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This study proposes a simple method of designing a high-speed liquid crystal variable retarder (LCVR) with reflection mode. First, a series of simple formulas is provided for analysing the effects of tilt incidence and birefringence of the liquid crystal on the phase retardation and response time of the LCVR. Then, a reflective LCVR is fabricated to validate the theoretical analysis. Measured results show that the response speed can reach 2.7 kHz with a phase retardation of 1 λ. Furthermore, the theoretical curve is close to the measured curve while the incident angle is less than 10°. However, the theoretical and measured values show a considerable difference under a large incident angle. This problem is discussed, and a modified method is given. This work is helpful for the design and fabrication of high-speed LCVR. [ABSTRACT FROM AUTHOR]

The isoplanatic patch of the human eye is a key parameter for the adaptive optics system (AOS) designed for retinal imaging. The field of view (FOV) usually sets to the same size as the isoplanatic patch to obtain high resolution images. However, it has only been measured at a specific wavelength. Here we investigate the wavelength dependence of this important parameter. An optical setup is initially designed and established in a laboratory to measure the isoplanatic patch at various wavelengths (655 nm, 730 nm and 808 nm). We established the Navarro wide-angle eye model in Zemax software to further validate our results, which suggested high consistency between the two. The isoplanatic patch as a function of wavelength was obtained within the range of visible to near-infrared, which can be expressed as: θ =0.0028 λ − 0 . 74 . This work is beneficial for the AOS design for retinal imaging. [ABSTRACT FROM AUTHOR]

The interaction matrix significantly affects the correction accuracy of wavefront correctors. For diffractive liquid crystal wavefront correctors (LCWFCs), the diffraction wavefront error must be taken into account to improve the accuracy of the interaction matrix. In this paper, a tunable Zernike polynomial coefficient method is demonstrated that decreases the effect of the diffraction wavefront error on the interaction matrix. Moreover, to eliminate the effect of the random error, a least squares method is combined with the tunable coefficient method. Experimental results show that, with the combined method, the error of the interaction matrix is decreased by a factor of 2.5 compared to that when the least squares method is used alone. Furthermore, an open loop adaptive correction experiment was performed and a considerable improvement of the image resolution was obtained by the novel method. Therefore, this method is very useful for liquid crystal adaptive optics systems to acquire the high correction accuracy. [ABSTRACT FROM AUTHOR]

We propose a voltage-tunable hyperbolic dispersion metamaterial (HDM) consisting of silver nanowires and nematic liquid crystals (NLCs). The metamaterial shows a novel electro-optic effect, in which the dispersion relation switches between elliptic and hyperbolic dispersions at the visible waveband with an external electric field. HDM functionality is analysed in detail based on the effective medium theory. 5CB NLC is chosen for the simulation, and tunability is achieved between 454 and 475 nm. The effects of filling ratio on tunability are also analysed. Tunable bandwidth widens from 21 to 45 nm with decreased filling ratio from 0.27 to 0.09. Finally, the effects of birefringence (Δn) are considered. Tunable bandwidth widens from 21 to 63 nm as Δnchanges from 0.2 to 0.6. These novel tunable performances of the HDM can be applied to optical devices and optical systems. [ABSTRACT FROM PUBLISHER]

An experimental analysis was conducted to investigate the fitting error of diffractive liquid crystal wavefront correctors (LCWFCs). First, an experiment was performed to validate the theoretical equations presented in our previous work Cao et al., 2009 [9] . The results showed an apparent discrepancy between the theoretical and measured results for the fitting error. This difference was examined and the influence of nonlinearities and rounding errors generated by the LCWFC was analyzed and discussed. Finally, the fitting error formula of the LCWFC was modified to obtain a more effective tool for the design of LCWFCs for atmospheric turbulence correction. These results will be useful for researchers who design liquid crystal adaptive optics systems for large-aperture ground-based telescopes. [ABSTRACT FROM AUTHOR]

Zonal method is widely used to reconstruct the wave-front. Up to now, the iterative algorithms have been used to reconstruct the arbitrary shape wave-front with high reconstruction accuracy. However, it has the shortcomings of long time consumption. To reduce the time delay, a shaped based method is proposed by adding the shape information into the geometry matrix. The simulated and experimental results indicate that the reconstruction accuracy of proposed method is similar to that of the iterative LS-based method, but the computation time of our method is 3 times less than that of the iteration method. Consequently, the high accuracy and low time consumption are simultaneously achieved with the proposed method. [ABSTRACT FROM AUTHOR]

Abstract: A novel liquid crystal wave front corrector (LCWFC) based open loop adaptive optics testbed for nonpolarized light was introduced in this paper. A polarizing beam splitter (PBS) was used behind the LCWFC to divide the reflected nonpolarized light into two orthogonal polarized lights with and without the modulation effect for high resolution imaging and aberration detection respectively. The alignment direction of LC molecules was parallel to the polarization direction of the light PBS reflected. This configuration was simple and easy for construction. It avoided the energy loss due to the use of polarizer for conventional LCWFC based closed loop adaptive optics system. Furthermore, it could be switched into closed loop configuration very easily, which was useful for quantitative system performance evaluation. The detailed construction and operation was reported in Section 2. In Section 3, this testbed was deployed at the coude focus of the 2.16m telescope at the Xinglong Station of the Beijing Astronomical Observatory. The observation of star Arcturus was done. The Full Width Half Maximum (FWHM) resolution was improved from 2.12″ to 0.64″ successfully. [Copyright &y& Elsevier]

