Dye-doped Polymer-dispersed Liquid Crystal Research Articles

Preparation of dye-doped polymer-dispersed liquid crystals using acrylic monomers, and enhancement of contrast by using THF solvent as a cleaning agent

Polymer-dispersed liquid crystal (PDLC), with characteristics of high contrast and low driving voltage, has found many product applications and is an important material for new generations. In this experiment, we used 2-phenoxy ethyl acrylate (2-PEA) and tripropylene glycol diacrylate, two acrylic monomers, to prepare PDLC, and we compared various preparation conditions in order to investigate the impact of structural properties on the optical behavior of PDLC. While maintaining the low driving voltage and high contrast of a PDLC, a dichroic dye was added to turn the device into a dye-doped PDLC. Using a polarized light microscope, it was observed that direct injection of the dye resulted in an overflow of the dichroic dye outside the thin film, reducing the contrast of the PDLC. Therefore, we utilized tetrahydrofuran solvent to wash away the liquid crystal (LC) from the PDLC, followed by re-addition of the dye-containing LC. This successfully solved the dye overflow problem and was found to enhance the PDLC contrast effectively.

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.

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.

The Improvement of Polymerization Characteristics for Reflective Dye-Doped Polymer Dispersed Liquid Crystal (PDLC) Display

The Improvement of Polymerization Characteristics for Reflective Dye-Doped Polymer Dispersed Liquid Crystal (PDLC) Display

Dichroic Dye Induced Nonlinearity in Polymer Dispersed Liquid Crystal Materials for Display Devices

The electro-optical characteristics of dye-doped polymer dispersed liquid crystals (PDLCs) have been investigated for display applications. The PDLC samples were obtained by polymerization induced phase separation of the nematic liquid crystal (LC)-dye-prepolymer mixtures under UV illumination with a constant intensity. The optimum conditions for the scattering characteristics of the dye-doped PDLC films as function of the dye concentration have been examined. It was seen that the phase separation and segregation of LC droplets is dependent on the amount of dye used. LC droplets in dye-doped PDLC films exhibit various configurations at lower and higher applied electric field when observed in situ under polarizing optical microscope. The effects of morphology on the electro-optical properties were examined. Experimental results indicate that the driving voltage and contrast ratio were affected considerably by the amount of dye. UV-VIS spectroscopy results showed that the molecular orientation of dye in LC droplets can be controlled to induce nonlinearity in these materials. The results showed that the dye concentration can be optimized to obtain promising electronic materials with minimum threshold and high contrast for display applications.

Random lasers in dye-doped polymer-dispersed liquid crystals containing silver nanoparticles

Abstract We report on the observation of random lasers for the first time in dye-doped polymer-dispersed liquid crystals (PDLCs) containing nano-sized liquid crystal droplets and silver nanoparticles. The dye-doped polymer-dispersed liquid crystal containing silver nanoparticles film is exposed by a collimated 532 nm Nd: YAG (yttrium aluminum garnet) laser beam, so that it is quickly cured. Under the excitation of a frequency-doubled Nd: YAG (yttrium aluminum garnet) laser operating at a wavelength of 532 nm, random lasing from dye-doped PDLCs containing Ag nanoparticles is observed as a result of cooperative effect due to light scattering of nano-sized liquid crystal droplets and the local field enhancement capabilities around silver nanoparticles. We show that the threshold of the random lasing is about 0.95 μ J / pulse which is lower than the lasing threshold of dye-doped polymer-dispersed liquid crystals containing nano-sized liquid crystal droplets. The linewidth of the lasing peaks is shown to be 0.2 nm. We also propose a possible mechanism to explain the random lasing from dye-doped polymer-dispersed liquid crystals containing silver nanoparticles.

