Liquid Crystal Gratings Research Articles

Optical edge detection with adjustable resolution based on liquid crystal polarization gratings

Optical edge detection, a part of image processing, plays an important role in extracting image information used in optical analog computation. In this Letter, we raise a new way to realize optical edge detection. This design is based on two liquid crystal polarization gratings with a period of 2.2 mm, which function as a spatial differentiator. We experimentally demonstrate broadband optical detection and real-time adjustable resolution. The proposed method takes advantage of the convenience to use, simple fabrication process, and real-time tunable resolution. It may guide more significant applications in the optical field and other practical scenarios like machine vision in computers.

Photopatterned azo poly(amide imide) layers as aligning substrates of holographic liquid crystal diffraction gratings for beam steering applications

Custom synthesized “T-type” azobenzene-functionalized poly(amide imide) allows for effective fabrication of a tunable liquid crystal photonic device for light beam steering.

Electro-optical characteristics of polymer-dispersed liquid crystal gratings doped with multi-walled carbon nanotubes

Electro-optical characteristics of polymer-dispersed liquid crystal gratings doped with multi-walled carbon nanotubes

Chiral Liquid Crystals: Tilted Chiral Liquid Crystal Gratings for Efficient Large‐Angle Diffraction (Advanced Optical Materials 22/2019)

The ability to steer light with miniaturized optical components has attracted great interest for next‐generation technologies. To efficiently diffract light over large angles, Inge Nys and co‐workers (article number 1901364) have developed a flat optical component with a sub‐micrometer period. Periodic photo‐alignment at the surface is used in combination with chiral liquid crystal and results in the self‐organization of a slanted unperturbed chiral structure in the bulk. The periodic structure at the interface can be matched perfectly with the chiral period by providing an appropriate slant angle. The resulting device achieves a high efficiency for reflective diffraction over large angles and shows strong polarization‐sensitivity.

Two-dimentional diffractive optical structure based on patterned photoalignment of polymerizable liquid crystal

This paper is devoted to the investigation of the diffraction characteristics of a multilayer optical structure, represented by a two-dimensional phase lattice formed in thin-film layers of nematic polymerizable liquid crystal (PLC). In order to generate periodically ordered liquid-crystal (LC) domains with dimensions of 5–10 µm, which form an anisotropic LC grating, the layer-by-layer patterned photoalignment technology of the water-soluble azo dye AbA-2522 was used. The possibilities of spatial-polarization control of light beams by means of developed one- and two-dimensional diffractive LC structures have been studied experimentally. Our results are promising from the point of view of development and creation of LC-devices for solving current problems of polarization photonics.

Tilted Chiral Liquid Crystal Gratings for Efficient Large‐Angle Diffraction

AbstractDeflecting light with high efficiency over large angles with thin optical elements is challenging but offers tremendous potential for applications, such as wearable displays and optical communication systems. Compared to the complex production of metasurfaces, the self‐organization of liquid crystal (LC) superstructures provides an elegant and flexible way to produce high‐quality thin optical components. The periodically varying dielectric tensor in short‐pitch chiral LC gives rise to a photonic bandgap, which can be exploited to realize efficient diffractive mirrors in the visible wavelength range. However, large‐angle diffractive devices require a small in‐plane period, leading to complex self‐assembly behavior in the bulk. This work demonstrates that by patterning photoalignment layers at the surfaces with a period comparable to the chiral pitch, the LC self‐assembles into a tilted, defect‐free helical structure. The director configuration is calculated by finite element simulations and it is experimentally demonstrated that a single photoaligned substrate is sufficient to template the tilted chiral structure in the bulk. This structure effectively (88%) diffracts light over large angles (≈46°) and enables novel micrometer‐thin (≈3 µm) optical components that can be produced with an elegant manufacturing process. Due to flexibility of photoalignment, this process could easily be implemented in emerging photonic applications.