Abstract: A method to improve the loop frequency of liquid crystal adaptive optics systems (LC AOSs) was investigated. Based on open-loop control, a concurrent control scheme was proposed and the correction frequency of the LC AOS was increased from 85Hz to 212Hz. A simulated correction analysis illustrates that the distortion rejection ability of our LC AOS was improved with −1.1dB by using the concurrent control technique. A 500m horizontal turbulence correction experiment was done to validate our considerations. Firstly, 500m horizontal turbulence was measured and its Greenwood frequency and atmospheric coherence length is 31.5Hz and 4–5cm, respectively. Then, 500m horizontal turbulence was corrected in real time by using consecutive and concurrent control methods. The corrected results indicate that the resolution and contrast of the image was significantly improved by the concurrent control method. These results show that the concurrent control method can be used for LC AOSs to improve the correction frequency. [Copyright &y& Elsevier]

Abstract: Temperature effect on the diffraction efficiency of the liquid crystal spatial light modulator is investigated. The birefringence of the liquid crystal as functions of the temperature is measured with and without the power supply. It is shown that the birefringence reduces while the temperature increases. And the change magnitude of the birefringence has an exponential decay relation with the applied voltage for different temperature intervals. The scalar diffractive theory is used to analyze this effect on the diffraction efficiency. It indicates that the diffraction efficiency decreases from 98.7% to 27.2% while the temperature increases from 10 to 90°C for 16 quantified levels. At last, temperature effect on its applications in optical testing and wavefront correction is discussed. It indicates that it has almost no effect on optical testing, but has an important effect on wavefront correction. And two solutions are given to eliminate this effect. [Copyright &y& Elsevier]

Abstract: The adaptive optics system (AOS) often operates in a discrete sampling process with finite closed-loop frequency. Reconstruction, detection, and time lag induced errors are the main correction errors of the system. An AOS that is based on a liquid crystal (LC) benefits from the LC’s high correction precision, thus the reconstruction error can be ignored. The primary error will be induced by the time lag from the time of detection to the time of compensation. In this paper, some theoretical simulations are introduced in order to evaluate the correction precision of AOS with an LC corrector. The main purpose is to compare the correction precision between the open-loop and closed-loop control. We attempt to find a method to ascertain the exact precision of the open-loop control and show whether it improves the correction precision. The conclusion is thus reached that the actual error rejection bandwidth for the closed-loop was lower than the −3dB error rejection bandwidth measured in practice. The increased refresh frequency of the open-loop control can improve the imaging performance to nearly −3dB bandwidth of the detector measured, which is the maximum possible bandwidth due to the time lag. [Copyright &y& Elsevier]

In the current solution for multiwavelength achromatic flat lenses, a one-to-one correspondence exists between the number of writing phase distributions and the number of achromatic wavelengths. Breaking this correspondence requires a complex phase design and parameter optimization. Here, we show that a dual-layer cascade liquid crystal Pancharatnam-Berry lens (CLCPBL) with two writing phase distributions and a specific coupled phase distribution between the layers can achieve three wavelength achromaticity without any parameter optimization process. Similarly, in a three-layer cascade, the number of achromatic wavelengths increases to seven through the permutations of the layers, with adjustable amplitude factors. We fabricate a three-layer CLCPBL with the design wavelengths of 396.8 nm, 1064 nm, and 1550 nm, which theoretically allows the light at 632.8, 532.8, 3383 and 450 nm to form a common focus, and test such structure. Our CLCPBL enables a wider range of applications than conventional achromatic flat lenses. Metalenses are lightweight and compact alternative to achromatic lens assemblies, but they usually fulfill the focusing requirements for a single wavelength. The authors design and fabricate a cascade liquid crystal Pancharatnam-Berry lens that enables a seven-wavelength achromatic focusing in a broad range of frequencies. [ABSTRACT FROM AUTHOR]

The poor time stability of surface-enhanced Raman scattering (SERS) substrates greatly limits their application potential. Although core–shell structures are commonly used to enhance stability, their complex preparation processes, high costs, and susceptibility under acidic or alkaline conditions result in serious disadvantages for practical applications. Here, we propose a new method of external oxygen barrier to improve spectral stability, in which SERS substrates are stored in an oxygen-free environment. Controlled experiments are carried out under air and vacuum. Raman spectrum intensity is measured 11 times within six months for each group. Using the attenuation formula, the Raman spectrum intensity decay results of each SERS substrate over time are obtained. The effectiveness of the external oxygen barrier method is demonstrated through curve fitting using the corresponding function. The substrate spectral attenuation rates of the vacuum group and the argon group within six months are <20%, proving the effectiveness of the external oxygen barrier method. [ABSTRACT FROM AUTHOR]