Applications of multidirectional reflective light-control films on reflective polymer-dispersed liquid crystal displays for enhancement in image quality at lower viewing angles

In this paper, multidirectional light-control reflective (LCR) films are developed in order to create an active reflective structure that will enhance the image brightness and contrast ratio of reflective dye-doped polymer-dispersed liquid crystal (D-PDLC) displays at lower viewing angles. Advantages of LCR films are that their production is low cost and they require a simple photolithographic fabrication method. The optimum design prism-type light-control reflective film succeeded in minimising the surface scattering effect; thus, the contrast ratio is much enhanced. The symmetric and asymmetric LCR films produced multidirectional scattering that enhances the reflectance at lower viewing angles, which has importance in future display applications. In particular, the prism LCR film has been found to be more influential on the reflectance of D-PDLC films due to multidirectional scattering of light by non-symmetric arrays. The improvement in contrast ratio has been confirmed by the enhancement of optical properties for reflective D-PDLC displays at lower viewing angles below 30°.

Investigation of nonionic diazo dye-doped polymer dispersed liquid crystal film

Sudan black B (SBB) was used to investigate as the nonionic diazo dye-doped in polymer dispersed liquid crystal (PDLC) display, by polymerization-induced phase separation (PIPS) method. The maximum absorbance, contrast ratio, dichroic ratio and the order parameter of nonionic diazo dye in nemetic host (TL203) were investigated using UV–Vis polarized spectroscopy. The orientation of the dye molecules was controlled by electric field, which enabled the contrast ratio of the dye to be obtained by electrically switching. The change occurring on droplet morphologies and electro-optical properties of PDLC film with the change in contents of Sudan black dye and liquid crystals (LC) contents was investigated. We found an increase in LC droplet sizes with the increase of diazo dye and LC contents. Moreover the addition of small amount of nonionic diazo dye reduced the threshold voltage (V $_{\boldsymbol{\rm th}})$ , increased off-state transmittance, enhanced the contrast ratio and decreased the response time of dye-doped PDLC. Additionally the change in transition temperature of LC and changes in LC droplet morphologies with the addition of dye were also observed. Such changes were observed with the images taken by polarized optical microscope (POM). The detail discussions on such behaviours were also made.

Wavelength tunable properties for distributed feedback lasing from dye-doped holographic polymer dispersed liquid crystal transmission grating

In this paper, we study the wavelength tunable properties for distributed-feedback lasing from 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) dye-doped holographic polymer dispersed liquid crystal grating. By preparing and pumping the DCM-doped HPDLC gratings with various grating spacings, we depict the tuning curve of the distributed-feedback lasing and confirm that only in a range from 574 nm to 685 nm, the lasing action can be observed. Then, we adjust the temperature of the sample and measure the lasing spectrum from the DCM-doped holographic polymer dispersed liquid crystal grating with a grating period of 610nm. There is an obvious 4.9 nm blueshift from 627.9 nm to 623 nm for the lasing wavelength with the temperature increasing from 20℃ to 65℃.

Organic dual-wavelength distributed feedback laser empowered by dye-doped holography

An organic dual-wavelength distributed feedback (DFB) laser empowered by a dye-doped holographic polymer dispersed liquid crystal (HPDLC) grating is reported for the first time. The dual-wavelength laser operates at 586.6 nm and 670.2 nm simultaneously in one laser beam via the seventh and eighth Bragg orders of a one dimensional (1D) HPDLC grating with a special period. We theoretically and experimentally demonstrate the mechanism of our dual-wavelength DFB laser and investigate the threshold and spectral properties of the output laser. Our organic dual-wavelength DFB laser shows the advantages of simple configuration, one-step fabrication, cost-effectiveness and the potential application to be integrated in all optical networks.