Influence of structural parameters on organic semiconducting polymer lasers with an external holographic grating as feedback layer

We report on the fabrication and characterization of an organic distributed feedback laser excited from a semiconducting polymer gain layer, poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenyl-envinylene) (MEH-PPV). A holographic polymer dispersed liquid crystal grating is fabricated on top of the homogeneous MEH-PPV layer as an external cavity. We established a model to calculate the confinement factor Γ. With the change of laser structural parameters (the refractive index of the grating layer, the film thickness of the MEH-PPV layer, etc.), the confinement factor Γ changes accordingly. It was found that a single longitudinal lasing mode could be observed only when the confinement factor Γ was in a specific range. Choosing a reasonable thickness of gain film and prepolymer with higher refractive index can obtain a lower threshold emission laser.

Electrically tunable dual-wavelength organic laser based on holographic polymer dispersed liquid crystal grating

Abstract An organic dual-wavelength surface-emitting distributed feedback (DFB) laser with low threshold and high conversion efficiency based on holographic polymer dispersed liquid crystal (HPDLC) grating is reported. In such a laser configuration, the dye blend 4-(dicyanomethylene)-2-methyl-6-( p -dimethylaminostyryl)-4H-pyran (DCM)/1,3,5,7,8-pen- tamethyl-2,6-diethylpyrromethene-difluoroborate (PM567) -doped HPDLC grating is formed on the top of an organic semiconductor poly(2-methoxy-5-(20 -ethyl-hexyloxy)-p- phenylenevinylene) (MEH-PPV) layer. The dual-wavelength laser located at 605.2 nm and 629.8 nm are obtained in a single beam. The dual-wavelength laser performance under an external electric field are investigated and illustrated for the first time. This study provides some new ideas for the improvement of tunable dual-wavelength laser and extends more applications in laser displays and integrated photonic circuits.

Device simulation of liquid crystal polarization gratings.

Liquid crystal polarization gratings manifest several unique features, such as high diffraction efficiency, polarization selectivity, and fast switching time. However, few works address the chiral-doped liquid crystal alignment issue in such gratings. Here, we develop an improved relaxation method to analyze the liquid crystal director distribution in chiral-doped polarization gratings. Our simulation result agrees well with experimental data on a polarization volume grating. The criteria for forming planar or slanted polarization grating are discussed.

Thermoplasmonic Activated Reverse-Mode Liquid Crystal Gratings

A new generation of reconfigurable optical components is conceived by bridging the photothermal properties of gold nanoparticles and the thermosensitivity of liquid crystalline materials. As such, gold nanorods (GNRs) heated using light are used to activate efficient hidden diffraction gratings realized in a blend made of a room temperature polymerizable liquid crystal (PLC) and nematic liquid crystal (NLC). Holographic liquid crystal polymer dispersed liquid crystal (HLCPDLC) gratings containing a small percentage of GNRs are fabricated with periodicity of 2.6 μm by means of a conventional UV holographic recording setup. The HLCPDLC structures are characterized using morphological, optical, and thermooptical techniques. Because of the initial refractive index match between the polymer-rich and LC-rich regions, the “grating” is hidden and the films appear transparent and nondiffractive. Illumination of these GNR-containing structures with a suitable light source (808 nm) induces a local heating due to the...

Diffusion kinetics investigations of Nano Ag-doped holographic polymer dispersed liquid crystal gratings

ABSTRACTIn this paper, we use multicomponent mutual diffusion method to derive a one-dimensional non-local diffusion dynamic model to describe the diffusion kinetics of a dynamic holographic polymer dispersed liquid crystal grating (H-PDLC) doped with nano-silver. The physical mechanism of diffusion between monomer and liquid crystal, monomer and nano-silver particles is analysed using this model. Using coupled-wave theory, the H-PDLC’s diffraction efficiency curve with the expose time are simulated due to the vivid changing of effective refractive index modulation caused by the movement of concentration of each component with the expose time. Correspondingly, in the experiment, the diffraction efficiency of the grating is measured in real time, which shows the improvement for the holographic properties because of nano-silver doped H-PDLC. The simulation results have a good agreement with experimental data by fitting the corresponding parameters of the model. In addition, through comparing with simulation and experimental results with doping different concentrations of nano-silver particles, the recipe and diffraction characteristics of H-PDLC grating can be improved. Thus, the diffusion Kinetics model can be used to optimise the phase separation of the PDLC grating, and finally to improve the opto-electrical properties of H-PDLC gratings.

