Liquid Crystal Polarization Gratings Research Articles

非等光强正交圆偏振光对液晶偏振光栅衍射特性的影响

非等光强正交圆偏振光对液晶偏振光栅衍射特性的影响

Complex liquid crystal superstructures induced by periodic photo-alignment at top and bottom substrates.

The formation of nematic liquid crystal (LC) superstructures in cells with non-uniform photo-alignment at the confining substrates is studied experimentally and by simulations. An interference pattern of left- and right-handed circularly polarized light is used to define the alignment at both substrates separately, so that the alignment varies along the x-coordinate on one substrate and along the y-coordinate on the other substrate. The interplay between the complex surface alignment and the liquid crystalline soft matter leads to the formation of interesting 3D configurations. The periodic LC structures that are formed in the bulk of the cell are analyzed experimentally by polarizing optical microscopy (POM) for different applied voltages. In the region with strong photo-alignment at both substrates, a 2D LC polarization grating (PG) with a complex 3D director configuration is formed. Distinct periodic structures with different symmetry properties are observed in the regions with weak illumination at the top and/or bottom substrate. The director configuration in the different regions was successfully simulated with the help of finite element (FE) Q-tensor simulations. The agreement between the simulations and the experiments was verified by comparing the POM images with simulated results for the transmission between crossed polarizers.

Designing an electro-optical module based on an LCPG switch for a 1064 nm Nd:YAG laser rangefinder

We designed an electro-optical module based on a liquid crystal polarization grating (LCPG) switch for a neodymium-doped yttrium aluminum garnet, Nd:Y3Al5O12, laser rangefinder with wavelength 1064 nm. Compared to other similar cases, the advantages of this design include maximization of the transmission percentage of the diffractive element (particularly first-order diffraction) and minimization of the light-leakage rate caused by zero order, and other unwanted diffractive orders. The conditions were prepared for using diffractive elements (gratings) with shorter periods and acquisition of diffraction orders with greater diffraction angles by benefitting from suitable compounds of LC in the given design. Similarly, the required measures were taken for the deletion of adverse effects caused by reflection from optic surfaces so that a very appropriate status was prepared for the function of the LCPG shutter.

Reflectance dependences of diffraction properties in Fabry–Perot liquid crystal polarization grating

The reflectance dependences of diffraction properties in a Fabry–Perot liquid crystal polarization grating (LCPG) were investigated. The Fabry–Perot LCPG was fabricated by combining two Au films and an orthogonal circular polarization grating (OCPG). The diffraction efficiency of our device strongly depends on the thickness of the Au films while polarization conversion properties are maintained to be the same as these of the general OCPG. Our results clarify that the Fabry–Perot OCPG has the potential to function as a multibranch polarization beam splitter whose diffraction efficiencies of respective diffraction directions are controlled in different ways by changing resonance conditions.

Large-aperture, wide-angle nonmechanical beam steering using polarization gratings

Liquid crystal polarization gratings (LCPGs) represent a relatively new technology capable of nonmechanically and efficiently steering light over a large field-of-regard in discrete steps. Due to their reliance on thin liquid crystal cells instead of mechanical moving parts, LCPG beam steering systems are attractive options for steering both active and passive optical sensors, especially in size, weight, and power (SWaP)-constrained platforms. This paper describes recent developments in large-aperture LCPG steering systems and summarizes the performance being achieved.

Oblique incidence effect on steering efficiency of liquid crystal polarization gratings used for optical phased array beam steering amplification

A liquid crystal polarization grating (LCPG) is proposed that amplifies the steering angle of a liquid crystal optical phased array for non-mechanical beam steering, taking advantage of its high steering efficiency under normal incidence. However, oblique incidence may play an important role in the overall steering efficiency. The effect of oblique incidence on steering efficiency of a LCPG was analyzed by numerically solving the extended Jones matrix and considering propagation crosstalk. The results indicate that the outgoing laser beam is amplitude-modulated under the effect of oblique incidence and behaves as a sinusoidal-modulated amplitude grating, which diffracts certain energies to non-blazed orders. Over-oblique incidence may even eliminate the steering effect of the incident beam. The modulation depth of the induced amplitude grating was found to be proportional to the product of sinusoidal value of oblique incidence angle and the LC layer thickness, and inversely proportional to the periodic pitch length of the LCPG. Both in-plane incidence and out-of-plane incidence behave similarly to influence the steering efficiency. Finally, the overall steering efficiency for cascaded LCPGs was analyzed and a difference of up to 11 % steering efficiency can be induced between different LCPG configurations, even without considering the over-oblique incidence effect. Both the modulation depth and final steering efficiency can be optimized by varying the LC birefringence and layer thickness.

