Polymer Dispersed Liquid Crystal Gratings Research Articles

Highly flexible and electrically controlled grating enabled by polymer dispersed liquid crystal

Liquid crystal (LC) materials have dielectric and optical anisotropy and are candidates for the preparation of electrically controlled tunable diffraction gratings. Flexible LC gratings have the advantages of high flexibility, light weight, small size, and attachability. At present, the electronically controllable flexible LC grating needs to be further studied. In this paper, polymer-dispersed liquid crystal (PDLC) is poured into the flexible cell made of ITO-PET, and 1D and 2D flexible PDLC gratings are prepared by the method of polymer-induced phase separation. It can realize the modulation of the grating diffraction order through an external electric field under different curvature radii, and it also has the characteristics of the low driving voltage, low saturation voltage, and fast response. The driving voltage and saturation voltage of the 1D flexible PDLC grating are 0.8 V and 12 V, the cell thickness is 5.2 μm, and the rise and decay times are 3.225 ms and 98.29 ms, respectively, the first-order diffraction efficiency at the center is 9.1%. For the 2D flexible PDLC grating, the driving voltage and saturation voltage are 2.5 V and 16.5 V, the cell thickness is 5.2 μm, the rise and decay times are 3.362 ms and 75.46 ms, respectively, the first-order diffraction efficiency at the center is 7.58%.

Electro-optical properties and frequency response of polymer-dispersed liquid crystal gratings doped with multi-walled carbon nanotubes

In this paper, a model developed for establishing the lowest possible driving voltage of Holographic polymer-dispersed liquid crystal (H-PDLC) based on the Maxwell–Wagner effect doped with specific nanotubes. The dependences of the turning point frequency on the doping concentration and the distribution of multi-walled carbon nanotubes (MWCNTs) are discussed. The MWCNTs can effectively adjust the relaxation frequency of grating, increase its LC droplet size, and decrease its dielectric permittivity. The diffraction efficiency and threshold voltage of the prepared switchable Bragg grating were found to reach 91% and 0.77 V μm−1, respectively, at 5 kHz. By comparing the results of experiments and simulations, we obtained the mass distribution ratio of MWCNTs in the LC and polymer regions. It was found that the MWCNTs diffuse into the LC regions during the polymerization process, and the use of a suitable driving frequency and doping concentration can minimize the driving voltage. This work provides a reference for studying the dynamic range optical tuning and polymerization kinetics of nanoparticles in switchable gratings.

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

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.

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.

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.

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.

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

Enhancement of conversion efficiency for an organic semiconductor laser based on a holographic polymer dispersed liquid crystal

In this paper, we report a high-conversion-efficiency organic semiconductor distributed feedback laser. The gain layer of the laser device is made from poly (2-methoxy-5-(20-ethylhexyloxy) p-phenyl-enevinylene) (MEH-PPV), and the holographic polymer dispersed liquid crystal (HPDLC) grating is used as the external light feedback layer. Thus the parameters of the laser device can be modulated independently. The solution of MEH-PPV in xylene (6 mg·mL-1) is deposited on the bottom glass substrate by spin-coating (2000 r/min). The MEH-PPV layer thickness is controlled at (80±2) nm by the spin-coating rate and confirmed by the Dektak profilometer. The HPDLC is made by the photo-induced phase separation method. To determine the orientations of LC molecules, the diffraction efficiency of each sample is measured by a He-Ne laser. The diffraction efficiency is defined as the diffracted light intensity in the first order divided by the incident light intensity. If p light diffraction efficiency (ηp) is much larger (smaller) than s light diffraction efficiency (ηs), it can be thought of as a symbol of a fairly good alignment of LC along the grating vector (grating grooves). When the period of HPDLC grating is larger than 450 nm, ηp is greater than ηs, and the averaged orientation of liquid crystal molecules is aligned along the grating vector direction, i.e., orthogonal to the holographic plane. For feedback light propagating along the grating vector, the refractive index modulation is dependent on the difference between the polymer refractive index np and the ordinary refractive index no of phase-separated LC. These two values are very close to each other, thus the effective light feedback for lasing output is not high. However, when the period of HPDLC grating is smaller than 450 nm, ηs is greater than ηp, and the orientation of phase-separated LC is altered. The refractive index modulation of feedback light originates from the difference between the polymer refractive index np and the extraordinary refractive index ne of phase-separated LC, thus the refractive index modulation can be improved and the HPDLC layer can provide better light feedback. The lasing threshold is 0.70 μJ/pulse, and the conversion efficiency is 2.5% for the sample with a grating period of 593 nm. However, the lasing threshold is lowered to 0.18 μJ/pulse, and the conversion efficiency increases to 6.4% for the sample with a grating period of 395 nm. These results show that the output lasing performance can be improved by using small period grating, since it has bigger refractive index in the grating vector direction (the lasing feedback direction). The laser performance of sample with small grating period is improved in some aspects such as threshold energy, conversion efficiency to some extent compared with those reported previously.

