Multiple-Color Reflectors Using Bichiral Liquid Crystal Polymer Films and Their Applications in Liquid Crystal Displays.

Multiple-color reflectors using bichiral liquid crystal polymer films (BLCPFs) are investigated. The BLCPFs consist of alternate layers of two different single-pitch cholesteric liquid crystal (CLC) layers, named CLC#A and CLC#B. The thickness of each CLC layer equals its single pitch length. The optical properties in terms of reflections, reflection-wavelength ranges, and distributions of reflection spectra of the BLCPFs that result from the fixed pitch length of CLC#A along with the decrease of the pitch length of CLC#B are qualitatively simulated and investigated. The results indicate that the above optical properties of the BLCPFs depend on the LC birefringence and pitch lengths of CLC#A and CLC#B layers. The concept of fabrication method of the BLCPFs by using polymerizable CLCs and thin films of poly(vinylalcohol) or photoalignment materials is discussed. They have potential practical applications in functional color filters, asymmetrical transmission systems, etc., owing to the multiple reflection bands of BLCPFs. Moreover, the BLCPFs, which can enhance the color gamut and light-utilization efficiency of light sources/LC displays, are reported herein.

Optical manipulation of nematic colloids at the interfaces in azo-dye-doped liquid crystals.

This study demonstrates the optical manipulation of colloids dispersed in azo-dye-doped liquid crystals (DDLCs) where the accumulation occurs at the interfaces of the phase domains. We explain the mechanism related to the formation of the domains and the movement of the colloids in DDLCs with respect to the isomerization of azo-dye molecules via the illumination of laser beams. The colloids are dragged to the interfaces of the isotropic/nematic domain and the air bubble/isotropic domain by molecular interaction and Marangoni flows.

Fabrication of optical vortex lattices based on holographic polymer-dispersed liquid crystal films.

This Letter demonstrates optical vortex lattices based on holographic polymer-dispersed liquid crystal (HPDLC) films. The fabrication method uses a phase-only reflective spatial light modulator with numerically calculated phase profiles loaded on it to simplify multi-helical-wave interference. The beam profiles of the diffraction beams are simulated using the Fourier transform and compared with experimental results. The topological charges of the 1st-order diffraction beams reconstructed from the HPDLC films are examined using a Michelson interferometer.

Complex beam shaping based on an equivalent q-plate system and analysis of its properties using digital holography polarization imaging.

In this study, we generate various complex beams carrying angular momentum (AM) by using a programmable beam shaping system to mimic typical q-plates. When a circularly polarized wave is incident onto the system, the emerging beam reverses its spin handedness and obtains a spatial phase factor. This phase factor can be engineered by designing a computer-generated hologram (CGH) and applying it to a spatial light modulator (SLM) to produce a beam with controllable spatially distributed orbital angular momentum (OAM) density. To determine the properties of the generated fields, we combine digital holography (DH) with the beam shaping system to yield visualizations of the beam intensity, phase, and AM distributions over the transverse plane at different propagation distances. Comparisons of the theoretically and experimentally obtained results show good qualitative agreement. This study advances our understanding and interpretation of AM characteristics produced by a programmable q-plate-like system.

Optical simulation of single-cell-gap transflective liquid crystal displays based on surface anchoring energy of periodical nano-grooved structures.

The single-cell-gap transflective liquid crystal display (TR-LCD) based on periodic distribution of surface anchoring energy (SAE) of periodical nano-grooved structures is reported in this study. Different SAEs of planar-aligned nematic LC cells are associated with the threshold and operation voltages of the adopted LCs. Thus, according to the transmittance versus applied voltage curves, the regions with strong and weak SAEs in LC cells can be the transmission and reflection regions of a TR-LCD, respectively. According to the simulation results using 1D-DIMOS software, the phase retardation of a strong SAE region is approximately twice as large as that of a weak SAE region when a specific voltage is applied, and a suitable difference in SAE exists between these two regions. Moreover, various SAEs based on periodical nano-grooved structures can be fabricated to demonstrate the TR-LCD.

Smart electro-optical iris diaphragm based on liquid crystal film coating with photoconductive polymer of poly(N-vinylcarbazole).

This study develops a light shutter whose transmittance can be tuned electro-optically. The liquid crystal (LC) film applies the photoconductive material of poly(N-vinylcarbazole) (PVK) based on twisted nematic (TN) liquid crystals (LCs). The hole-transport layer of PVK could reduce the built-in electric field of the LC film under the exposure of UV light. The driving voltage is considerably decreased with the aid of UV light exposure. The repeating optical switching is performed under sunlight illumination with an external bias of electric field ∼5 V. Further, it could be operated under a parallel/cross-polarizer to change the light beam/ring as an iris and used to automatically block the UV light to protect an optical integrated system.

