (a) Preparation of a chiral surface layer through spin-coating the CRM mixture on a patterned ITO glass with a homogeneous alignment layer. (b) Photo-polymerization patterning upon the UV exposure through a photomask. (c) Removal process of the unpolymerized CRM region. (d) Assembling process of an empty cell with anti-parallel rubbed bottom and top substrates. (e) Injection of the LC into the cell by capillary action. Here, d denotes the total thickness of the cell and d2 is the thickness of the chiral pattern.
The nematic LC on top of the CRM may align along a different direction from the top alignment layer. However, under the condition that the cell gap of the LC layer should satisfy to yield a half-wave retarder, the cell gap was thin enough (about 3 μm) to be dictated by the strong anchoring imposed on the top alignment layer. Thus, the relatively weak anchoring on the CRM surface was not likely to produce appreciable twist of the LC director.
The phase retardation through the LC layer is given by
Note that for the half-wave retardation at the wavelength 550 nm, d1 = 1.2 μm, which is identical to the thickness of the LC layer used in our sample cell. For different coloration, a different thickness of the LC layer is needed. The LC thickness can be simply varied with the spin-coating rate of the CRM solution in our case.4. Results and discussion
4.1. Electro-optical properties
We performed the numerical simulations of the reflectance spectra of our device according to the polarization state of light in the formalism of the Berreman 44 propagation matrix [23]. In our simulations, the helicity of the CRM pattern was set to be right-handed as in our sample cell. The peak wavelength of 550 nm at the Bragg reflection corresponds to the green color. The ordinary and extraordinary refractive indices of the CRM pattern were 1.52 and 1.75, respectively, and the pitch was taken as 340 nm. The material parameters of the LC were those of E7. The thickness of the LC layer was set to be 1.2 μm. The reflection spectra were measured using a spectrometer (USB2000 + ; Ocean Optics) with un-polarized incident light.
Figures 3(a)-3(c) show the simulation data and the experimental results of the reflection spectra under no optical filter, the RHC filter, and the LHC filter, respectively. As a reference, the reflection spectra of a cell with only LC and no CRM pattern were first measured. Using the reflection spectra of the reference cell, the reflection spectra of our device were normalized. Since the reflectance from the interface between the LC layer and the CRM pattern was not taken into account, the experimental results showed slightly higher reflectance than the simulation data. Note that the reflectance of our device at 550 nm (35%) is lower than the cases of typical bulk cholesteric LC layers [24–28]. It is well known that more than 10 pitches yield the full reflection [29] from the helical structure. In our case, the CRM pattern of 1.8 μm thick contains about 6 pitches, and thus the nature of such imperfect photonic crystal results in the relatively low reflectance. For the CRM material we used, the patterns with 10 pitches often involved the uneven surface and the thickness inhomogeneity. In general, the uniform alignment of the CRM in a thick case is barely achieved in bulk throughout the whole sample. Thus, the uniform CRM with 6 pitches were constructed in our work. Under no optical filter, the reflectance with the peak wavelength of 550 nm stays essentially constant irrespective of the applied voltage. This is because the change of the polarization state of light by the reorientation of the LC molecules cannot be detected without any optical filter. In contrast, through the RHC filter, the reflectance of 35% was obtained at 1.9 V (on-state) whereas essentially no reflectance was produced at 0 V (off-state). In the case of the LHC filter, the reflectance behavior exactly opposite to the case of the RHC filter was observed as expected. In other words, no reflectance occurred in the ‘on-state’ while about 35% reflectance was obtained in the ‘off-state’. In the insets of Fig. 3, the corresponding microscopic images, taken with a polarizing optical microscopy (Eclipse E600; Nikon), were shown. The experimental results were found to be well consistent with the simulations. Basically, our device is capable of selecting one of two stored images in a proper combination of the polarization state of light and the optical filter. The orthogonality between the RHC polarization and the LHC polarization enables to eliminate the crosstalk between two different images in a single device.
Fig. 3 Simulation data and experimental results for the reflection spectra according to the applied voltage and the optical filter type. Reflection spectra under (a) no filter, (b) the RHC filter, and (c) the LHC filter. The insets in the figures show the microscopic images of our device depending on the applied voltage. Scale bars represent 500 μm.