We present a weighted method of wavefront reconstruction for multiple laser guide stars (LGS) adaptive optics (AO) system. Unlike simple averaging method in the traditional ground layer adaptive optics (GLAO), the weighted method generates different weighting factors according to LGSs’ different positions for the reconstructed wavefront from different LGSs. And then the target aberration to correct could be obtained with summation of these wavefronts multiplied by their correspondent weighting factors. Wide-field simulations are presented and analyzed. Experiments in the lab are conducted to validate the method. The weighted method is better than the average method especially near the center area of FOV. At center of FOV, the SR is improved by about 0.2 compared to the average method and the SR decreases with enlargement of FOV. For the first time, the weighted wavefront reconstruction method effectively decreases the reconstruction error in multi-LGSs AO system. The simulation and experimental result show that weighted method is also promising to improve the imaging resolution of LGS adaptive optics systems in astronomical observations. [ABSTRACT FROM AUTHOR]

• The performance degradation and compensation method of the beam-steering device based on liquid crystal polarization grating (LCPG) and liquid crystal adjustable half-wave plate (LCAHWP) is studied. • This beam-steering device can still work at a laser power density of 16 W/cm2 irradiation. • The laser tolerance of the device is higher than that of liquid crystal optical phased array (LCOPA) and liquid crystal micro-lens array (LCMLA). The effects of laser irradiation thermal deposition on the performance degradation of liquid crystal (LC) devices in high-power laser systems are extremely vital. In this study, the first-order beam-steering system (FBS) was developed using a passive liquid crystal polarization grating (LCPG) and liquid crystal adjustable wave-plate (LCAW). The LCPG was prepared by holographic exposure technology, and the LCAW was prepared using the standard LC cell preparation process. We investigated the influences of diffraction efficiency of FBS under a 1070-nm continuous-wave (CW) laser irradiation. The energy is concentrated in the −1st order at first. When the half-wave retardation was reached, all the energy is concentrated in the +1st order (∼97 %), and the beam is deflected. Once the phase retardation decreases below the half-wave retardation, the energy gradually reconcentrates in the −1st order. When the LCAW fails, the diffraction the energy is refocused to the −1st order by the LCPG. The results indicated that both thermal deposition and driving voltage reduce the phase retardation of the LCAW, generating a variation of energy between in ± 1 st orders. By reducing the driving voltage, this half-wave retardation can be recovered under a certain half-wave voltage and laser power density, and the diffraction efficiency of the +1st order can be compensated to ∼97 %. If the reduced phase retardation is not less than half-wave, the FBS can still be modulated in high-power continuous lasers. Our research provides a reference for the development of beam-steering technology based on LC materials in high-power laser systems. [ABSTRACT FROM AUTHOR]

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-ΔnLCs in a specific temperature range. Therefore, LC compounds with low viscosity could become a component of LC mixtures with fast response performance. [ABSTRACT FROM AUTHOR]

Abstract: For an open-loop liquid-crystal adaptive optics system, its performance is mainly limited by the time delay. We propose a new modal recursive least-squares (RLS) predictive algorithm to overcome this problem. The RLS algorithm has a simple architecture, low computational complexity and a high converging speed. The impact of the number of the foreword prediction frame N and the number of the predictor order M of the RLS predictor is analyzed in detail. The results show that the best foreword prediction frame N must be equal to the system loop delay frame SLDF. The appropriate predictor order M of the RLS predictor is equal to 2 or 3 when there is no measurement noise and it depends on noise ratio N R when the measurement error cannot be neglected. We present numerical simulations to show the significant improvements brought by the RLS predictor. [Copyright &y& Elsevier]

We propose a thermal modulation structure made of two identical SiO2 slabs coated by Dirac semimetal (DSM) films and separated by a nanoscale vacuum gap. The energy transmission probability reveals that the coupled surface plasmon polaritons (SPPs) between the two DSM films, and the surface phonon polaritons (SPhPs) supported by the SiO2 substrate can vary sensitively with the Fermi level, the degenerate factor of 3D Dirac points and the thickness of the DSM film, thus providing the possibilities for modulating the radiative heat transfer by tuning these parameters. Based on Maxwell's equations incorporating fluctuational electrodynamics, the effects of these parameters on the heat transfer coefficient and the thermal modulation contrast are numerically analyzed. Under proper parameters, higher modulation contrasts are obtained by continuously tuning the Fermi level from 0.05 eV to 0.3 eV. The obtained results might be helpful in designing a DSM-based thermal modulator with higher modulation contrasts. [ABSTRACT FROM AUTHOR]