Tri-color composite volume H-PDLC grating and its application to 3D color autostereoscopic display.

A tri-color composite volume holographic polymer dispersed liquid crystal (H-PDLC) grating and its application to 3-dimensional (3D) color autostereoscopic display are reported in this paper. The composite volume H-PDLC grating consists of three different period volume H-PDLC sub-gratings. The longer period diffracts red light, the medium period diffracts the green light, and the shorter period diffracts the blue light. To record three different period gratings simultaneously, two photoinitiators are employed. The first initiator consists of methylene blue and p-toluenesulfonic acid and the second initiator is composed of Rose Bengal and N-phenyglycine. In this case, the holographic recording medium is sensitive to entire visible wavelengths, including red, green, and blue so that the tri-color composite grating can be written simultaneously by harnessing three different color laser beams. In the experiment, the red beam comes from a He-Ne laser with an output wavelength of 632.8 nm, the green beam comes from a Verdi solid state laser with an output wavelength of 532 nm, and the blue beam comes from a He-Cd laser with an output wavelength of 441.6 nm. The experimental results show that diffraction efficiencies corresponding to red, green, and blue colors are 57%, 75% and 33%, respectively. Although this diffraction efficiency is not perfect, it is high enough to demonstrate the effect of 3D color autostereoscopic display.

Paper No P09: Electrically Tunable Grating Using Holographic Polymer Templated Blue Phase Liquid Crystal

AbstractIn this paper, we demonstrate an alternative approach to fabricating an electrically tunable holographic polymer‐templated blue phase liquid crystal grating. This grating is obtained by preforming a polymer grating template, and then refilling it with the blue phase liquid crystal. Compared with conventional holographic polymer dispersed liquid crystal gratings, its driving voltage can be reduced to 43 V, while maintaining sub‐millisecond response time.

Electrically tunable holographic polymer templated blue phase liquid crystal grating**Project supported by the National Basic Research Program of China (Grant No. 2013CB328804), the National Natural Science Foundation of China (Grant No. 61307028), the Funds from the Science and Technology Commission of Shanghai Municipality (Grant Nos. 11JC1405300, 13ZR1420000, and 14ZR1422300), and the Fundamental Research Funds for the Central Universities, China (Grant

In this paper, we demonstrate an alternative approach to fabricating an electrically tunable holographic polymer templated blue phase liquid crystal grating. This grating is obtained by preforming a polymer template comprised of periodic fringes, and then refilling it with a blue phase liquid crystal. Compared with conventional holographic polymer dispersed liquid crystal gratings, our grating can remarkably reduce its switching voltage from 200 V to 43 V while maintaining a sub-millisecond response time. The holographic polymer templated blue phase liquid crystal (HPTBPLC) grating is free from electrode patterning, thus leading to a lower cost and more flexible applications.

Formation of holographic gratings in polymer-dispersed liquid crystals using off-resonant light

This paper presents the electro-optical characteristics of polarization-independent holographic gratings (HGs) recorded in polymer-dispersed liquid crystals (PDLCs) using off-resonant laser beam. The key mechanism is based on the low light absorbance by the materials used, which enables a slow polymerization-induced phase separation. The off-resonant light can penetrate through the cell without much energy loss (absorbance) and can be uniformly absorbed across the LC cell to produce uniform PDLC structures. The intensity-modulated interference field, which is generated by two linearly polarized off-resonant laser beams, is adopted to record the HGs. The fabricated HGs are electrically switchable and polarization independent. Moreover, the diffractions of the HGs fabricated using off-resonant light is better than those produced by resonant light.

Improvement on the Performance of Holographic Polymer-Dispersed Liquid Crystal Gratings with Surface Plasmon Resonance of Ag and Au Nanoparticles

An investigation and comparison of the characteristics of holographic polymer-dispersed liquid crystal (H-PDLC) gratings doped with Ag and Au nanoparticles (NPs) is presented. The experimental results show that the diffraction efficiency and electro-optical properties of holographic gratings are improved through the addition of Ag and Au NPs. It is notable that Ag NP-doped gratings have far superior performance than those containing Au NPs. Furthermore, we find that the diffraction efficiency improvement can be explained as being due to the surface plasmon resonance of the metal NPs embedded in the PDLC. We theoretically simulate the surface plasmon resonance (SPR) wavelengths of the Ag and Au NPs within the PDLC and find that the laser wavelength used to fabricate the holographic gratings is much closer to the SPR wavelength of Ag NPs than that of Au NPs. Atomic force microscopy images show that the phase separation of the Ag-doped grating is improved, and the grating structure is much smoother than those of the undoped samples and the Au NP-doped samples, because of the SPR properties of the Ag NPs in the PDLC microstructure during the holographic recording process.

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.

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.