Figure 4 shows the simulations and the experimental results for the EO reflectance illustrating the switching capability by the polarization state of light as a function of the applied voltage. The simulations were performed using a commercial program (LCD Master; Shintech). The simulations were performed in a homogeneous LC layer. In Fig. 4(a), the values of one of the Stokes parameters, the fourth parameter (S3), representing the degree of the circular polarization were shown as a function of the applied voltage. At each voltage, S3 was determined as the difference (between the RHC reflectance and the LHC reflectance) divided by the sum of the RHC reflectance and the LHC reflectance.
Fig. 4 The simulations and the experimental results for the EO reflectance as a function of the applied voltage. (a) The values of the Stokes parameter, S3, as a function of the applied voltage. (b) The simulation data under the RHC filter (red solid line), the experimental results under the RHC filter (red dashed line with dots), the simulation data under the LHC filter (yellow solid line), and the experiment results under the LHC filter (yellow dashed line with dots) as a function of voltage.
Note that S3 = −1 for the perfect LHC polarization and S3 = 1 for the perfect RHC polarization. In our case, the values of S3 were varied from –0.87 to 0.87 in the range of the applied voltage between 0 to about 2 V, meaning that the LHC polarization was transformed into the RHC polarization as the applied voltage was increased. In the presence of the applied voltage, the LC of E7, having the positive dielectric anisotropy (), was reoriented parallel to the applied electric field along the direction perpendicular to the substrate, and then no phase retardation through the LC layer was produced. In other words, the vertically reoriented LC layer was optically isotropic along the propagation direction of light incident perpendicular to the cell substrate. The values of the EO reflectance of our device at the peak wavelength of 550 nm under the RHC filter and the LHC filter were presented as a function of the applied voltage in Fig. 4(b). As expected, the reflectance under the RHC filter is reciprocal to that under the LHC filter. The degree of the polarization conversion between the LHC polarization and the RHC polarization through the LC layer is electrically tunable.
4.2. Pixel selection from different color pixels
Figure 5 shows the pixel selection from two color (green and blue) pixels made of two different CRMs under un-polarized incident light. For the blue pixel, a mixture of RMS11-066 and RMS11-067 at the weight ratio of 4:6 was used. The mixture was spin-coated at a relatively slow rate of 1500 rpm on the substrate, yielding a relatively thick CRM pattern so that the LC layer satisfies the condition for the half-wave retardation at the peak wavelength of blue. The thickness of the blue CRM pattern was 2 μm, allowing for the LC layer to be 1 μm thick. These values assured the thickness uniformity of the CRM and the requirement for the LC layer to behave as a half-wave retarder. Note that both the green and blue color pixels had the same cell gap of 3 μm between two substrates. In Fig. 5, the voltage was applied to the left pixels only in our device. Figures 5(a)-(c) show the green pixel selection by the applied voltage and the optical filter. The blue pixel selection by the applied voltage and the optical filter are presented in Figs. 5(d)-(f).
Fig. 5 Microscopic images showing the pixel selection from two types of color (green and blue) pixels made of two different CRMs. The green pixel selection by the applied voltage and the optical filter in (a)-(c). The blue pixel selection by the applied voltage and the optical filter in (d)-(f). In all cases, the voltage was applied to the left pixels only. Reflection of (a) green color and (b) blue color under no filter, reflection of (b) green color and (d) blue color through the RHC filter, and reflection of (c) green color and (f) blue color through the LHC filter in the presence of the applied voltage of about 2V. Scale bars are 500 μm.
As shown in Figs. 5(a) and 5(d), under no optical filter, all the pixels were appeared irrespective of the applied voltage. However, the reflected images of green color and blue color were observed through the RHC filter in Figs. 5(b) and 5(e). Similarly, in the presence of the applied voltage of about 2 V, the images of green color and blue color were retrieved under the LHC filter as shown in Figs. 5(c) and 5(f). It should be noted that the low fidelity of the CRM pattern observed inside a certain dotted rectangle was attributed to the patterning error involved in the pixel fabrication but not associated the intrinsic problem of the CRM. In principle, for given optical filter (the RHC or the LHC filter), the application of the applied voltage can select only one of two types of the pixels as clearly shown in Figs. 5(c) and 5(f). This concept can be further extended to an array of multi-color pixels similar to a color filter array of a common LC display [30].