Liquid crystal polarization gratings (LCPGs) possess unique optical properties and therefore are widely used in laser systems. Therefore, developing our understanding of the laser damage resistance of LCPGs is critical for future high-intensity laser applications. In this study, the LCPGs (Λ = 8 μm) were fabricated from liquid crystal polymer (LCP) films with photo-alignment technology, which had a small-angle (∼7.51°) diffraction order with high diffraction efficiency (∼92 %) under chiral polarized incidence at 1064 nm. The diffraction efficiency and polarization of each diffraction order were determined by the polarization of the incident light. The damage characteristics of the LCPGs were investigated using a chiral polarized nanosecond laser at 1064 nm in a one-on-one test. The laser-induced damage thresholds (LIDTs) of left-handed circular polarization (LHCP) and right-handed circular polarization (RHCP) were 8.0 J/cm2 and 8.5 J/cm2, respectively. For comparison, the LIDTs of S-polarization and P-polarization were 7.8 J/cm2 and 8.4 J/cm2, respectively. All the damage occurred at the interface between the LCP layer and the substrate when the laser fluence was near the LIDT, and the damage morphology was observed with a scanning electron microscope (SEM), including flat pits, cracks, and bumps at the edge of the pits. The results indicate that the LIDTs and damage morphology of the LCPGs were insensitive to the polarization of the irradiation. The electrical effects of differently polarized irradiation on the LCPGs were simulated using the finite element method (FEM). The electric field distribution was symmetrical under irradiation by LHCP and RHCP lasers. However, the distribution of the electric field under different linearly polarized irradiation conditions were distinct. Based on these findings, we believe that the contribution of chiral polarized irradiation to the damage was consistent and that the LIDTs were similar. This study has important implications for the application and development of LCPGs in high-intensity laser systems. • The laser-induced damage of liquid crystal polarization gratings (LCPGs) is first proposed. • We investigated the laser-induced damage of the LCPGs using a nanosecond pulsed laser with chiral polarized irradiation. • This study provides reference for the application and development of LCPGs in high-intensity laser systems. [ABSTRACT FROM AUTHOR]

We demonstrate stopband modulation in a corrugated parallel plate waveguide (PPWG) in terahertz (THz) frequency. Different from the traditional approaches to achieve narrow band-pass filter, a method based on the junction of two stopbands is provided. Two stopbands can come closer through adjustment of plate distance and controlling the grooves depth, which generate a narrowband transmission region. It is found that plate distance can control the interval of two stopbands. On optimization of plate distance, the filter can achieve a range of zero-sideband, high transmissivity (T=99.8%), narrow linewidth (5.8 GHz), and high-quality factor (Q ≈ 220). Furthermore, we also present the designs of tunable filter and modulate performance when liquid crystal (LC) is used as waveguide material. So, an excellent tunable filtering characteristic has been achieved. Finally, this work provides a new possibility for the designing of narrow band-pass filter devices. [ABSTRACT FROM AUTHOR]

• The PI alignment layer is pinned on the ITO film to imitate the structure of an LCD. • The damage of the samples are attributed to the high absorption of the ITO film. • The mechanical properties of the PI are primarily responsible for the LIDT result. This report focuses on the damage characteristics of the indium-tin-oxide (ITO) layer and the polyimide (PI) layer, which are two constituent components of a LCD. This investigation is different from the previous study, in which the alignment layer was deposited directly on a glass substrate. The PI alignment layer is pinned on the ITO film to imitate the structure of the LCD as much as possible in our current study. The damage process of the ITO/Glass sample involves melting, vaporization near the laser-induced damage threshold (LIDT), and removal at a higher fluence. However, the damage process of the PI/ITO/Glass sample involves thermally induced plastic deformation, followed by cooling when the irradiation fluence is near the LIDT, and rupture when the irradiation fluence is higher. The LIDTs of the PI/ITO/Glass samples, as determined by the on-line CCD detection technique, are higher than those of the ITO/Glass samples. The favorable mechanical properties of the PI are primarily responsible for this result. [ABSTRACT FROM AUTHOR]

Abstract A set of liquid crystal adaptive optics system for visible light is installed for 2-meter telescope in Changchun, China. To get higher resolution images after adaptive optics, phase diversity (PD) is applied. Two liquid crystal spatial light modulators were used for two polarized states of light, and they were also used for two channel correction of focused image and diversity image. Observations about star HD 29139 were performed on March 23, 2018. The results of the star with the PD show that the full width half maximum was decreased from about 10 pixels to 4 pixels, corresponding to angular resolution improved from 0.5" to 0.2". The peak intensity was improved by 66.7% after the PD. The root mean square of the wavefront estimated using the PD agrees with the Strehl ratio of observed image after adaptive optics. The observed results indicate that the PD technique can significantly improve the image resolution after liquid crystal adaptive optics. [ABSTRACT FROM AUTHOR]

Abstract Two-photon light-sheet microscopy (TP-LSM) system performance is greatly degraded by specimen-induced aberrations in illumination path, which limit the field of view, axial resolution and excitation efficiency of the system. Adaptive optics (AO) is an effective method for attenuating these effects. For the design and evaluation of an AO system, a comprehensive analysis of the effects of aberrations is needed. In this paper, a TP-LSM system is simulated, and new indexes based on integral intensity are introduced for the evaluation of an aberrated light-sheet. Then, the influences of each Zernike mode and random aberrations on the illumination path of the TP-LSM system are investigated with a numerical simulation method. Results show that high-order aberrations have little effect on the axial resolution and excitation efficiency of the system and only low-order components require correction. The random aberrations varied in strength with the depth of the specimens, so the number of corrected Zernike modes is variable. A general formula is generated for the estimation of the number of modes that should be detected and corrected under different aberrations and different numerical aperture of the objective. The results can provide important guidance in the design and evaluation of AO units for TP-LSM systems. [ABSTRACT FROM AUTHOR]

