Study on dynamic response characteristics of the scanning angle in a liquid crystal cladding waveguide beam scanner.

This paper studies the dynamic response characteristics of the scanning angle in a liquid crystal cladding waveguide beam scanner. Based on liquid crystal dynamic theory, finite element analysis and vectorial refraction law, a dynamic response calculation model of scanning angle is constructed. The simulation results show that the dynamic responses of the scanning angle during the electric field-on and field-off processes are asymmetric, and exhibit "S"-shape and "L"-shape changing trends, respectively. In addition, by comparing with the bulk phase modulation response process of traditional liquid crystal devices, the intrinsic physical reason for the rapid light regulation of the liquid crystal cladding waveguide beam scanner is clarified to be that the liquid crystal close to the core layer has a faster rotation speed during the electric field-off process. Moreover, the liquid crystal cladding waveguide beam scanner is experimentally tested, and the experiment results are in good agreement with theoretical simulations.

Collecting, transporting and sorting micro-particles via the optical slings generated by a liquid crystal q(φ)-plate.

We disclose a transporting/collecting optical sling generated by a liquid crystal geometric phase optical element with spatial variant topological charge, which shows the intriguing repelling/indrawing effect on the micro-particle along the spiral orbit. Two proof-of-concept prototypes, i.e., an optical conveyor and a particle collector, are demonstrated. Based on the distinct dynamic characteristics of the micro-particles in different sizes, we conceptually propose a design for particle sorting. Thus, our proposed method to generate a spiral optical sling with spatial variant orbital angular momentum for on-demand collecting, transporting and sorting micro-particles is substantiated, which can find extensive applications in bio-medicine, micro-biology, etc.

Design of prism coupling structure for liquid crystal cladding waveguide beam steerer.

This paper proposes an extended prism coupling analysis method to accurately analyze the coupling structure of liquid crystal (LC) cladding waveguide beam steerer. We analyze the effects of LC anisotropy on the coupling of transverse electric (TE) and transverse magnetic (TM) modes and derive the expression of the optical field distribution that perfectly matches the given coupling structure. Based on this method, we present the optimal coupling structure for Gaussian beam. Taking into account the practical manufacturing process, we propose a simplified coupling structure and perform a detailed analysis of its performance based on numerical simulations. Experimental results show a coupling efficiency of 91% and a coupling angle full width at half maximum (FWHM) of about ±0.02°, demonstrating the effectiveness of the proposed method in predicting the coupling performance of anisotropic cladding waveguides.

Ultrawide-view achromatic circular polarization dynamic converter using an easy-to-integrate multi-layer structure.

What we believe to be a novel integrated circular polarization dynamic converter (CPDC) is proposed based on the four-layer mirror symmetry structure. By designing the twisted structure and rearranging the orientation direction of liquid crystal molecules for each layer, the application wavelength range could be broadened. For the viewing angle expansion, negative birefringent films are selected to compensate for the retardation deviation under oblique incidence. Finally, the particle swarm algorithm is used to optimize the whole configuration, and the polarization conversion efficiency calculated by the finite element method (FEM) can achieve 90% in the wavelength range from 320 nm to 800 nm at an ultrawide view of 160°. Compared with traditionally active liquid crystal waveplates, the design has potential advantages in both wavelength and field of view (FOV) and provides the possibility for the integrated and flimsy fabrication of devices.

Liquid-crystal-based fiber laser sensor for non-invasive gas detection.

This Letter reports a new optical fiber gas sensor for measuring breath acetone. The sensor is based on photonic bandgap (PBG) mode laser emission sensing technology using liquid crystal (LC), which is combined with silica fiber and chiral nematic liquid crystal (CNLC), thus providing an ultra-compact, fast-response and simple-to-produce sensing system with a fast response that can accurately and quantitatively determine the concentration of respiratory acetone within the normal oral temperature range (35-38°C). Since LCs are affected by temperature, we propose a method that eliminates the influence of the temperature to solve the problem of the temperature influence when measuring gas. The detection of acetone leads to splitting of the dual laser peaks, with a linear correlation of 0.99. The sensor has a limit of detection of 65 ppm for acetone vapor and thus is suitable for breath acetone detection in diabetic patients.