4.3. Optical switching in array and image selection by polarization
Figure 6 shows the optical switching capability in 22 array by the polarization state according to the applied voltage. The voltage map in the array of 2x2 pixels was given in the top. The pixels under the applied voltage were represented by ‘1’ and those under no voltage were denoted by ‘0’. The pixel selection under the RHC filter was shown in Figs. 6(a)-(c) and those under the LHC filter were in Figs. 6(d)-(f).
Fig. 6 Microscopic images showing the optical switching capability in 2x2 array by the polarization state according to the applied voltage. The pixel selection under the RHC filter in (a)-(c) and under the LHC filter in (d)-(f). The voltage map in the array of 2x2 pixels was given in the top. Scale bars are 500 μm.
Depending the type of the optical filter (the RHC or the LHC filter) employed, the information stored in either the on-state (‘1’) of the off-state (‘0’) can be retrieved. The incident light was un-polarized. As clearly seen in Fig. 6, the information in the off-state is complementary to that in the on-state. Under the RHC filter, the selection of the desired pixels (or the reflection from the desired pixels) by the application of the applied voltage was performed in sequence in Figs. 6(a)-(c). The complementary cases were demonstrated in Figs. 6(d)-(f). Of course, all ‘0’ states and all ‘1’ states in 22 array can be easily achieved. As explained above, the pixel fidelity in our case was not sufficiently high to fully block or transmit the reflected light through the individual pixels. However, the underlying concept behind the optical selection mechanism is valid for a wide range of applications including optical logic, image processing, and anti-counterfeiting. The microscopic physical origins of the light leakage and/or blocking come from the disruption of the LC alignment, the variations of the pretilt angles of the LC molecules, and the development of the defects including disclinations and point defects around the CRM patterns in protrusion. In fact, such leakage of light near the protrusions in the LC device was observed previously [31].
Finally, let us examine the image selection from two images stored in our device by the polarization state according to the applied voltage. This is an extended version of the pixel selection described above. Two types of the characters (‘SNU’ and ‘MIPD’) were stored using the CRM patterns at the wavelength of green color as shown in Fig. 7(a).
Fig. 7 Image selection by the polarization state according to the applied voltage. (a) Microscopic images of ‘SNU’ and ‘MIPD’ stored in our LC device. The voltage was applied to the upper region where the characters of ‘SNU’ were patterned. (b) Under the applied voltage, the retrieval of only the image of ‘SNU’ through the RHC filter and (c) under no applied voltage, the retrieval of only the image of ‘MIPD’ through the LHC filter. Scale bars are 500 μm.
In a similar fashion to the pixel case, for given optical filter (the RHC or the LHC filter), only one of the two images can be independently retrieved according to the applied voltage. In the presence of the applied voltage, only the image of ‘SNU’ was observed through the RHC filter as shown in Fig. 7(b). On the other hand, under no applied voltage, the image of ‘MIPD’ was appeared through the LHC filter in Fig. 7(c). Although our prototype LC device presents a limited number of pixels/images, the chiral surface approach would be applicable for a variety of the symbols, images, or even videos created using two orthogonal states of the circular polarization.
5. Concluding remarks
We demonstrated the concept of the chiral image storage and retrieval based on the LC by two orthogonal states of the circular polarization. Our prototype LC device was simply constructed using solidified CRMs for the chiral surface images and the LC layer for tunable phase retardation. Depending on the applied voltage, the polarization conversion from the LHC polarization to the RHC polarization or vice versa was achieved. According to the polarization conversion, one of two images created by the CRM patterns was retrieved from the LC device. Basically, the combination of the chiral nature of the image patterns and the polarization switching capability of the LC layer allows the image storage and selection by two orthogonal states of the circular polarization. By multi-stacking different color CRM patterns and/or reducing the size of the CRM pixel below the sensitivity of the human eye, our concept presented here provides a new way of storing multiple images in the same area and selecting the desired image by the combination of optical and electrical retrieval schemes. This versatile scheme for the image selection will also to the development of high-performance reflective-type 3D displays and sophisticated anti-counterfeiting devices.
Funding
This work was supported by the BK 21 Plus Project funded by the Ministry of Education of Korea and Samsung Display Company.