Abstract The performance of free-space laser communication (FSLC) is greatly decreased by horizontal atmospheric turbulence. Adaptive optics is used to overcome horizontal turbulence. This study investigated the effect of the 3 dB bandwidth of an adaptive optics system (AOS) on FSLC performance by using power-in-bucket (PIB) method, which considers the effect of receiving aperture and then, can accurately evaluate of the effect of system bandwidth on the communication performance. A 9 km horizontal maritime link experiment was performed through adaptive correction to validate the theoretical analysis. Results indicate that the effect of system bandwidth can be accurately evaluated using PIB method. This work provides a basis for designing and evaluating AOSs for FSLC. [ABSTRACT FROM AUTHOR]

Adaptive optics systems (AOSs) rely on good estimation of a distorted wavefront. Two problems make centroid detection difficult, namely, the problem of a relatively large subaperture that brings high-order aberrations and that of a large laser guide-star spot that brings in a large photon noise. To solve these problems, we proposed an estimation method based on Poisson statistics to reduce the photon noise, and the high-order aberrations are partly separated from a spot image to well estimate the slope in the subaperture. Simulation results indicate that our method performs better than the weighted center-of-gravity (WCOG) method and correlation method (CM). Under the condition of SNR =20 and r 0 =13cm The root-mean-square values of the residual wavefront using the proposed method are approximately 10%, 50%, and 35% lower than those of the COG, WCOG, and CM, respectively. The proposed method would be highly beneficial for ground-based astronomical AO systems intended to provide moderate to high Strehl ratios at near infrared wavelengths on larger than 10m-aperture telescopes. [ABSTRACT FROM AUTHOR]

Flowerlike silver nanostructure substrates were fabricated in liquid crystalline phase and the distance dependent property of metal enhanced fluorescence for such substrate was studied for the first time. The distance between silver nanostructures and fluorophore was controlled by the well-established layer-by-layer (LbL) technique constructing alternate layers of poly (allylamine hydrochloride) (PAH) and poly (sodium 4-styrenesulfonate) (PSS). The Rhodamine 6G (R6G) molecules were electrostatically attached to the outmost negative charged PSS layer. The fluorescence enhancement factor of flowerlike nanostructure substrate increased firstly and then decreased with the distance increasing. The best enhanced fluorescence intensity of 71 fold was obtained at a distance of 5.2 nm from the surface of flowerlike silver nanostructure. The distance for best enhancement effect is an instructive parameter for the applications of such substrates and could be used in the practical MEF applications with the flowerlike nanostructure substrates fabricated in such way which is simple, controllable and cost-effective. [ABSTRACT FROM AUTHOR]

The average orientation of a liquid crystal (LC) director to the grating formation, morphology, and switching properties of a holographic polymer-dispersed liquid crystal (HPDLC) grating was systematically investigated in this study. The grating possessed high diffraction efficiency and low scattering with the LC director being parallel to the grating vector. The scanning electron microscope confirmed the well-defined morphology with the LC director being parallel to the grating vector. The grating was easily switched when the LC director was perpendicular to the grating vector. Moreover, polarization excitation was performed to investigate the polarization dependence behavior of the HPDLC-distributed feedback (DFB) laser. The results confirmed that the HPDLC grating is suitable as a laser oscillation when the LC director is parallel to the grating vector. Finally, the tuning range was obtained for the HPDLC DFB laser by applying an external electric field. The tunability, ease of fabrication, and mass production make the HPDLC DFB lasers suitable as smart laser sources for spectroscopy and communication. [ABSTRACT FROM AUTHOR]

Highly effective surface-enhanced fluorescence substrates with roughened 3D flowerlike silver nanostructures were fabricated by electrodeposition in liquid crystalline template which is simple and controllable. Due to the localized surface plasmon resonance of silver nanostructures, the substrates were used as surface enhanced fluorescence substrates. The morphology and optical properties of the substrates were studied. The fluorescence experiments of the Rhodamine 6G on the substrates for different growth times were carried out and the best enhancement factor of 181 was achieved. Eight substrates with the same growth conditions were used to study the reproducibility of the substrate which shows that the fluctuations are within 9%. This substrate was used in organic distributed feedback lasers and the amplified spontaneous emission of poly(2-methoxy-5-(2′-ethyl-hexyloxy)- p -phenylenevinylene) was enhanced dramatically which means the reduced threshold and improved slope efficiency. Such easily fabricated flower-like silver nanostructure substrates with strong surface enhanced fluorescence effect and good reproducibility are good candidate for potential applications in optical imaging, biotechnology and material detections. [ABSTRACT FROM AUTHOR]

Thermal annealing effect on an organic distributed feedback (DFB) laser excited from a semiconducting polymer gain layer, poly(2-methoxy-5-(2’-ethyl-hexyloxy)-p-phenyl-envinylene) (MEH-PPV), is reported. The morphology, absorption and photoluminescence (PL) spectral characteristics of the MEH-PPV film annealed at different temperatures were analyzed. The amplified spontaneous emission (ASE), the optical gain and loss coefficients were also investigated. The organic lasing behaviors including threshold, energy conversion efficiency and polarization state in a DFB laser device were studied. The results show that the optical properties of the organic semiconducting laser can be enhanced by thermal annealing effect. The single mode laser emission at 622.4 nm with lower lasing threshold 0.2 μJ/pulse and higher energy conversion efficiency 6.71% was achieved with thermal annealing at 120 °C. The thermal annealing treatment decreases laser threshold and increases laser energy conversion efficiency dramatically, which shows the potential in ultra-low cost organic semiconducting polymer DFB lasers. [ABSTRACT FROM AUTHOR]