Laser emission from tapered fiber-based liquid-crystal microsphere for sensing.

This Letter introduces a novel laser emission probe for liquid-crystal microspheres based on a tapered fiber. A cholesteric liquid crystal (CLC) is injected into a hollow glass microsphere (HGM) attached at the front end of a tapered fiber in order to produce laser. Tapered fibers are preferable to rectangular fibers for liquid-crystal microsphere laser emission. The whispering gallery mode (WGM) laser is significantly suppressed by the tapered fiber-based liquid-crystal microsphere, which also displays an apparent single-mode photonic bandgap (PBG) laser peak. The stimulation response of tapered fiber-based liquid-crystal microspheres to organic vapors causes a modification of the laser peak wavelength with increasing gas concentration. In addition, laser emission generated by tapered fiber-based liquid-crystal microspheres is expected to be used in fields such as microenvironmental biosensing.

Temperature-compensated optical fiber sensor for volatile organic compound gas detection based on cholesteric liquid crystal.

External temperature variations inevitably affect the accuracy of a liquid crystal sensor. Therefore, we propose a novel temperature-compensated fiber volatile organic compound (VOC, using acetone as a model compound) gas sensor. The proposed sensor consists of a short segment of hollow-core fiber (HCF), which is spliced on a multimode fiber. Cholesteric liquid crystal (CLC) is sealed into HCF to sense the temperature, and another type of CLC is coated on the end face of HCF for VOC gas detection. The VOC gas concentration and ambient temperature can be simultaneously measured by monitoring the wavelength shifts of two Bragg reflection peaks caused by two types of CLCs. The effects of the CLC thickness on the sensitivities of temperature and acetone concentration are investigated, and optimal parameters are chosen. An optimal sensor can reach a temperature sensitivity of 2.53 nm/°C and acetone concentration sensitivity of 48.46 nm·L/mmol at 8-44°C. In addition, temperature compensation capability, repeatability, response time, and stability are also researched. The experimental results prove this sensor has great application potential in high-precision real-time VOC gas monitoring and detection.

Improved bandwidth of open loop liquid crystal adaptive optics systems with a proportional-derivative controller.

Open loop liquid crystal adaptive optics (LC AO) has overcome the disadvantage of low energy efficiency after years of research, and its use is very promising in ground-based large aperture telescopes for visible band imaging. However, the low system bandwidth of open loop LC AO still limits its application. In order to solve this problem, we bring the concept of proportional-derivative control (which is widely used in closed loop systems) into open loop LC AO in this paper. Experiment results verified that the system -3 dB rejection bandwidth could improve from 75 Hz to 112 Hz when tip-tilt aberration is introduced, and the mean relative contrast ratio of imaging results could improve 80% when high-order aberrations are introduced. The proposed control method has significant meaning in promoting the application of open loop LC AO in ground-based large aperture telescopes for visible imaging.

Multi-plane augmented reality display based on cholesteric liquid crystal reflective films.

To address the accommodation-convergence conflict problem in conventional augmented reality (AR) head-mounted displays, we propose a compact multi-plane display design based on cholesteric liquid crystal (CLC) reflective films and a polarization switch. Because of the polarization selectivity of CLC films, circularly-polarized light with different handedness is reflected by different CLC films, resulting in different optical path lengths and different image depths by the lens. A flicker-free dual-plane prototype with correct focus cues and relatively low operating voltage has been implemented. Moreover, a multi-plane AR display scheme with more than 2 depth planes is proposed by stacking multiple CLC films and polarization switches together.

[A Study of Lasing Spectrum of Dye-Doped Chiral Nematic Liquid Crystal Cell under Electric Field].