Independent Manipulation of Topological Charges and Polarization Patterns of Optical Vortices.

We present a simple and flexible method to generate various vectorial vortex beams (VVBs) with a Pancharatnam phase based on the scheme of double reflections from a single liquid crystal spatial light modulator (SLM). In this configuration, VVBs are constructed by the superposition of two orthogonally polarized orbital angular momentum (OAM) eigenstates. To verify the optical properties of the generated beams, Stokes polarimetry is developed to measure the states of polarization (SOP) over the transverse plane, while a Shack-Hartmann wavefront sensor is used to measure the OAM charge of beams. It is shown that both the simulated and the experimental results are in good qualitative agreement. In addition, polarization patterns and OAM charges of generated beams can be controlled independently using the proposed method.

Isothermal electrically and optically induced phase separation of liquid crystal and poly(N-vinylcarbazole) films.

This paper reports the electro- and photo-isomerization-induced isothermal phase separation of liquid crystals (LCs) and poly(N-vinylcarbazole) (PVK). The proposed phase separation process determines reformed PVK films on substrates to obtain switchable LC light valves. UV illumination induces simultaneous isothermal phase transition of the mixture and dissolution of PVK into the LCs. Phase separation of PVK and LCs occurs by the reversed phase transition via rapid electro-isomerization and slow dark-relaxation. During rapid phase separation, micron-sized LC domains (branch-like PVK structures) are generated to develop stable light scattering; during slow dark-relaxation, a uniform PVK film is obtained, thereby providing stable transparency.

Spiral phase plate based on polymer dispersed liquid crystal for wide visible band applications.

This study demonstrates helical wave fronts via a spiral phase plate based on polymer dispersed liquid crystals (PDLCs). Because the PDLC is electric tunable, the plate can be used in a wide visible band. In addition, if the probe beam deviates from the center of the sample, some of the light propagates out of the sectors. We propose some of the applications for the results.

Optical filter with tunable wavelength and bandwidth based on phototunable cholesteric liquid crystals.

An optical filter with tunable wavelength and bandwidth is demonstrated using two phototunable cholesteric liquid crystals (CLCs) configured in a reflection mode. In this mode, incident light is first reflected by one Azo-chiral-doped CLC and then by another one. The tuning mechanism, which is based on the pitch modulation of the CLCs that contain an Azo-chiral dopant, can be controlled by two pumping laser beams. The central wavelength can be tuned from 510 to 628 nm, and the bandwidth can be changed from 13 to 79 nm.

Photo- and electro-isomerization of azobenzenes based on polymer-dispersed liquid crystals doped with azobenzenes and their applications.

We report the photo-isomerization and electro-isomerization effects in azobenzenes-doped polymer-dispersed liquid crystals during the switching of the liquid crystal (LC) device between transparent (cis-isomers dominant) and scattering states (trans-isomers dominant). The isothermal phase transition, which is a result of the illumination of UV light and the application of DC voltage, was the main mechanism to switch the LC device between transparency, scattering, and gray scales. This study discusses in detail the variations in the population of cis-isomers as functions of the period and the amplitude of the applied DC voltage.

Direct optical switching of bistable cholesteric textures in chiral azobenzene-doped liquid crystals.

The direct optical switching of bistable cholesteric textures (i.e., planar and focal conic textures) in chiral azobenzene-doped liquid crystals (LCs) is demonstrated. Chiral azobenzene is a chiral dopant with optically tuned helical twisting power that results from the photo-isomerization between trans- and cis- isomers via exposure to UV or visible light. The pitch length of the material can be optically and repeatedly elongated and shortened. With regard to free energy, LCs tend to be stable at planar (focal conic) textures when pitch length is elongated (shortened) by exposure to UV (visible) light. Thus, direct optical switchable LC displays are investigated.

All-optically controllable and highly efficient scattering mode light modulator based on azobenzene liquid crystals and poly(N-vinylcarbazole) films.

The present study reports that isothermal phase transition induced by photoisomerization of azobenzene liquid crystals (azo-LCs) from trans- to cis-isomers results in the dissolution of poly(N-vinylcarbazole) (PVK) into azo-LCs. Transparent (scattering) states can be demonstrated using uniform (rough) morphologies of PVK generated by slow (rapid) phase separation of PVK and azo-LCs from cis- to trans-isomers. The PVK films were examined in detail using scanning electron microscopy. Scattering performance resulting from the rough PVK surface induced micron-sized LC domains, and transparent performance resulting from the reformed uniform PVK surface can be optically and reversibly switched. Finally, all-optically controllable and highly efficient (contrast ratio of 370:1) scattering mode light modulators based on azo-LCs and PVK films were demonstrated.

