Lishuang Yao

Theory and characteristics of holographic polymer dispersed liquid crystal transmission grating with scaffolding morphology

We have performed a detailed characterization of the optical properties of a holographic polymer dispersed liquid crystal (LC) transmission grating with polymer scaffolding morphology, which was fabricated with conventional high-functionality acrylate monomer under low curing intensity. Temporal evolution of the grating formation was investigated, and the amount of phase-separated LC was determined by birefringence investigation. A grating model combined with anisotropic coupled-wave theory yielded good agreement with experimental data without any fitting parameter. The results in this study demonstrate the non droplet scaffolding morphology grating is characterized by a high degree of phase separation (70%), high anisotropy, low scattering loss (<6%), and high diffraction efficiency (95%).

Electrically Tunable Distributed Feedback Laser Emission from Scaffolding Morphologic Holographic Polymer Dispersed Liquid Crystal Grating

Electrically tunable distributed feedback (DFB) laser emission from a dye-doped holographic polymer dispersed liquid crystal (HPDLC) transmission grating is demonstrated. The homogenously aligned liquid crystal (LC) in this polymer scaffolding morphologic grating enables a large refractive index (RI) change for the TM wave under applied electric field, which in turn leads to a red-shift of 8 nm in the output laser emission. The tuning behavior of the DFB laser is well explained on the basis of DFB waveguide theory and amount of phase-separated LCs. Copyright 2013 The Japan Society of Applied Physics.

Electrooptical properties of new type fluorinated phenyl-tolane isothiocyanate liquid crystal compounds

ABSTRACT A series of 4ʹ-(4-alkyl-phenyl)-2ʹ,6ʹ-difluorotolane isothiocyanate liquid crystal (LC) compounds were synthesised, and their phase transitions and electrooptical properties were investigated. These compounds exhibit high birefringence, about 0.47–0.52. As the number of carbon atoms in the alkyl chains increases, these LC compounds can exhibit smectic phases. When these LCs were mixed into the commercial LCs, the birefringence values of LC mixtures become higher than pure commercial LCs, and the visco-elastic coefficients of five LC mixtures are very close to each other at every test temperature. The results of response properties indicate that the compounds with a tri-fluorine substitution and n-propyl end group possess better response performance than the others. These LC compounds are particularly attractive for achieving fast response times in LC optic devices. Graphical Abstract

Improvement of the switching frequency of a liquid-crystal spatial light modulator with optimal cell gap

In the application of a nematic liquid-crystal (LC) spatial light modulator, we derived the formula of retardation dynamic response of the device by solving the Erickson-Leslie equation. Then, the response time of the 2π phase change can be expressed as a function of the LC cell gap. The theoretical and experimental results all indicate that the response time of 2π first decreases and then increases with the LC cell gap increasing, and there is an optimal cell gap to obtain the shortest response time. Therefore, the method of optimizing the cell gap shows potential to improve the switching frequency for all type of nematic LC optical device with specific modulation quantity.

Photoalignment of liquid crystals in a hydrogen‐bonding‐directed layer‐by‐layer ultrathin film

In this paper, a new type of hydrogen‐bonding‐directed layer‐by‐layer (LBL) ultrathin film with photosensitive cinnamoyl chromophores was prepared. It could induce uniform alignment of liquid crystals (LCs) after being irradiated by linearly polarised ultraviolet light (LPUVL). The hydrogen‐bonding‐directed formation of a photochromic multilayer film was achieved on the basis of an alternating assembly of poly(4‐vinylpyridine) (PVPy) and poly(4‐acryloyloxy)cinnamic acid (PCA). The assembly process was followed by UV–visible spectroscopy. The nature of the interaction between the two polymers was identified as a hydrogen bond through Fourier transform infrared spectroscopy. When the film was irradiated by LPUVL, the cinnamoyl units in the film underwent [2+2] cycloaddition along the electric vector direction of the LPUVL. The polarised UV–visible spectra proved that the film was anisotropic, and then the film could induce homogeneous alignment of nematic LCs. With the number increment of the bilayers, the alignment effect of the multilayer film was enhanced, and the contrast ratio became higher. The orientation direction of the LCs was always perpendicular to the electric vector of the irradiating LPUVL.