Optical control of the rotation of cholesteric liquid crystal gratings.

Two photoalignment-based methods to achieve orientational control of optical diffractions by cholesteric liquid crystal (CLC) fingerprint gratings are proposed and demonstrated. A trace of methyl red in the CLC host can effectively induce surface alignment upon linearly polarized green exposure and enable optically rewritable alignment. An effective rotation of the photo-aligned CLC grating is attained by changing the surface alignment axis. Using axially symmetric photoalignment, electrically tunable radial and concentric gratings are also realized. 1D grating diffraction is produced by operating off-axis and can be rotated by mechanically moving the axially symmetric grating. Such optical gratings have great potential for practical use in vibration detection, multi-directional optical modulations, and beam steering.

Diffractive and Refractive Liquid Crystal Devices Based on Multilayer Matrices

Tunable photonic devices based on self-organized materials like liquid crystals are opening new alternatives for photonic applications, especially when several optical functions can be implemented into one device. Beam steering devices based on liquid crystals usually work either as digital beam steerers (phase gratings), or as analog steerers (prisms). Digital steerers move laser beams to fixed positions; they cannot be tuned continuously. Analog beam steerers produce a continuous angular deviation of laser beams but with very limited deviation angles. In this paper, we developed a tunable grating prism, that can work as a prism and as a phase grating at the same time. This combination of refractive and diffractive effects is achieved with multilayer indium tin oxide (ITO) electrode matrices in a liquid crystal device. This solution is completely different from previous approaches, like blaze gratings or Fresnel configurations - the tunability range here is completely continuous, or completely stepped, or a combination of both at the same time - as there are no interpixels or dead areas in the multilayer configuration. The proposed grating prism is able to deviate a light beam to a large fixed angle by diffraction, and then fine tune the beam over small angle ranges by refraction. Additionally, the light losses produced by ITO diffraction in conventional liquid crystal gratings are significantly reduced using the layered configuration.

Electrically tunable holographic waveguide display based on holographic polymer dispersed liquid crystal grating

In this Letter, we present an electrically tunable holographic waveguide display (HWD) based on two slanted holographic polymer dispersed liquid crystal (HPDLC) gratings. Experimental results show that a see-through effect is obtained in the HWD that both the display light from HWD and the ambient light can be clearly seen simultaneously. By applying an external electric field, the output intensity of the display light can be modulated, which is attributed to the field-induced rotation of the liquid crystal molecules in the two HPDLC gratings. We also show that this electrically tunable performance enables the HWD to adapt to different ambient light conditions. This study provides some ideas towards the development of HWD and its application in augmented reality.

Dynamic Control of Light Direction Enabled by Stimuli-Responsive Liquid Crystal Gratings.

The ability to control light direction with tailored precision via facile means is long-desired in science and industry. With the advances in optics, a periodic structure called diffraction grating gains prominence and renders a more flexible control over light propagation when compared to prisms. Today, diffraction gratings are common components in wavelength division multiplexing devices, monochromators, lasers, spectrometers, media storage, beam steering, and many other applications. Next-generation optical devices, however, demand nonmechanical, full and remote control, besides generating higher than 1D diffraction patterns with as few optical elements as possible. Liquid crystals (LCs) are great candidates for light control since they can form various patterns under different stimuli, including periodic structures capable of behaving as diffraction gratings. The characteristics of such gratings depend on several physical properties of the LCs such as film thickness, periodicity, and molecular orientation, all resulting from the internal constraints of the sample, and all of these are easily controllable. In this review, the authors summarize the research and development on stimuli-controllable diffraction gratings and beam steering using LCs as the active optical materials. Dynamic gratings fabricated by applying external field forces or surface treatments and made of chiral and nonchiral LCs with and without polymer networks are described. LC gratings capable of switching under external stimuli such as light, electric and magnetic fields, heat, and chemical composition are discussed. The focus is on the materials, designs, applications, and future prospects of diffraction gratings using LC materials as active layers.