Fast-response and high-efficiency optical switch based on dual-frequency liquid crystal polarization grating

A dual-frequency liquid crystal polarization grating is fabricated by photoalignment and demonstrated as an optical switch. A high diffraction efficiency up to 95% is obtained for a single first order with circular incident polarization. Via merely alternating the frequency of applied electric field, the switch On and Off time reach 350 μs and 550 μs, respectively. This work supplies a new design for fast-response and high-efficiency optical switch with the merits of easy fabrication and low power consumption.

Simple system for measuring optical rotation of glucose solution using liquid-crystal grating

We demonstrate an optical system for measuring the concentrations of optically active media using liquid-crystal polarization gratings (LCPGs). The optical rotation angle is determined by measuring the intensities of two diffracted light beams from an LCPG combined with a quarter-wave plate (QWP). The intensity ratio is used to evaluate the optical rotation angle, minimizing the dependence on changes in light source intensity and wavelength and reducing the influence of temperature-drift-induced LC birefringence shifts. We demonstrate the system by measuring the concentration of a glucose–water solution. The measurement error caused by the slight wavelength dependence of the QWP’s retardation is assessed numerically.

A Simplified Model for the Optimization of LC Photonic Elements

Recently, liquid crystal (LC) polarization gratings (PGs), because of several advantages, have become extremely popular although the process for analysis and optimization of different optical characteristics is still very complicated. Here, we reveal a simplified model (SM) to evaluate and optimize the performance of LCPGs. Unlike the existing extended Jones matrix (EJM) method, the SM excludes the layer-by-layer analysis and considers an effective twist that reduces the complexity of the calculations. In addition, obtained results from the SM have been compared with the EJM model for variety of conditions and found in good agreement. Furthermore, the proposed SM can also be used for variety of LC photonic elements viz., polarization convertors, polarization rotators, and so on.

P.92: Characterization of Complex Liquid Crystal Polarization Gratings at Oblique Incidence Using Extended Jones Matrix Method

AbstractThe validity of combing extended Jones matrix and vectorial Fourier coefficients to characterize a complex type polarization grating (PG) at oblique incidence was proved. More specifically, the transmitted field of a complex polarization grating at oblique incidence is characterized through the extended Jones matrix. The diffraction properties of the mth order of the grating are determined by the vectorial Fourier coefficients of the transmitted field, under the Fraunhofer approximation. The measured diffraction efficiencies of a complex PG using RGB laser light show very good agreement with the calculated data.

Polarization gratings using ferronematics—An elastic continuum theory

In our paper we apply the elastic continuum theory in order to describe the working of a polarization grating device based on the ferronematic liquid crystal and driven by magnetical field. Solving the Euler–Lagrange equations we obtain analytical expressions for the critical thickness and threshold magnetic field in strong and weak anchoring conditions. We discuss the influence of the ferroparticle presence and properties on the behavior of a device based on the ferronematic liquid crystal polarization gratings. We study also the dynamic behavior of this kind of device and we find the expression for switching on and off times. Our study may be useful in designing magneto-optical devices which include nanoparticles or microprocessed surfaces.

Extended Jones matrix method for oblique incidence study of polarization gratings

A fast and accurate extended Jones matrix method for characterizing the transmitted field of polarization gratings at oblique incidence is reported. The far field diffraction properties of the mth order of the grating are determined by vectorial Fourier coefficients of the transmitted field. With this method, polar plots of the various transmission orders can be obtained. The dependence of viewing angle performance on the grating pitch and grating thickness is studied. Liquid crystal polarization gratings with different pitches were fabricated with azo-dye SD-1 as photoalignment material, by polarization holography. The experimental results show very good agreement with the calculated data.

Highly efficient twisted nematic liquid crystal polarization gratings achieved by microrubbing

We demonstrate twisted nematic liquid crystal (LC) polarization gratings (TNPGs) with a high first-order diffraction efficiency (>90%) that have the twist angle as constant, whereas the azimuthal angle of the mid-plane liquid-crystal director varies linearly in space. The high diffraction-efficiency design scheme is derived by means of the Jones matrix method. The designed first-order diffraction efficiency theoretically reaches 100% for the thick LC layer in which the retardation equals (n + 1/2)λ (n is an integer and λ is the wavelength). We also present a simple model for the grating period- and twist angle-dependent threshold voltage of the Freedericksz transition in TNPGs.

Efficient and monolithic polarization conversion system based on a polarization grating

We introduce a new polarization conversion system (PCS) based on a liquid-crystal polarization grating (PG) and louvered wave plate. A simple arrangement of these elements laminated between two microlens arrays results in a compact and monolithic element, with the ability to nearly completely convert unpolarized input into linearly polarized output across most of the visible bandwidth. In our first prototypes, this PG-PCS approach manifests nearly 90% conversion efficiency of unpolarized to polarized for ±11° input light divergence, leading to an energy efficient picoprojector that presents high efficacy (12 lm/W) with good color uniformity.