The lasing spectrum of dye-doped chiral nematic liquid crystal under electric filed was investigated. Two kinds of electrodes were designed to apply transverse electric field to positive liquid crystal cell and longitudinal electric field to negative liquid crystal cell. A 532 nm Nd∶YAG pulsed solid-state laser was used to pump the cell. When transverse electric field is applied to positive liquid crystal device, multi-wavelength laser output is obtained in the range of 630~660 nm. When longitudinal electric field is applied to negative liquid crystal cell, 18.5 nm tunable output lasing is obtained. The output characteristic of cell was analyzed from the texture of device and the photonic band gap. In the positive liquid crystal cell, the competition of electric moment with twisting moment causes the flow of LC molecule, and the flow of LC molecule leads to a floating photonic band gap. For this reason, not only at the edge of photonic band gap but in the photonic band gap can produce lasing. For negative liquid crystal cell, the pitch shrinks with the increase of electric field. The photonic band gap shows blue-shift with the decrease of pitch. and Lasing wavelength is blue shifted from 681.0 to 662.5 nm and the lasing at the edge of photonic band gap. Negative liquid crystal cell has better stability under electric field effect.

Experimental realization of hyperbolic dispersion metamaterial for the whole visible spectrum based on liquid crystalline phase soft template.

We experimentally demonstrated a metamaterial composed of hexagonal arrays of silver nanowires that exhibits hyperbolic dispersion and negative refraction in the entire visual wavelength range. The nanowires with extremely small size of 10 nm diameter and 15 nm center-to-center distance were fabricated using the reverse hexagonal liquid crystalline phase template containing AgNO(3) solution. Through the experiments of angle dependent reflectance for s-polarization and p-polarization, the dielectric constants were measured in several wavelengths. Calculations and experiments both show hyperbolic dispersion relations from 370 nm to 750 nm which indicates the presence of all-angle negative refraction. For all the experimental wavelengths, the permittivities of the material are in good agreement with the theoretical calculations.

Metasurface-enabled polarization-independent LCoS spatial light modulator for 4K resolution and beyond.

With the distinct advantages of high resolution, small pixel size, and multi-level pure phase modulation, liquid crystal on silicon (LCoS) devices afford precise and reconfigurable spatial light modulation that enables versatile applications ranging from micro-displays to optical communications. However, LCoS devices suffer from a long-standing problem of polarization-dependent response in that they only perform phase modulation on one linear polarization of light, and polarization-independent phase modulation-essential for most applications-have had to use complicated polarization-diversity optics. We propose and demonstrate, for the first time, an LCoS device that directly achieves high-performance polarization-independent phase modulation at telecommunication wavelengths with 4K resolution and beyond by embedding a polarization-rotating metasurface between the LCoS backplane and the liquid crystal phase-modulating layer. We verify the device with a number of typical polarization-independent application functions including beam steering, holographical display, and in a key optical switching element - wavelength selective switch (WSS), demonstrating the significant benefits in terms of both configuration simplification and performance improvement.

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.

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.

Does a Widespread Species Have a Higher Competitive Ability Than an Endemic Species? A Case Study From the Dongting Lake Wetlands.

The distribution range of plants is usually related to their competitiveness. The competitive ability between common widespread, which are generally considered to be invasive, and common endemic species, is still not very clear. Five plant communities were monitored in the field to compare the competitive abilities of widespread species, Phragmites australis, and endemic species, Triarrhena lutarioriparia, in the Dongting Lake wetlands. The ratios of individual numbers of T. lutarioriparia to P. australis per square meter were found to be 9:0, 14:1, 10:5, 7:6, and 0:11 in the five respective communities. A manipulation experiment was then performed with five planting modes (T. lutarioriparia: P. australis was 4:0, 3:1, 2:2, 1:3, and 0:4, respectively). Results from field monitoring showed that the two plant species exhibited similar decreased survival percentages during flooding. P. australis had higher aboveground biomass before the flooding and a higher relative elongation rate, whereas T. lutarioriparia had higher aboveground biomass after flooding and a higher relative growth rate (RGR). P. australis had a higher competitive ability than T. lutarioriparia before and after the flooding. The manipulation experiment revealed that P. australis had a higher survival percentage than T. lutarioriparia, with no differences in plant biomass, RGR, and the relative elongation rate between the two species. P. australis was found to have a higher competitive ability than T. lutarioriparia in the early growing stage and a lower competitive ability in the middle and later stages. The relative yield total in the field monitoring and manipulation experiment was 1, indicating that T. lutarioriparia and P. australis occupied different niches in the experimental conditions. It was concluded that, compared with T. lutarioriparia, P. australis has a higher competitive ability in submerged habitats and a lower competitive ability in the non-submerged habitat. The niche differences between the two species enabled their coexistence in the Dongting Lake wetlands with seasonal flooding.