References
1. X. Li, J. W. Chon, S. Wu, R. A. Evans, and M. Gu, “Rewritable polarization-encoded multilayer data storage in 2,5-dimethyl-4-(p-nitrophenylazo)anisole doped polymer,” Opt. Lett. 32(3), 277–279 (2007). [CrossRef] [PubMed]
2. D. McPhail and M. Gu, “Use of polarization sensitivity for three-dimensional optical data storage in polymer dispersed liquid crystals under two-photon illumination,” Appl. Phys. Lett. 81(7), 1160–1162 (2002). [CrossRef]
3. H. H. Pham, I. Gourevich, J. K. Oh, J. E. N. Jonkman, and E. Kumacheva, “A multidye nanostructured material for optical data storage and security data encryption,” Adv. Mater. 16(6), 516–520 (2004). [CrossRef]
4. A. B. Taylor, P. Michaux, A. S. Mohsin, and J. W. Chon, “Electron-beam lithography of plasmonic nanorod arrays for multilayered optical storage,” Opt. Express 22(11), 13234–13243 (2014). [CrossRef] [PubMed]
5. P. Zijlstra, J. W. Chon, and M. Gu, “Five-dimensional optical recording mediated by surface plasmons in gold nanorods,” Nature 459(7245), 410–413 (2009). [CrossRef] [PubMed]
6. W. Yue, S. S. Lee, and E. S. Kim, “Angle-tolerant polarization-tuned color filter exploiting a nanostructured cavity,” Opt. Express 24(15), 17115–17124 (2016). [CrossRef] [PubMed]
7. T. Ellenbogen, K. Seo, and K. B. Crozier, “Chromatic plasmonic polarizers for active visible color filtering and polarimetry,” Nano Lett. 12(2), 1026–1031 (2012). [CrossRef] [PubMed]
8. L. Duempelmann, D. Casari, A. Luu-Dinh, B. Gallinet, and L. Novotny, “Color rendering plasmonic aluminum substrates with angular symmetry breaking,” ACS Nano 9(12), 12383–12391 (2015). [CrossRef] [PubMed]
9. H. Yun, S. Y. Lee, K. Hong, J. Yeom, and B. Lee, “Plasmonic cavity-apertures as dynamic pixels for the simultaneous control of colour and intensity,” Nat. Commun. 6(1), 7133 (2015). [CrossRef] [PubMed]
10. E. Heydari, J. R. Sperling, S. L. Neale, and A. W. Clark, “Plasmonic Color Filters as Dual-State Nanopixels for High-Density Microimage Encoding,” Adv. Funct. Mater. 27(35), 1701866 (2017). [CrossRef]
11. X. M. Goh, Y. Zheng, S. J. Tan, L. Zhang, K. Kumar, C. W. Qiu, and J. K. Yang, “Three-dimensional plasmonic stereoscopic prints in full colour,” Nat. Commun. 5(1), 5361 (2014). [CrossRef] [PubMed]
12. C.-T. Lee, H.-Y. Lin, and C.-H. Tsai, “Designs of broadband and wide-view patterned polarizers for stereoscopic 3D displays,” Opt. Express 18(26), 27079–27094 (2010). [CrossRef] [PubMed]
13. W. T. Chen, K. Y. Yang, C. M. Wang, Y. W. Huang, G. Sun, I. D. Chiang, C. Y. Liao, W. L. Hsu, H. T. Lin, S. Sun, L. Zhou, A. Q. Liu, and D. P. Tsai, “High-efficiency broadband meta-hologram with polarization-controlled dual images,” Nano Lett. 14(1), 225–230 (2014). [CrossRef] [PubMed]
14. H. Kim, J. Kim, J. Kim, B. Lee, and S.-D. Lee, “Liquid crystal-based lenticular lens array with laterally shifting capability of the focusing effect for autostereoscopic displays,” Opt. Commun. 357, 52–57 (2015). [CrossRef]
15. H. Kang, S. Roh, I. Baik, H. Jung, W. Jeong, J. Shin, and I. Chung, “A novel polarizer glasses-type 3D displays with a patterned retarder,” SID Symp. Digest Tech. Pap. 41, 1–4 (2010). [CrossRef]
16. K.-S. Bae, U. Cha, Y.-K. Moon, J. W. Heo, Y.-J. Lee, J.-H. Kim, and C.-J. Yu, “Reflective three-dimensional displays using the cholesteric liquid crystal with an inner patterned retarder,” Opt. Express 20(7), 6927–6931 (2012). [CrossRef] [PubMed]