Electro-optical characteristics of dye-doped polymer dispersed liquid crystals

The electro-optical properties of dye-doped Polymer Dispersed Liquid Crystal (PDLC) films were examined as a function of the polymer UV curing temperature, injection temperature and applied voltage. In addition, the effects of the morphology on the electro-optical properties were examined. The contrast ratio and driving voltage of the PDLC films were affected considerably by changes in the UV curing temperature but the injection temperature had a much lesser effect. The color difference and electro-optic characteristics of the dye doped PDLC film were investigated, where the PDLC films were doped with RGB dyes for flexible display applications. The red-dye-doped PDLC film with small-sized particles showed good color differences. In this dye-doped PDLC system, the film driving voltage ( V on) of the PDLC increased with increasing liquid crystal droplet size, except for the TL205–prepolymer–red dye. In particular, the red-dye doped PDLC composition showed the lowest V on in the electro-optic characteristics of the dye-doped PDLC film.

Distributed-feedback lasing from dye-doped holographic polymer dispersed liquid crystal transmission grating

In this paper,we report the fabrication of dye-doped holographic polymer dispersed liquid crystal(HPDLC) transmission grating and the property of distributed-feedback lasing from dye-doped HPDLC transmission grating. Under the excitation of a frequency-doubled Nd:yttrium-aluminum-garnet laser operating at a wavelength of 532 nm, optically pumped lasing with narrow linewidth and low threshold was observed from a DCM dye-doped HPDLC transmission grating with the grating period of 586 nm. The results showed that the emitted lasing peak centered at about 603 nm with a full width at half maximum (FWHM) of only 1.4 nm, and the threshold pumping intensity was about 17.3 μJ, which is evidently lower than the previously reported values.

Dye-Doped Polymer Dispersed Liquid Crystal Films for Flexible Displays

Red, green, and blue dyes were doped to polymer dispersed liquid crystal (PDLC) films for flexible display applications. Dichroic dye-doped liquid crystal droplets had a bipolar configuration. The E7–DG6071–dye composition showed better chromaticity data than other compositions. The small-particle-size red-dye-doped PDLC film showed good color differences. To improve the color difference, the dye particle size has to be small, and the bead milling process can make dye particles small. In this system, the bigger the liquid crystal droplet size, the higher the PDLC film driving voltage (Von), except in the LC–DG6071–red dye composition. This is the reason that the splay deformation increase is greater than the droplet size increase. In the electro-optic characteristics of dye-doped PDLC film, the TL205–DG6071–red dye composition had the lowest Von and the TL205–DG7052–red dye had the highest contrast ratio.

All-optically controllable random laser based on a dye-doped polymer-dispersed liquid crystal with nano-sized droplets

This study elucidates for the first time an all-optically controllable random laser in a dye-doped polymer-dispersed liquid crystal (DDPDLC) with nano-sized LC droplets. Experimental results demonstrate that the lasing intensity of the random laser can be controlled to decrease by increasing irradiation time/intensity of one green beam, and increase by increasing the irradiation time of one red beam. The all-optical controllability of the random laser is attributed to the green (red)-beaminduced isothermal nematic-->isotropic (isotropic-->nematic) phase transition in LC droplets by trans-->cis (cis-->trans back) isomerization of azo dyes. This isomerization may decrease (increase) the difference between the refractive indices of the LC droplets and the polymer, thereby increasing (decreasing) the diffusion constant (or transport mean free path), subsequently decreasing the scattering strength and, thus, random lasing intensity.

Polarizer-free, electrically switchable and optically rewritable displays based on dye-doped polymer-dispersed liquid crystals

This work demonstrates the feasibility of a polarizer-free, electrically switchable and optically rewritable display based on dye-doped polymer-dispersed liquid crystals (DD-PDLCs). Experimental results indicate that the doped dyes are homeotropically adsorbed onto the polymer film when an appropriate AC voltage is applied during patterning. The adsorbed dyes in the illuminated region then align the liquid crystals homeotropically, and produce a transparent pattern in the scattering background without any polarizer. Notably, the adsorbed dyes can be erased and readsorbed using thermal and optical treatments, respectively. The switching time of the fabricated display is of the order of milliseconds, and the contrast ratio is approximately 30.