Electron multiplying charge-coupled-device (EMCCD) applied in Shack–Hartmann wavefront sensor (S–H WFS) makes the wavefront sensing more efficient for adaptive optics (AO). However when the brightness of the observed target changes in large ranges in a few minutes, a fixed electron multiplying (EM) gain may not be optimum. Thus an auto-gain-control (AGC) method based on the spots image of the S–H WFS is proposed. The designed control value is the average value of the maximum signals of all the light spots in a frame. It has been demonstrated in the experiments that the control value is sensitive to the change of the target brightness, and is stable in the presence of detecting noises and turbulence influence. The goal value for control is predetermined based on the linear relation of the signal with the EM gain and the number of photons collected in sub-apertures. The conditions of the self-protection of the EMCCD are also considered for the goal value. Simulations and experiments indicate that the proposed control method is efficient, and keeps the sensing in a high SNR which reaches the upper SNR limit when sensing with EMCCD. The self-protection of the EMCCD is avoided during the whole sensing process. [ABSTRACT FROM AUTHOR]

The laser performance of liquid-crystal devices (LCDs) must be considered because of their applications in laser systems. The phase modulation variation mechanisms, as well as the key absorption factor under continuous-wave (CW) laser loading, remain unclear. In this study, the influence of indium tin oxide (ITO) layers on the laser tolerance of LCDs under CW laser loading was investigated. Comparing the phase modulation variations and thermal effects of passive and active LCDs, the influence of the ITO absorbing layer and the decisive role of temperature on the laser tolerance of the LCDs were confirmed. The results showed that the laser tolerance of the LCD with ITO layers was approximately one-seventh of that of the LCD without ITO layers. ITO layers have proven to be the main absorption factor for active LCDs. However, the variations in phase modulation and morphology under CW laser loading were similar for LCDs with or without ITO layers. The phase modulation variations of the LCDs increased with increasing temperature in the irradiated area. For LCDs with or without ITO, once the CW laser loading induced a similar temperature increase, their phase modulation variations were almost the same. The temperature rise of LCDs is confirmed to be an effective parameter for evaluating the phase modulation degradation of LCDs in high-power CW laser applications. The coincidence between the theoretical and experimental phase modulation variations for LCDs with or without ITO indicates that the temperature-induced refractive index change was the main reason for the LCD phase modulation variation under CW laser loading. These results clarify the basic mechanisms of the phase modulation variation of LCDs under CW laser loading and provide guidance for process optimization and application of LCDs in high-power laser systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • ENZ and LSPR modes in corrugated ITO films are designed for broadband perfect absorption. • Nanoparticle size and plasmonic frequency can tune the absorption bandwidth and location. • Polarization-independent broadband light absorption (A > 98%) can be obtained below 20°incident angle. • The device is compatible with self-assembly technology, enabling low-cost, large-area fabrication. Epsilon-near-zero (ENZ) optical materials, whose real part of the dielectric constant crosses zero, have been demonstrated to enhance linear and nonlinear optical responses in the ENZ region. Here, a new broadband light absorption design concept based on the coupling of ENZ and localized surface plasmon resonance (LSPR) modes is proposed. It is verified by a single-layer semi-sphere patterned indium tin oxide (ITO) film based on large-area and low-cost self-assembly technology. Simulation and experimental results show that the 91 nm thick ITO film on the two-dimensional microsphere array can simultaneously excite the ENZ and LSPR modes without complex coupling structures, which enable >98% light absorption in the range of 1435–1680 nm. The perfect absorber based on a corrugated ITO film has typical polarization-independent, large-angular-spectrum, adjustable-absorption-bandwidth, and field-enhanced resonance absorption characteristics. These findings provide a new design solution for low-cost broadband perfect absorbers based on ENZ materials, which have wide prospects for application in near-infrared sensors, photoelectric detection, and ENZ photonics. [ABSTRACT FROM AUTHOR]

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 andn-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. [ABSTRACT FROM AUTHOR]

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(orπ) 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. [ABSTRACT FROM PUBLISHER]