Numerical Analysis of H-PDLC Using the Split-Field Finite-Difference Time-Domain Method.

In this work, an accurate numerical modeling of the diffraction properties of transmission holographic polymer dispersed liquid crystal (H-PDLC) gratings is presented. The method considers ellipsoid geometry-based liquid crystal (LC) droplets with random properties regarding size and location across the H-PLDC layer and also the non-homogeneous orientation of the LC director within the droplet. The direction of the LC director inside the droplets can be varied to reproduce the effects of the external voltage applied in H-PDLC-based gratings. From the LC director distribution in the droplet, the permittivity tensor is defined, which establishes the optical anisotropy of the media, and it is used for numerically solving the light propagation through the system. In this work, the split-field finite-difference time-domain method (SF-FDTD) is applied. This method is suited for accurately analyzing periodic media, and it considers spatial and time discretisation of Maxwell’s equations. The scheme proposed here is used to investigate the influence on the diffraction properties of H-PDLC as a function of the droplets size and the bulk fraction of LC dispersed material.

Hidden Gratings in Holographic Liquid Crystal Polymer-Dispersed Liquid Crystal Films.

Dynamic diffraction gratings that are hidden in the field-off state are fabricated utilizing a room-temperature photocurable liquid crystal (LC) monomer and nematic LC (NLC) using holographic photopolymerization techniques. These holographic LC polymer-dispersed LCs (HLCPDLCs) are hidden because of the refractive index matching between the LC polymer and the NLC regions in the as-formed state (no E-field applied). Application of a moderate E-field (5 V/μm) generates a refractive index mismatch because of the NLC reorientation (along the E-field) generating high-diffraction efficiency transmission gratings. These dynamic gratings are characterized by morphological, optical, and electrooptical techniques. They exhibit a morphology made of oriented LC polymer regions (containing residual NLC) alternating with a two-phase region of an NLC and LC polymer. Unlike classic holographic polymer-dispersed LC gratings formed with a nonmesogenic monomer, there is index matching between the as-formed alternating regions of the grating. These HLCPDLCs exhibit broad band and high diffraction efficiency (≈90%) at the Bragg angle, are transparent to white light across the visible range because of the refractive index matching, and exhibit fast response times (1 ms). The ability of HLCPDLCs not to consume electrical power in the off state opens new possibilities for the realization of energy-efficient switchable photonic devices.

Thermally and optically tunable lasing properties from dye-doped holographic polymer dispersed liquid crystal in capillaries

In this paper, we investigated tunable lasing properties from the dye-doped holographic polymer dispersed liquid crystal (HPDLC) gratings in capillaries with thermal and optical manners. The thermally tunable range of the lasing from the dye-doped HPDLC reached 8.60 nm with the temperature ranging from 23 °C to 50 °C. The optically tunable laser emission was achieved by doping azo-dye in HPDLC. The transition of azo-dye from trans- to cis-state could induce the reorientation of LC molecules after UV light irradiation, which resulted in the variation of refractive index contrast of LC-rich/polymer-rich layer in HPDLC. Experimentally, the emission wavelength of lasing showed a blueshift (about 2 nm) coupled with decreasing output intensities. The tunable laser based on HPDLC may enable more applications in laser displays, optical communication, biosensors, etc.

基于液晶/聚合物光栅的可调谐双波长有机激光器

基于液晶/聚合物光栅的可调谐双波长有机激光器

Three Dimensional Surface Profile Measurement Based on Grating Pattern Projection Method Using Liquid Crystal Grating

Three Dimensional Surface Profile Measurement Based on Grating Pattern Projection Method Using Liquid Crystal Grating