Switchable Random Laser From Dye-Doped Polymer Dispersed Liquid Crystal Waveguides

A dye-doped polymer-dispersed liquid crystal (PDLC) film has been fabricated for random lasing action. In this PDLC film, the sizes of most liquid crystal (LC) droplets ranged from 200 to 500 nm. When the sample is optically pumped, ultrahigh Q (>10 000) lasing modes and a collimated laser beam can be observed. The threshold of the random laser is shown to be 0.23 mJ/cm2. Additionally, a 9.2-V/mum external electric field was applied to control the orientations of LC molecules, thereby obtaining a switchable random laser. Consequently, the linewidth, intensity, and polarization of the emitted random laser are controlled

Gain narrowing and random lasing from dye-doped polymer-dispersed liquid crystals with nanoscale liquid crystal droplets

Dye-doped polymer-dispersed liquid crystals have been studied for random lasing. The dye-doped polymer-dispersed liquid crystal film was fabricated by photoinitiated polymerization with a collimated 514.5nm Ar+ laser beam. Scanning electron microscopy analysis showed that most liquid crystal droplets in polymer matrix ranged from 20to80nm. Gain narrowing and random lasing from dye-doped polymer dispersed liquid crystals were observed under the excitation of a frequency-doubled Nd:YAG (yttrium aluminum garnet) laser operating at a wavelength of 532nm. The possible mechanism was proposed to explain the random lasing. The threshold of the random lasing was about 25μJ/pulse. The linewidth of the lasing peaks was about 1nm. With the film thickness of 6.5μm, the lasing mode was nearly transverse electric polarized.

High contrast switching of distributed-feedback lasing in dye-doped H-PDLC transmission grating structures

Electrically switched distributed-feedback (DFB) lasing action is presented in a Pyrromethene 580 lasing dye-doped holographic polymer dispersed liquid crystal (H-PDLC) transmission grating structure. This design, when compared with the previously utilized H-PDLC reflection grating structure, has the advantage of a greatly enlarged gain length (10 mm) and a low concentration of liquid crystal (20%) while maintaining sufficient refractive index modulation. The experimental results demonstrate that the emitted laser bandwidth (~5 nm) can be obtained with a pump energy threshold of ~0.3 mJ at three different wavelengths, 561 nm, 569 nm and 592 nm, corresponding to three different grating spacings. The near- and far-field measurements have shown a high directionality of the lasing output. The lasing can be electrically switched off by an applied field of 30V/mum. The temporal, spectral, and output/input properties of the laser output are also presented.

Quasi-Two-Dimensional Diffusive Random Laser Action

We report on random lasing in a disordered system in which the multiple scattering feedback mechanism can be switched from a three-dimensional random walk to a quasi-two-dimensional type of transport. The emission from this system is anisotropic, extraordinary polarized, and is controlled via an external electric field. The phenomenon is observed in dye-doped polymer dispersed liquid crystals and makes use of the strong scattering anisotropies in these materials.

Enhanced Photorefractivity from Ion-Doped Polymer-Dispersed Liquid Crystals

Dynamic near-field scanning optical microscopy (NSOM) and asymmetric two-beam coupling experiments are performed on thin films of photorefractive polymer-dispersed liquid crystals (PDLCs). Results obtained from films doped with perylene (serving as the photoexcitable electron donor) and N,N‘-dioctyl-1,4,5,8-naphthalene diimide (NDI) (the electron acceptor) are compared to those obtained from films that have also been doped with tetrabutylammonium tetrafluoroborate (TBATFB). Both the dynamic NSOM and two-beam coupling results provide clear evidence for enhanced photorefractivity in films doped with permanent ions such as TBATFB. Computer simulations of the NSOM results lend supporting evidence and also provide valuable information on the origins of this enhancement. It is concluded that the enhanced optical response arises from the nonlinear dependence of the liquid crystal orientation state on the electric fields within individual liquid crystal droplets and, hence, the concentrations of both permanent and photogenerated ions. These observations suggest a possible alternative means for enhancing the response of photorefractive devices employing dye-doped polymer-dispersed liquid crystals.