Liquid crystal devices (LCDs) are widely used in several applications involving high-power lasers, such as laser processing, laser shaping, laser communication, laser radar and so on. Investigation on the performance of the LCDs working in high-power laser has always been of concern. In this study, a modified nearly common-path interferometry technique is proposed to investigate the phase modulation characteristics of the LCD irradiated by 1064 nm continuous-wave (CW) laser with Gaussian beam profile and an effective area of 0.1 cm2. In addition, the temperature rise of the LCD induced by laser irradiation was synchronously measured with an infrared thermal imager. A quantitative and dynamic investigation of the relation between the thermal effect caused by the laser and the phase-modulation variation is reported. Experimental results showed that the trend of temperature increase of the LCD was similar to that of phase shift variation. The theoretical phase shift calculated using the temperature-dependent LC refractive index coincided with the experimental results. This indicated that the refractive index change induced by temperature was the main reason for the phase-modulation variation of the LCD under laser irradiation. When the laser power was further increased, interference fringe distortion appeared, and morphological changes in the LC were observed. This implied that the laser-induced temperature rise had reached the clearing point of the LC and that the LCD was inactive. The temperature simulation along the laser incidence direction implied that, under the irradiation CW laser, indium tin oxide (ITO) layer absorbs laser energy, resulting in a temperature rise and a small temperature gradient between the ITO, polyimide (PI) and LC layers. The rising temperature is too low to induced damage of ITO layer and PI layer, but influences the refractive index of the LC. Compared with other layers of the LCD, the LC material will fail first when the irradiated laser power is further risen. The transmission variation of an LCD induced by laser irradiation was found. Possibly owing to the refractive index change of the LC as well as the multilayer film structure of the LCD, there seemed to be a cyclical transmission variation. • In our study, a modified nearly common-path interferometry technique was proposed to investigate the phase modulation characteristics of the LCD under continuous-wave (CW) laser irradiation. • A quantitative and dynamic investigation of the relation between the thermal effect caused by the laser and the phase-modulation change was reported. • This indicated that the refractive index change induced by temperature was the main reason for the phase-modulation variation of the LCD under laser irradiation. • The temperature simulation along the laser incidence direction implied that, under the irradiation CW laser, ITO layer absorbed laser energy, resulting in a temperature rise and a small temperature gradient between the ITO, PI and LC layers. • Besides, the transmission variation of the LCD without polarizer induced by laser irradiation was first reported. [ABSTRACT FROM AUTHOR]

Abstract: To process real-time wavefront signal in our liquid crystal adaptive optics system (LCAOS), a real-time wavefront processor (RTWP) with graphics processing unit (GPU) accelerator is developed. A series of parallel acceleration algorithm is implemented to reconstruct wavefront, compute gray map and process lookup table (LUT). Especially, an algorithm by using symmetry of Zernike polynomial is designed to compute gray map and it provides a speedup of 3.16. Finally, the total computing time in our RTWP is 109μs and temporal bandwidth in our LCAOS is 45Hz; it is shown that our RTWP has the ability to real time process wavefront signal in our LCAOS. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Abstract: Base on the Lorentz–Lorenz formula, magnetic response in metamaterial composed of randomly dispersive SiC microspheres is demonstrated theoretically around 13μm mid-infrared wavelength. By adjusting the microspheres volume filling fraction the permeabilities both less than one and more than one can be obtained. Through parameters optimization, isotropic negative permeability is represented. Electromagnetic simulation shows the mechanism for magnetic effects and negative permeability in SiC microspheres. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Abstract: The optimal cell gap of nematic liquid–crystal wavefront corrector (LCWFC) is proved theoretically by numerical computation of the Ericksen–Leslie equation based on finite differences and confirmed experimentally. The numerical results of optimizing cell gap overcome the constraints of many approximations e.g. the small angle approximation and agree much better with the experimental data than those of analytical solution. Therefore, this method supplies a useful tool to achieve the optimal cell gap of a nematic LCWFC with high precision. Together with high birefringence and low rotational viscosity liquid crystal (LC) material effect, a nematic LCWFC with response time of sub-millisecond can be achieved in the visible spectral region. [Copyright &y& Elsevier]

Surface-enhanced Raman scattering (SERS) has been widely reported to improve the sensitivity of Raman spectra. Ordinarily, the laser is focused on the sample to measure the Raman spectrum. The size of the focused light spot is comparable with that of micro-nano structures, and the number of micro-nano structures contained in the light spot area (defined as duty cycle) will severely affect the spectrum intensity. In this study, flower-like silver nanostructures were fabricated with a soft lyotropic liquid crystal template in order to investigate the effect of duty cycle. They were observed under a scanning electron microscope, and their spectrum enhancement factor was computed with the obtained Raman spectrum. Then, their duty cycles were measured using a SERS substrate at different locations. A formula was derived to represent the relation between the duty cycle of the nanoflowers and the Raman spectral intensity. This work could promote the actual applications of SERS in high-sensitivity spectrum testing. [ABSTRACT FROM AUTHOR]

Abstract: Reverse hexagonal lyotropic liquid crystal template is proposed to achieve optical hyperbolic metamaterial composed of silver nanowire arrays. All angle negative refraction is obtained during the wide wavelength range from near-infrared to visible light. Hyperbolic metamaterial incorporated with gain dye molecules is calculated to show the possibility of eliminating energy losses and the transmittance can be enhanced in the selected wavelength range. [Copyright &y& Elsevier]

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 Δnof 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. [ABSTRACT FROM AUTHOR]

We extended Berenger’s split-field perfectly matched layer (PML) absorbing boundary conditions (ABCs) to cylindrical coordinates for the use in body-of-revolution finite-difference time-domain (BOR FDTD) method that is suitable for the analysis of axially symmetric subwavelength diffractive optical elements (SWDOE). As application, we use our algorithm in the analysis of a multilevel microlens and its counterpart of binary SWDOE. Numerical results illustrate the efficiency of our PML ABCs. [Copyright &y& Elsevier]

