Zenghui Peng

Preliminary use of nematic liquid crystal adaptive optics with a 2.16-meter reflecting telescope.

A nematic liquid crystal adaptive optics system (NLC AOS) was assembled for a 2.16-m telescope to correct for atmospheric turbulence. LC AOS was designed and optimized with Zemax optical software. Second, an adaptive correction experiment was performed in the laboratory to test the performance of the NLC AOS. After the correction, the peak to valley (PV) and root mean square (RMS) of the wavefront were down to 0.2 lambda (lambda=633 nm) and 0.05 lambda, respectively. Finally, the star of Pollux (beta Gem) was tracked using the 2.16-m Reflecting Telescope, and real time correction of the atmospheric turbulence was performed with the NLC AOS. After the adaptive correction, the average PV and RMS of the wavefront were reduced from 11 lambda and 2.5 lambda to 2.3 lambda and 0.6 lambda, respectively. Although the intensity distribution of the beta Gem was converged and its peak was sharp, a halo still existed around the peak. These results indicated that the NLC AOS only partially corrected the vertical atmospheric turbulence. The limitations of our NLC AOS are discussed and some proposals are made.

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

Anisotropic waveguide theory for electrically tunable distributed feedback laser from dye-doped holographic polymer dispersed liquid crystal

Anisotropic waveguide theory is developed for electrically tunable distributed feedback (DFB) laser from dye-doped holographic polymer dispersed liquid crystal (HPDLC) grating. The period grating structure, optical anisotropy of the liquid crystal (LC) and practical light propagation path in the HPDLC have been considered. The emitted lasing wavelength is deduced on basis of the dielectric anisotropy of the LC, transverse-magnetic (TM) light wave propagation in the core layer and DFB laser theory. An experimental method to determine the tilt angle of the LC and the lasing behaviours under different electric fields are used to verify the validity of the anisotropic numerical analysis. The results show that a more accurate agreement between the theoretical calculations and the experimental data is achieved. The anisotropic numerical analysis presented here is very useful when designing and optimising tunable lasers for optical communications and integrated optics.

Advanced single-frame overdriving for liquid-crystal spatial light modulators

A single-frame overdriving scheme was employed to improve the temporal response of the active matrix addressing liquid-crystal spatial light modulator used in an open-loop adaptive optics system (OLAOS). Optimal time distribution giving minimum wavefront residual error for the OLAOS was demonstrated. As a result, the measured -3 decibels rejection frequency was increased from 26 to 35 Hz, and the image quality was significantly improved.

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.

Low threshold of distributed feedback lasers based on scaffolding morphologic holographic polymer dispersed liquid crystal gratings: reduced losses through Forster transfer

The threshold of distributed feedback (DFB) lasing from dye-doped holographic polymer dispersed liquid crystal (HPDLC) gratings was reduced using a guest–host (G–H) system. Different doping concentrations of the guest emitter 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) in the host dye 1,3,5,7,8-pentamethyl-2,6-diethylpyrromethene-difluoroborate (PM567) were dispersed in the HPDLC transmission gratings. DFB lasing was observed in the pure host, the pure guest and in the G–H system. With the increase in doping concentration of the guest, the gain spectrum was redshifted. Through effective Forster energy transfer, we found that the lasing threshold was lowered to 1.35 μJ/pulse, which reduced by factor 2 compared to that without energy transfer process. This effect is attributed to the suppression of self-absorption at the lasing wavelength.

Photoalignment of liquid crystals by a covalently attached self-assembled ultrathin film

In this paper, the polyanion-containing cinnamoyl group (PACSS-CF3) was self-assembled with diazoresin (DR) to form a kind of stable covalent ultrathin film by irradiation with 365 nm UV light. The photoalignment properties of the DR/PACSS-CF3 covalent film were investigated. The covalent film was found to have anisotropy after irradiation by 297 nm linearly polarised ultraviolet light (LPUVL), and could induce uniform alignment of liquid crystals (LCs). The pretilt angle of the LC was 2.5°. The stability of the film was enhanced by the covalent bonds. The films were thermally stable to 180°C. Polarised UV-Vis spectroscopy was utilised to investigate the photochemical process of the covalent film. It was found that cinnamoyl moieties parallel to the polarisation direction of the LPUVL were consumed by the photoreaction faster than those perpendicular to the polarisation direction. It can be concluded that the selective photoreaction induced the anisotropy of the films. The anisotropic films induced the homogeneous alignment of LC.

Alignment of liquid crystals induced by a photopolymerized self-assembled film

An alignment film derived from a photopolymerized self‐assembled film may be used to orient nematic liquid crystals after irradiating the film with linearly polarized UV (LPUV). A photosensitive cationic amphiphile was first synthesized containing two double bonds and which could be polymerized by UV. A layer‐by‐layer self‐assembled multilayer film was next prepared in an aqueous solution of the cationic amphiphile and poly(sodium 4‐styrenesulphonate); the UV‐Vis spectra showed that each layer of the LBL multilayer film was uniform. When the film was irradiated by LPUV, the photosensitive double bonds underwent [2+2] cycloaddition along the vector direction of LPUV. The polarized UV‐Vis absorption spectra also provided evidence that the film was anisotropic, i.e. the photopolymerization was along a certain direction. The anisotropic film was used as an alignment layer for nematic liquid crystals, and observations under a polarizing microscope indicated that the alignment of the liquid crystals was good, as expected, and that the orientation direction of the liquid crystals was always perpendicular to the electric vector of the irradiating LPUV.

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.