Bichromatic optical switch of diffractive light from a BCT photonic crystal based on an azo component-doped HPDLC.

This study demonstrates all optical switches between the four diffractive light levels of a body-centered tetragonal photonic crystal. The sample is based on holographic polymer-dispersed liquid crystals that are fabricated using a two-beam interference with multiple exposures. The switching mechanism bases on the effective index modulation of the PC that contains a liquid crystal/azo-dye mixture could be controlled by two pumping laser beams. The switching time between the blue-laser-pumped and the blue-and-green-laser-pumped levels is fast. The study also discusses the switching time of the various levels.

Optically and thermally controllable light scattering based on dye-doped liquid crystals in poly(N-vinylcarbazole) films-coated liquid crystal cell.

This paper presents the optically controllable light scattering based on dye-doped liquid crystals (DDLCs) in a cell, whose substrates are coated with poly(N-vinylcarbazole) (PVK) films. The optical control mechanism is the light-induced dissolution of PVK in DDLCs, which reforms the disordered LC distribution into multiple and micron-sized LC domains. The induced thermal effect on the process is investigated in detail. Scanning electron microscopy images are obtained to show the surface structures of the produced PVK films. The generated scattering can be switched back to the original one by particular thermally induced phase separation. Results indicate that the light-induced thermal effect and photoisomerization lead to the dissolution of PVK in DDLCs. Finally, scattering mode light shutter with different transmission is successfully achieved by illuminating the cell under various light intensities.

High-efficiency Fresnel lens fabricated by axially symmetric photoalignment method.

In this study, a Fresnel lens with radial and azimuthal liquid crystal (LC) alignments in the odd and even zones was fabricated using the photoalignment technique based on an azo dye doped in LC cells. The lens has approximately 35% focusing efficiency as determined using a linearly polarized probe beam. In addition, the lens converts the input linear polarization into axially symmetrical polarization at the focal plane. The fabricated Fresnel lens is polarization-independent and has electrically controllable focusing efficiency. Moreover, the far-field pattern of a probe beam through the device placed between the polarizers agrees with the pattern obtained from the simulation.

Multimode lasing from the microcavity of an octagonal quasi-crystal based on holographic polymer-dispersed liquid crystals.

An eightfold photonic quasi-crystal (PQC) sample is fabricated holographically using two-beam interference with multi-exposure based on polymer-dispersed liquid crystals. The transmission spectra from the finite-difference time-domain (FDTD) simulation prove the photonic stop band of the rotational symmetry structure of the sample. The resonant mode of the circular microcavity formed in the PQC is calculated. Amplified spontaneous emission and multimode lasing action are demonstrated from the pumped laser-dye-doped PQC microcavity using a Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) pulse laser.

All-optical controlling of the focal intensity of a liquid crystal polymer microlens array.

The current work demonstrates a liquid crystalline polymer microlens array (LCP MLA) with an all-optically tunable and multistable focal intensity through photochemical phase transition. The operational mechanism of the optical tuning is associated with the photoisomerization effect. The proposed LCP MLA device has a focusing unit based on a birefringence LCP and a tuning unit with a light responsive material to control the polarization state of the incident probe beam. The optically variable refractive indices of LCP enable a positive or negative MLA that can control the polarization of incident light to be realized.

Optical switch of diffractive light from a BCT photonic crystal based on HPDLC doped with azo component.

This study demonstrates an optical switch of the diffractive light from a body-centered tetragonal photonic crystal based on holographic polymer-dispersed liquid crystals that are fabricated using two-beam interference with multiple exposures. The liquid crystal-rich regions form the lattice points of the PC, which contains a liquid crystal/azo-dye mixture. The concentration of the cis isomer changes under laser light exposure; this change, in turn, modulates the effective index of the LCs, and then switches diffractive light.

Dual liquid crystal alignment configuration based on nanoparticle-doped polymer films.

This paper proposes an approach for producing dual liquid crystal (LC) alignment configuration based on nanoparticle-doped polymer films. Experimental results indicate that illuminating a nanoparticle-doped pre-polymer film, coated onto a substrate with a homogeneous alignment layer, with unpolarized UV light through a photomask causes the polymerization of pre-polymer, ultimately generating homogeneous and vertical alignment layers in unpolymerized and polymerized regions, respectively. The dual LC alignment configuration of the homogeneous (vertical) and hybrid alignments can be achieved by combining the treated substrate with another substrate that has a homogeneous (vertical) alignment layer. Additionally, the applications of the dual LC alignment layer in phase gratings and transflective LC displays are demonstrated.