The adaptive optics system (AOS) has been used to eliminate the effect of atmospheric turbulence for free-space optical communications. The AOSs are designed based on the statistical average performance of the turbulence. In reality, the amplitude of turbulence changes drastically, and the AOS cannot always detect and correct the aberrations appropriately. To prevent the amplitude of turbulence from exceeding the designed range of the Shack–Hartmann wavefront sensor (SH WFS), the combination of the wavefront sensorless and SH WFS methods is proposed to extend the detection range. First, the control scheme is demonstrated, and the numerical simulation is performed. Then, the validation experiments are performed with static and dynamic turbulence. Results show that the proposed method is valid and that the effective communication time is greatly lengthened. • Turbulence amplitude often exceeds the detection range of adaptive optics systems. • Strong turbulence is detected with the combination of SPGD and wavefront sensor. • Wavefront sensor may detect the turbulence with the correlativity more than 0.81. • Communication BER is reduced from 10 − 1 to 10 − 6 with proposed correction method. [ABSTRACT FROM AUTHOR]

During quasi-CW (QCW) 1064-nm laser annealing process, one excess oxygen could capture two free electrons, forming one O2− and then occupied one neighboring site of the oxygen vacancy, resulting in the annihilation of the oxygen vacancy. In addition, one Sn4+ ion was combined with two free electrons to create a Sn2+ ion. The two free electrons were transformed to form two bound electrons, indicating a reduction in free electrons. The oxygen vacancies annihilation and tin oxide reduction after QCW 1064-nm laser annealing reduced the number of free electrons, decreased the carrier density and enhanced the optical transmittance in the NIR region. • Quasi-CW (QCW) laser annealing (LA) notably prompts the transparency in NIR region. • Increase of transmittance after LA attributes to reduction of carrier absorption. • Oxygen vacancy elimination and tin oxide reduction result in carrier reduction. • More tin oxide reduction occurs in oxygen-poor zone than that in oxygen-rich zone. In this study, a 1064-nm quasi-continuous-wave laser was employed to anneal the indium tin oxide (ITO) films, thereby prominently prompting transparency in the near-infrared region with insignificant variation in resistivity. The decrease in carrier density due to laser annealing contributed to the improvement in transparency. Interestingly, the depth-resolved X-ray photoelectron spectroscopy distinguishes between two different mechanisms for tailoring the properties of the free carriers. Tin oxide reduction in the oxygen-deficient zone of the ITO films at a depth of approximately more than 80 nm was more significant than that in the oxygen-rich zone within the top 80-nm thin layer. In addition, the elimination of oxygen vacancies in the oxygen-rich environments did not indicate apparent advantage when compared with that in the oxygen-deficient environments owing to the oxygen supplied during the tin oxide reduction process. These results provide an alternative laser annealing technologies for tailoring the properties of the free carriers and elucidating the fundamental mechanisms in oxygen-rich and oxygen-deficient environments in an annealing process. [ABSTRACT FROM AUTHOR]

We have analyzed and discussed a flat-top Brewster filter that works in full conical mounting. Results reveal that flat-top reflection band can be formed by merging double peaks that corresponded to guided modes with different polarization states. Merging is achieved by adjusting the incident angle in the design. When the proposed single-layer waveguide grating structure is irradiated with transverse magnetic (TM) polarized plane wave at the Brewster angle, the filter has a highly efficient flat-top reflection spectral response with central wavelength of 547 nm, bandwidth of 15.5 nm (R > 90%), and sideband reflectance of <2.3%. • A flat-top Brewster filter that works in full conical mounting. • Flat-top reflection band can be formed by merging double peaks. • Brewster angle achieves sideband suppression. • The filter presents high-efficiency flat-top filter at the Brewster angle. [ABSTRACT FROM AUTHOR]

An eigenmode method is proposed to estimate the correction performance of atmospheric turbulence based on a deformable mirror (DM). On the basis of the eigenmode, a formula that describes the relationship among residual error, eigenmode N , and turbulence strength D/r 0 (D , telescope aperture, r 0 , atmospheric coherence length) is derived. The eigenmode and the Zernike polynomial methods are compared, too. An experiment is conducted, and results validate the theoretical equations. Hence, the correction performance of atmospheric turbulence may be estimated with the eigenmode method. This result is helpful for conveniently designing and evaluating the adaptive optics system used for turbulence correction. [ABSTRACT FROM AUTHOR]

Large-scale controllable fabrication of highly roughened flower-like silver nanostructures is demonstrated experimentally via electrodeposition in the liquid crystalline phase. Different sizes of silver flowers are fabricated by adjusting the deposition time and the concentration of the silver nitrate solution. The density of the silver flowers in the sample is also controllable in this work. The flower-like silver nanostructures can serve as effective surface-enhanced Raman scattering and surface-enhanced fluorescence substrates because of their local surface plasmon resonance, and they may have applications in photoluminescence and catalysis. This liquid crystalline phase is used as a soft template for fabricating flower-like silver nanostructures for the first time, and this approach is suitable for large-scale uniform fabrication up to several centimetres. [ABSTRACT FROM AUTHOR]