17. Y. J. Wu, Y. S. Jeng, P. C. Yeh, C. J. Hu, and W. M. Huang, “Stereoscopic 3D display using patterned retarder,” SID Symp. Digest Tech. Pap. 39, 260–263 (2008). [CrossRef]
18. Y. Cui, R. S. Hegde, I. Y. Phang, H. K. Lee, and X. Y. Ling, “Encoding molecular information in plasmonic nanostructures for anti-counterfeiting applications,” Nanoscale 6(1), 282–288 (2014). [CrossRef] [PubMed]
19. Y. Heo, H. Kang, J. S. Lee, Y. K. Oh, and S. H. Kim, “Lithographically Encrypted inverse opals for anti-counterfeiting applications,” Small 12(28), 3819–3826 (2016). [CrossRef] [PubMed]
20. H. Hu, Q.-W. Chen, J. Tang, X.-Y. Hu, and X.-H. Zhou, “Photonic anti-counterfeiting using structural colors derived from magnetic-responsive photonic crystals with double photonic bandgap heterostructures,” J. Mater. Chem. 22(22), 11048–11053 (2012). [CrossRef]
21. Y. Liu, Y. H. Lee, Q. Zhang, Y. Cui, and X. Y. Ling, “Plasmonic nanopillar arrays encoded with multiplex molecular information for anti-counterfeiting applications,” J. Mater. Chem. C Mater. Opt. Electron. Devices 4(19), 4312–4319 (2016). [CrossRef]
22. H. Ditlbacher, J. R. Krenn, B. Lamprecht, A. Leitner, and F. R. Aussenegg, “Spectrally coded optical data storage by metal nanoparticles,” Opt. Lett. 25(8), 563–565 (2000). [CrossRef] [PubMed]
23. D. W. Berreman, “Optics in stratified and anisotropic media: 4×4-matrix formulation,” J. Opt. Soc. Am. 62(4), 502–510 (1972). [CrossRef]
24. Y. C. Hsiao and W. Lee, “Electrically induced red, green, and blue scattering in chiral-nematic thin films,” Opt. Lett. 40(7), 1201–1203 (2015). [CrossRef] [PubMed]
25. Y. Mohri, J. Kobashi, H. Yoshida, and M. Ozaki, “Morpho-butterfly-inspired patterning of helical photonic structures for circular-polarization-sensitive, wide-angle diffuse reflection,” Adv. Opt. Mater. 5(7), 1601071 (2017). [CrossRef]
26. A. C. Tasolamprou, M. Mitov, D. C. Zografopoulos, and E. E. Kriezis, “Theoretical and experimental studies of hyperreflective polymer-network cholesteric liquid crystal structures with helicity inversion,” Opt. Commun. 282(5), 903–907 (2009). [CrossRef]
27. M. E. McConney, V. P. Tondiglia, J. M. Hurtubise, L. V. Natarajan, T. J. White, and T. J. Bunning, “Thermally induced, multicolored hyper-reflective cholesteric liquid crystals,” Adv. Mater. 23(12), 1453–1457 (2011). [CrossRef] [PubMed]
28. J.-D. Lin, C.-L. Chu, H.-Y. Lin, B. You, C.-T. Horng, S.-Y. Huang, T.-S. Mo, C.-Y. Huang, and C.-R. Lee, “Wide-band tunable photonic bandgaps based on nematic-refilling cholesteric liquid crystal polymer template samples,” Opt. Mater. Express 5(6), 1419–1430 (2015). [CrossRef]
29. D. Y. Kim, C. Nah, S. W. Kang, S. H. Lee, K. M. Lee, T. J. White, and K. U. Jeong, “Free-standing and circular-polarizing chirophotonic crystal reflectors: photopolymerization of helical nanostructures,” ACS Nano 10(10), 9570–9576 (2016). [CrossRef] [PubMed]
30. R. W. Sabnis, “Color filter technology for liquid crystal displays,” Displays 20(3), 119–129 (1999). [CrossRef]
31. S.-U. Kim, B.-Y. Lee, J.-H. Suh, J. Kim, J.-H. Na, and S.-D. Lee, “Reduction of gamma distortions in liquid crystal display by anisotropic voltage-dividing layer of reactive mesogens,” Liq. Cryst. 44, 1 (2016). [CrossRef]