Differences in nitrogen and phosphorus sinks between the harvest and non-harvest of Miscanthus lutarioriparius in the Dongting Lake wetlands.

Plant non-harvest changes element circulation and has a marked effect on element sinks in the ecosystem. In this study, a field investigation was conducted on the fixation of nitrogen and phosphorus in Miscanthus lutarioriparius, the most dominant plant species in the Dongting Lake wetlands. Further, to quantitatively compare the difference in nitrogen and phosphorus sinks between harvest and non-harvest, an in situ experiment on the release of the two elements from two types of litters (leaves and stems) was studied. The nitrogen concentrations in the plant had no significant relationship with the environmental parameters. The phosphorus concentrations were positively related to the plot elevation, soil organic matter, and soil total potassium and were negatively related to the soil moisture. The leaves demonstrated a higher decomposition coefficient than that of the stems in the in situ experiment. The half decomposition time was 0.61 years for leaves and 1.12 years for stems, and the complete decomposition time was 2.83 years for leaves and 4.95 years for stems. Except for the nitrogen concentration in the leaves, all the concentrations increased during the flood period. All concentrations unsteadily changed in the backwater period. Similarly, except for the relative release index of nitrogen in the leaves, all the relative release indices decreased in the flood period. At the end of the in situ decomposition experiment, the relative release indices of both the nitrogen and phosphors were greater than zero, indicating that there was a net release of nitrogen and phosphorus. Under the harvest scenario, the aboveground parts of the plant were harvested and moved from the wetlands, thus increasing the nitrogen and phosphorus sinks linearly over time. The fixed nitrogen and phosphorus in the aboveground parts were released under the non-harvest scenario, gradually accumulating the nitrogen and phosphorus sinks from the first year to the fifth year after non-harvest, reaching a maximum value after the fifth year. This study showed that the nitrogen and phosphorus sinks greatly decreased after the non-harvest of M. lutarioriparius compared to that after harvest. It is recommended to continue harvesting the plant for enhancing the capacity of element sinks.

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.

Novel spectral range expansion method for liquid crystal adaptive optics.

Energy loss is a main problem of liquid crystal adaptive optics systems (LC AOSs). It is caused by the polarization dependence and narrow spectral range. The polarization dependence has been avoided by Love and Mu et al. [Appl. Opt. 32, 2222 (1993); Appl. Opt. 47, 4297 (2008)]. In this paper, a novel method was proposed to extend the spectral range of LC AOSs using multiple liquid crystal wavefront correctors (LCWFCs) to improve the energy utilization. Firstly, the chromatism of an LCWFC was measured and analyzed. The calculated results indicate that one LCWFC is only suitable to perform adaptive correction for a narrow waveband; therefore, multiple LCWFCs must be used to achieve a broadband correction. Secondly, based on open-loop control, a novel optical layout consisting of three LCWFCs was proposed to extend the spectral range of LC AOSs and thus achieve correction in the whole waveband of 520-810 nm. Thirdly, a broadband correction experiment was conducted and near diffraction-limited resolution was achieved in the waveband of 520-690 nm. Finally, a 500 m horizontal turbulence correction experiment was performed in the waveband of 520-690 nm. With adaptive correction, the resolution of the optical system was improved significantly and the image of the single fiber was clearly resolved. Furthermore, compared with a sub-waveband system, the system energy was improved. The energy of the whole waveband is equal to the sum of all the sub-wavebands. The experiment results validated our method and indicate that the chromatism in a broad waveband of LC AOSs can be eliminated. And then, the system energy can be improved greatly using the novel method.

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.

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.