Xinghai Lu

Wavefront correction based on a reflective liquid crystal wavefront sensor

A novel wavefront sensor (WFS) based on liquid crystal on silicon (LCOS) is demonstrated. The design of the liquid crystal microlens array with LCOS is given in detail. To save the energy of the incident light, an optimized optical design of the WFS is also considered. In addition, closed loop wavefront correction experiments were conducted based on the designed LC WFS with square and hexagonal microlens arrays, respectively. In the experiment, the corrector was a tilt and tip mirror. The results indicated that the Zernike coefficients of the tilt and tip were decreased to less than 0.1λ(633 nm) after correction.

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.

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.

A simple method for evaluating the wavefront compensation error of diffractive liquid-crystal wavefront correctors

A simple method for evaluating the wavefront compensation error of diffractive liquid-crystal wavefront correctors (DLCWFCs) for atmospheric turbulence correction is reported. A simple formula which describes the relationship between pixel number, DLCWFC aperture, quantization level, and atmospheric coherence length was derived based on the calculated atmospheric turbulence wavefronts using Kolmogorov atmospheric turbulence theory. It was found that the pixel number across the DLCWFC aperture is a linear function of the telescope aperture and the quantization level, and it is an exponential function of the atmosphere coherence length. These results are useful for people using DLCWFCs in atmospheric turbulence correction for large-aperture telescopes.

Modal interaction matrix measurement for liquid-crystal corrector: precision evaluation

A modal interaction matrix (IM) measurement procedure is introduced for a liquid-crystal (LC) corrector for use in a phase-wrapping technique. Zernike modes are used to reconstruct the aberration wavefront and to drive the LC corrector. Usually the driving area is different from the active area. This difference induces a coupling effect on Zernike modes, which may have an impact on correction precision. In this paper the coupling effect is evaluated due to area difference and decentration, respectively. Then, a simulated turbulence wavefront is used to simulate the reconstruction process to evaluate its influence on reconstruction precision. We present simulation results that show that this method can be used to measure the IM with very high reconstruction precision under proper configuration. In order to maintain precision, the permissible eccentricity distance is also simulated with a result of no more than 5% of the LC corrector diameter.

Analysis of display defects in the multi-domain vertical alignment mode liquid crystal display

Abstract Two types of display defects were found at special positions near the pixel indium tin oxide (ITO) edge in the multi-domain vertical alignment (MVA) mode liquid crystal display (LCD). Through simulated and experimental results, the formation mechanisms of these two types of display defects, the detailed configurations of liquid crystal (LC) orientations at the display defect regions and the principle to suppress the defect dark line were analyzed and discussed. It was further found that the occurrence of the dark line was related to the positioning accuracy of substrate assembly. The reason of the dark line occurrence related with the positioning accuracy and the manufacturing margin were provided here.

A polarization independent liquid crystal adaptive optics system

A liquid crystal device is normally limited to use with polarized light, resulting in a loss of 50% of the incident unpolarized light. In this paper, a polarization independent liquid crystal adaptive optics wavefront correction system is demonstrated. In the demonstration system, a quarter-waveplate (QWP) and mirror combination are used to rotate the plane of polarization by 90° on the return pass. The result is that both orthogonal components of the incident unpolarized light are phase modulated by the liquid crystal on silicon (LCOS) device. Theoretical analysis shows that the optical throughput of the novel system is about 19.7% higher than that of a standard, polarized LC adaptive optics system. As a demonstration, we used the LC adaptive optics system to correct the aberration of unpolarized light, and obtained a clear image of the unpolarized light source.

Optical hyperbolic metamaterials based on nanoparticles doped liquid crystals

Self-aligned liquid crystals containing randomly dispersed nanoparticles are proposed to realise hyperbolic metamaterials at visible spectrum. Opposite signs for anisotropic permittivity tensors originated from the scattering of metallic nanoparticles in liquid crystals are achieved. Tunable effective wavelength properties are also demonstrated by choosing dielectric-metal core-shell nanoparticles.

A high precision phase reconstruction algorithm for multi-laser guide stars adaptive optics*

Adaptive optics (AO) systems are widespread and considered as an essential part of any large aperture telescope for obtaining a high resolution imaging at present. To enlarge the imaging field of view (FOV), multi-laser guide stars (LGSs) are currently being investigated and used for the large aperture optical telescopes. LGS measurement is necessary and pivotal to obtain the cumulative phase distortion along a target in the multi-LGSs AO system. We propose a high precision phase reconstruction algorithm to estimate the phase for a target with an uncertain turbulence profile based on the interpolation. By comparing with the conventional average method, the proposed method reduces the root mean square (RMS) error from 130 nm to 85 nm with a 30% reduction for narrow FOV. We confirm that such phase reconstruction algorithm is validated for both narrow field AO and wide field AO.

The optimal cell gap determination of a liquid crystal wavefront corrector from a single photoelectric measurement

It has been a crucial technique to improve the dynamic response characteristics of a liquid crystal wavefront corrector (LCWFC) with optimal cell gap since the LCWFC needs at least 2π (or π) phase modulation in adaptive optics systems (AOSs). We have given a complete process for obtaining the optimal cell gap accurately from a single photoelectric measurement, which can be conducted with a liquid crystal (LC) cell of any known thickness. This method has been analysed theoretically and confirmed experimentally by using a wedge-shaped cell; the experimental results match very well with the theoretical analysis. The response time of an optimal gap cell can be a novel evaluation method of response performance of LC materials.