Liquid Crystal Gratings Research Articles

Switchable beam steering with zenithal bistable liquid-crystal blazed gratings.

Switchable beam steerers based on zenithal bistable liquid crystal (LC) gratings are designed and theoretically investigated. The nematic orientation profiles and the optical transmittance properties of the gratings are rigorously calculated, respectively, via a tensorial formulation of the Landau-de Gennes theory and the full-wave finite-element-method. By proper design of the grating geometry, beam steering with high diffraction efficiency is demonstrated between the two stable LC states. The tolerance of the device performance with respect to material parameters is assessed, evidencing spectral operation windows of more than 50 nm in the visible for a beam steering efficiency higher than 90%.

Monochromatic visible light "photoinitibitor": Janus-faced initiation and inhibition for storage of colored 3D images.

Controlling the kinetics and gelation of photopolymerization is a significant challenge in the fabrication of complex three-dimensional (3D) objects as is critical in numerous imaging, lithography, and additive manufacturing techniques. We propose a novel, visible light sensitive "photoinitibitor" which simultaneously generates two distinct radicals, each with their own unique purpose-one radical each for initiation and inhibition. The Janus-faced functions of this photoinitibitor delay gelation and dramatically amplify the gelation time difference between the constructive and destructive interference regions of the exposed holographic pattern. This approach enhances the photopolymerization induced phase separation of liquid crystal/acrylate resins and the formation of fine holographic polymer dispersed liquid crystal (HPDLC) gratings. Moreover, we construct colored 3D holographic images that are visually recognizable to the naked eye under white light.

Simulation and optimization of liquid crystal gratings with alternate twisted nematic and planar aligned regions.

Electro-optical properties of liquid crystal (LC) gratings with alternate twisted nematic (TN) and planar aligned (PA) regions are simulated. Three typical steps are introduced: first, the LC director distributions of the two different regions are simulated. Then, the phase and amplitude of the emergent light in each region are calculated through Jones matrix. Based on this information, the voltage-dependent diffraction efficiency is achieved by Fourier transformation, finally. It gives an exact explanation for the mechanism of this kind of gratings. Experiments with optimized parameters are carried out through photopatterning. The trend of the measured voltage-dependent efficiency fits the simulation result very well. This method can be used to optimize the performance of LC gratings with alternate TN and PA regions, and exhibits great potential in the simulation of corresponding photonics and display applications.

The Study of a Novel Optical Communication Device

A novel optical communication device is investigated in this paper. The device is a kind of optical switch. The intensity of emergent light is tunable continuously by introducing liquid crystal technique. Holographic technique is used to fabricate this optical device. According to optical theories and through many experiments, the ideal optical switching device can be obtained when the photosensitive monomers are mixed together with liquid crystal with certain weight ratio. The emergent light intensity of liquid crystal grating can be modulated by electric field because of the electric control characteristics of liquid crystal. The output light is only 5% of the incident light when the electric field is more than 15V. Besides, the liquid crystal also has the advantages of large electro-optic coefficient, fast response speed and so on. Consequently, the liquid crystal grating has immense potential productivity in passive optical communication device such as optical switching and variable optical attenuator.

Gelator-doped liquid-crystal phase grating with multistable and dynamic modes

We demonstrate a gelator-doped nematic liquid-crystal (LC) phase grating, which can be operated in both the multistable mode and the dynamic mode. Thermoreversible association and dissociation of the gelator molecules can vary and fix the multistable diffraction efficiencies of the gratings. A voltage (V) can also be applied to modulate dynamically the diffraction efficiencies of the grating, which behaves as a conventional LC grating. Experimental results show that the variations of the diffraction efficiencies in the multistable and dynamic modes are similar. The maximum diffraction efficiency is approximately 30% at V = 2 V.

Highly diffractive, reversibly fast responsive gratings formulated through holography

Versatile and reversibly rapid responsive one-dimensional photonic crystals with a diffraction efficiency of 97%, which consisted of uniformly 273 ± 48 nm wide liquid crystal belts within transmission holographic polymer dispersed liquid crystal (HPDLC) gratings, were formulated by a facile single step holography based on a hyperbranched monomer. The effect of the monomer average functionality on the photopolymerization kinetics and the electro-optical performances as well as the grating morphologies was investigated. The results show that the low intrinsic viscosity of the hyperbranched monomer accounts for a well-structured morphology in terms of providing a prolonged gelation time for the liquid crystals to diffuse from the light illumination region during the holographic polymerization induced phase separation. Another intriguing observation is that with an increase in the hyperbranched monomer loading, the diffraction efficiency of the HPDLC gratings gradually increases from zero to an average of 94% and then levels off. This is quite different from previous results that gave less than a 50% diffraction efficiency when the monomer average functionality was larger than 4.

Nanosecond switching of photo-responsive liquid crystal diffraction gratings

The diffraction efficiency of periodic microstructures composed of photoresponsive liquid crystals and polymer gratings is modulated using visible CW or pulsed radiation. A polymeric template (static grating) enabled by holographic photopatterning was infiltrated by capillary flow with a high performance photosensitive (green) liquid crystal after the initial formation step. The infiltration realizes high quality diffractive structures containing well-aligned photosensitive LC molecules and layers separated by polymer walls in a transmission grating geometry. The optical anisotropy present due to the oriented liquid crystals induces a strong polarization dependence of the grating diffraction efficiency. Exposure of the periodic structure to green CW or pulsed excitation modulates both the spectral and amplitude transmission behavior of the grating structure. In particular, under the influence of single pulse excitation, the diffraction efficiency can be modulated from 87% to 17% in 4 ns with a spontaneous back relaxation time three orders of magnitude (microsecond) faster than the one observed for the same material conventionally aligned in standard glass cells. The ability to remotely and quickly change the diffraction properties of soft-composite periodic microstructures marks a breakthrough towards the realization of ultra-fast all-optical devices.