Polarization-Insensitive Variable Optical Attenuator and Wavelength Blocker Using Liquid Crystal Polarization Gratings

We demonstrate a variable optical attenuator (VOA) based on liquid crystal polarization gratings (LCPGs), which eliminates the need for complex polarization management found in competing LC technologies. We then configure the VOA as a multi-channel wavelength blocker resulting in a simple, compact architecture with high performance and low cost. Together with a dual fiber collimator, relay lenses, a diffraction grating, a quarter wave plate, and a mirror we achieve optical attenuation of ~50 dB with minimal polarization dependent loss (≤ 0.3 dB) and insertion loss ( ≤ 2.5 dB). The device also manifests competitive wavelength flatness (≤ 0.35 dB variation), response times ( ~ 40 ms), and temperature dependent loss (≥ 47 dB maximum attenuation up to 85°C). We describe the principle of operation, explain the fabrication process and optimization challenges, and finally present the system design and experimental results for a four-channel, 100 GHz wavelength blocker in the C-band.

High efficiency reflective liquid crystal polarization gratings

We experimentally demonstrate a reflective-mode liquid crystal polarization grating with high reflectance, small grating period, and subms switching times. This switchable optical element can diffract ∼100% into a single order, have highly polarization-sensitive first orders, and have a polarization-insensitive zero order. Here we introduce an absorbing layer that overcomes the reflection of the (ultraviolet) holographic beams, which otherwise prevents high quality fabrication. At a grating period of 2.1 μm, we report 98% diffraction efficiency, 90% reflectance, ∼600:1 contrast-ratio, and ∼3000:1 polarization contrast. These elements can therefore be configured as polarization-independent modulators or switchable polarizing beam splitters, for use in telecommunications, displays, spatial-light modulators, and polarimetry.

Polarization insensitive imaging through polarization gratings

Liquid crystal polarization gratings exhibit high diffraction efficiency (approximately 100%) in thin material layers comparable to the radiation wavelength. We demonstrate that they can be combined for polarization-insensitive imaging and optical switching applications. A pair of closely spaced, parallel oriented, cycloidal polarization gratings is capable of canceling the diffractive property of an individual grating. As a result, the phase of the beam is not distorted, and holographic images can be formed through them. An anti-parallel arrangement results in a broader effective diffraction band and doubles the diffraction angle. Broadband diffraction spanning from 480 nm to beyond 900 nm wavelengths has been obtained for a pair of gratings with 500 nm and 633 nm peak diffraction wavelengths. Liquid crystal polymer cycloidal gratings were used in the study showing 98% diffraction efficiency over a large area, and allowed for the use of laser beams expanded to 25 mm. The characteristics of combined cycloidal gratings were tested with laser beams at both UV and red wavelengths.

18:3: Late‐News Paper: Polarization Independent Liquid Crystal Microdisplays

AbstractWe demonstrate a polarization‐independent, diffractive, liquid crystal microdisplay on a reflective 256×256 pixel silicon backplane using nematic liquid crystal polarization gratings. Striking results are observed in a single‐panel projector with remarkably simple optics, where the technology supports up to ∼75% throughput (reflectance) of unpolarized light, contrast ratios approaching 1000:1, and < 800 μs total switching time.

Polarization‐independent modulation for projection displays using small‐period LC polarization gratings

Abstract— Progress in the use of liquid‐crystal polarization grating (LCPG) to modulate unpolarized (and polarized) light with a grating period as small as 6.3 μm is reported. Similar to LCPGs formed at larger periods (11 μm) reported previously, polarization‐independent switching, predominantly three diffraction orders, maximum contrast ratios of ∼100:1 for unpolarized broadband light, very low scattering, and diffraction efficiencies >98% continue to be observed. The smaller period led to an expected lower threshold voltage, even though the thickness was greater. Because the smaller grating period enables a brighter result from a Schlieren projection scheme for a microdisplay using the LCPG light valve, the inherent tradeoffs involved with both material and design parameters are discussed, and prospects for a polarization‐independent projection display are commented upon.

Diffraction characteristics of a liquid crystal polarization grating analyzed using the finite-difference time-domain method

This work studies the polarization characteristics of diffracted beams from a liquid crystal polarization grating. The grating is fabricated by exploiting the photo-alignment effect on a substrate that is coated with an azo dye-doped polyvinyl alcohol (PVA) film. The mechanism is induced by the irradiation of this film with suitably polarized light, which reorients the dyes. The reoriented dyes then align the liquid crystals (LCs). An LC polarization grating is fabricated using this approach. The LC alignment of the grating on one substrate is uni-directionally parallel to the surface, while that on the other is rotated. The polarization and the intensity of the diffracted beams are measured. A simulation based on the finite-difference time-domain (FDTD) method is performed and is very consistent with the experimental results.