Optimal energy-splitting method for an open-loop liquid crystal adaptive optics system.

A waveband-splitting method is proposed for open-loop liquid crystal adaptive optics systems (LC AOSs). The proposed method extends the working waveband, splits energy flexibly, and improves detection capability. Simulated analysis is performed for a waveband in the range of 350 nm to 950 nm. The results show that the optimal energy split is 7:3 for the wavefront sensor (WFS) and for the imaging camera with the waveband split into 350 nm to 700 nm and 700 nm to 950 nm, respectively. A validation experiment is conducted by measuring the signal-to-noise ratio (SNR) of the WFS and the imaging camera. The results indicate that for the waveband-splitting method, the SNR of WFS is approximately equal to that of the imaging camera with a variation in the intensity. On the other hand, the SNR of the WFS is significantly different from that of the imaging camera for the polarized beam splitter energy splitting scheme. Therefore, the waveband-splitting method is more suitable for an open-loop LC AOS. An adaptive correction experiment is also performed on a 1.2-meter telescope. A star with a visual magnitude of 4.45 is observed and corrected and an angular resolution ability of 0.31″ is achieved. A double star with a combined visual magnitude of 4.3 is observed as well, and its two components are resolved after correction. The results indicate that the proposed method can significantly improve the detection capability of an open-loop LC AOS.

Wide-angle switchable negative refraction in high birefringence nematic liquid crystals

Negative refraction (NR) is a promising technique that provides an opportunity to manipulate beam behaviour. We have demonstrated experimentally the controllable NR with large refraction angle at visible wavelength in the novel high birefringence (Δn) liquid crystals. When the Δn of liquid crystal reaches 0.42, the NR angle and critical incident angle reach –14° and 23.2°, respectively, which are much larger than those achieved before by other liquid crystals. By applying the electric field, we control the switch from NR to positive.

Fast response property of low-viscosity difluorooxymethylene-bridged liquid crystals

In this study, three kinds of low-viscosity difluorooxymethylene-bridged (CF2O) liquid crystals (LCs) were synthesised. These materials exhibit low melting point and low viscosity at room temperature. The viscosity of the final LC mixture significantly decreased when LC compounds were mixed with high-birefringence (Δn) isothiocyanato LCs. The tested curves with varied temperatures of LC mixtures showed that some CF2O LC compounds could enhance the response performance of high-Δn LCs in a specific temperature range. Therefore, LC compounds with low viscosity could become a component of LC mixtures with fast response performance.

Single-cell-gap transflective liquid-crystal display and the use of photoalignment technology

— In this paper, transflective liquid-crystal-display (LCD) technology will be reviewed, and several new single-cell-gap transflective LCD configurations are proposed. Photoalignment technology is studied especially for transflective-LCD applications. In order to realize the optimal performance of the display as well as a matched transmittance/reflectance voltage curve (TVC/RVC) for the transflective configurations, two different single-cell-gap transflective-LCD approaches will be discussed. The first one is the dual-mode single-cell-gap approach, in which different liquid-crystal modes are applied to the transmissive and reflective subpixels of the transflective LCD. The other approach is the single-mode s ingle-cell-gap approach, in which an in-cell retardation film is applied to adjust the performance and TVC/RVC matching of a transflective LCD. Photoalignment technology is used to fabricate the dual-mode liquid-crystal cell in the first approach and also the in-cell retardation film in the second approach. Prototypes of the proposed configurations have been fabricated, which show good performance and a matched TVC/RVC.

Ultrafast switchable wide angle negative refraction in novel dual-frequency liquid crystal mixture

Ultrafast switchable wide angle negative refraction in a novel dual-frequency liquid crystal mixture is demonstrated experimentally. Laterally fluorinated isothiocyanato phenyl-tolane liquid crystal compound is doped into Merck MLC-2048 to acquire the high birefringence dual-frequency liquid crystal. The maximum negative refraction angle achieved is about 12° and the switching time is only 25 ms for a 40-μm cell which is two orders faster than switchable negative refraction achieved before in nematic liquid crystals. Such properties provide a feasible approach to design negative refraction devices based on liquid crystals.