纳米银掺杂的液晶/聚合物全息光栅中的表面等离子体共振

纳米银掺杂的液晶/聚合物全息光栅中的表面等离子体共振

Low threshold distributed feedback laser based on scaffolding morphologic and holographic polymer dispersed liquid crystal gratings

We have made a low scattering holographic polymer dispersed liquid crystal (HPDLC) transmission grating with polymer scaffolding morphology, which was fabricated under the condition of low curing intensity and no liquid crystal droplet. We studied the amplified spontaneous emission (ASE) thresholds and relative intensities of PM567(4, 4-difluoro-1, 3, 5, 7, 8-pentamethyl-2, 6-diethyl-4-bora-3a, 4a-diaza-s-indacene), DCM (4-Dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran), and DCJTI(4-dicyanomethylene)-2-isopropyl-6-(1, 1, 7, 7-tetramethylzulolidyl-9-enyl)-4H-pyran). It is shown that the dye DCJTI has the best properties of ASE, the lowest threshold and the highest relative intensity among the three dyes. Results suggest that DCJIT is a promising material for low threshold, high slope efficiency lasers. Each dye was doped in HPDLC grating with polymer scaffolding morphology individually, and lasers with different wavelengths can be obtained by changing the period of the gratings. The excellent laser property is obtained from the DCJIT-doped laser. A spectral linewidth of 0.3 nm is observed at pump energy threshold 0.65 J/pulse and a conversion efficiency of 1.6% is achieved at the operating wavelength 635 nm. The laser performance is improved in some aspects such as threshold energy, conversion efficiency, and linewidth to some extent as compared with those reported previously.

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

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

Electro-optical characteristics of holographic polymer dispersed liquid crystal gratings doped with nanosilver

We report on the synthesis and characteristics of a holographic polymer dispersed liquid crystal (H-PDLC) switchable grating based on nano-Ag particles. The influence of doping different concentrations of nano-Ag on the diffraction efficiency, driving voltage, and response time of the H-PDLC grating is investigated. The best grating characteristics were achieved with 0.05% nano-Ag doping. Calculated and experimental results reveal that the improvement of the characteristics is likely due to the surface plasmon effect of nano-Ag.

Polarization independent blue phase liquid crystal gratings based on periodic polymer slices structure

A polarization independent switchable phase grating based on polymer stabilized blue phase liquid crystal is proposed. A high efficiency of the phase grating has been achieved because of the sharp rectangular phase profile which shows good agreement with the simulation results. The diffraction efficiency of the 1st order is 38%, the response time is in the submillisecond range, and the phase grating is independent of the polarization of the incident light. The voltage-induced hysteresis characteristics are also investigated.

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

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

Paper No 9.3: Liquid Crystal Gratings Technologies and Ferroelectric Liquid Crystal‐Based Gratings

AbstractIn this article, existing technologies of switchable liquid crystal gratings and their limitations are discussed. A new mode ferroelectric liquid crystal with its unique characteristics is presented, which shows great potential for grating use. Some possible fabrication procedures and results are also presented in this paper.

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.

Liquid Crystal Gratings with Twisted Alignment Produced by One-Step Polarizer-Rotation Exposure on Photocrosslinkable Polymer Liquid Crystal Films

Liquid crystal (LC) gratings with twisted alignment were fabricated by simple one-step polarizer-rotation exposure. An empty glass cell coated with photocrosslinkable polymer liquid crystal (PCLC) films was exposed by UV light with modulation of intensity or polarization. LC grating cells were obtained by filling the resultant cells with a nematic LC and the LC alignment structures were well controlled by both the intensity and polarization of the UV light. The diffraction properties of the LC gratings were explained by considering the periodic director distribution in the cells, estimated based on the elastic continuum theory of nematic LCs and the photoalignment effects of the PCLC films.

Optically rewritable ferroelectric liquid-crystal grating

This paper describes the fabrication procedure and optical characterization of ferroelectric liquid-crystal (FLC) gratings based on photo-alignment. The fabrication procedure includes only one side photo-alignment substrate, while the other substrate does not have any alignment layer. Both 1D and 2D gratings have been fabricated. The proposed diffraction element shows high diffraction efficiency ∼ 65% and fast response time of 50 μs, which is much faster than the existing technologies. Such gratings can be operated with high frequency of around 2 kHz at the electric field of 6.67 V/μm. Moreover, the proposed grating can be erased and rewritten optically for different grating vector in simple steps. Therefore, with these advance features, such gratings have high potential to be applied in verity of devices and for the improvement of some important existing devices.

纳米银掺杂的高效率全息聚合物分散液晶光栅制备

纳米银掺杂的高效率全息聚合物分散液晶光栅制备

Holographically Formed, Acoustically Switchable Gratings Based on Polymer-Dispersed Liquid Crystals

We report holographic polymer-dispersed liquid crystal (H-PDLC) gratings driven by surface acoustic waves (SAWs). Our experiments show that upon applying SAWs, the H-PDLC grating exhibited switchable properties: The diffraction of the H-PDLC grating decreased, whereas the transmission increased. This acoustically switchable behavior is due to the acoustic streaming-induced realignment of liquid crystals as well as absorption-resulted thermal diffusion. Such SAW-driven H-PDLC gratings are potentially useful in many photonic applications, such as optical switches, spatial light modulators, and switchable add/drop filters.

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.