Dye-doped Polymer-dispersed Liquid Crystal Films Research Articles

Advances in the display quality of dye-doped polymer-dispersed liquid crystal films by addition of disc-shaped multidirectional light-control film

Typically, the addition of a dye is considered to increase the contrast ratio (CR) of a polymer-dispersed liquid crystal (PDLC) display; however, with the addition of a small amount of dye, the contrast ratio (CR) unexpectedly decreased as compare to the CR of a normal white PDLC display, attributed to various reasons. In this study, an additional multidirectional light scattering polymer film is developed over a conventional dye-doped polymer-dispersed liquid crystal (DPDLC) film to enhance the CR. For this, a disc-shape (DS) pattern on PN393 film is coated. This structure is believed to enhance the scattering properties of DPDLC displays via multiple-directional scattering nature of DS pattern film. Fabrications of such devices are simple and have advantageous of its low-cost production. This optimal designed disc-shaped light-scattering (DSS) film resulted in the enhancement of the CR of the DPDLCs.

Random lasing in dye-doped polymer dispersed liquid crystal film**Project supported by the National Natural Science Foundation of China (Grant No. 61378042), the Colleges and Universities in Liaoning Province Outstanding Young Scholars Growth Plans, China (Grant No. LJQ2015093), and Shenyang Ligong University Laser and Optical Information of Liaoning Province Key Laboratory Open Funds, China.

A dye-doped polymer-dispersed liquid crystal film was designed and fabricated, and random lasing action was studied. A mixture of laser dye, nematic liquid crystal, chiral dopant, and PVA was used to prepare the dye-doped polymer-dispersed liquid crystal film by means of microcapsules. Scanning electron microscopy analysis showed that most liquid crystal droplets in the polymer matrix ranged from 30 μm to 40 μm, the size of the liquid crystal droplets was small. Under frequency doubled 532 nm Nd:YAG laser-pumped optical excitation, a plurality of discrete and sharp random laser radiation peaks could be measured in the range of 575–590 nm. The line-width of the lasing peak was 0.2 nm and the threshold of the random lasing was 9 mJ. Under heating, the emission peaks of random lasing disappeared. By detecting the emission light spot energy distribution, the mechanism of radiation was found to be random lasing. The random lasing radiation mechanism was then analyzed and discussed. Experimental results indicated that the size of the liquid crystal droplets is the decisive factor that influences the lasing mechanism. The surface anchor role can be ignored when the size of the liquid crystal droplets in the polymer matrix is small, which is beneficial to form multiple scattering. The transmission path of photons is similar to that in a ring cavity, providing feedback to obtain random lasing output.

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.

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.

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.

A Method for Improving Contrast Ratio of Polymer Dispersed Liquid Crystal Film Using the Oriented Azo-Dye Molecules in Polymer Matrix

An improvement in the contrast ratio (CR) of transmittance characteristic of polymer dispersed liquid crystal (PDLC) was achieved by doping the polymer matrix with azo-dye and then orienting the dye molecules with the photo pumping technique. The contrast ratio was measured by the light scattering technique. We investigated the optimum conditions for the scattering characteristics of the dye-doped PDLC films as a function of the dye concentration, of the UV curing intensity, and of the micro-droplets size. The light scattering characteristics of PDLC films with aligned dye molecules are compared with films with dyes randomly oriented. Experimental results indicate that the contrast ratio of these aligned films are much higher than those with nonaligned dye-doped PDLC films. The scattered light in the transparent mode was reduced by factor of 5 at the scattering angle of 10°.

Fast optical recording in dye-doped polymer-dispersed liquid-crystal films

We describe a fast optical recording material based on a dye-doped polymer-dispersed liquid-crystal film. A holographic grating is written in this film with a single Q -switched Nd:YAG laser pulse that has a duration of ~6 ns . Such a grating is due to the reorientation effect of the liquid-crystal molecules through interaction with the photoinduced adsorption of the doped azo dyes. Experimental results indicate that the grating thus formed is permanent but electrically switchable.

Electrooptical Properties of Aligned Dye-doped Polymer Dispersed Liquid Crystal Films

We investigate how plane-parallel and twisted alignment treatments influence the electrooptical properties of dye-doped polymer dispersed liquid crystal (PDLC) films. These films are transparent in the electric field-on state and opaque in the field-off state. The relationship between field-off state transmittance of such dye-doped PDLC films and polarization of the incident light is discussed. Experimental data confirm that, even for unpolarized incident light, the contrast ratio of these aligned films surpasses that of a nonaligned dye-doped PDLC film.

Effects of Dichroic Dye in UV-Cured Polymer Dispersed Liquid Crystal Films

We study the effects of dichroic dye in “ultraviolet” polymer dispersed liquid crystal (PDLC) films. Experimental data show that for a dye-doped PDLC film the absorption coefficient for He-Ne laser light is different from that for UV light. Transmittance of a PDLC film changes when the UV curing intensity or dye concentration is varied. It is found that under UV the illumination with a constant intensity the dye concentration can be optimized to obtain minimum transmittance and to improve the contrast ratio. Correspondingly, for a fixed dye concentration the UV curing intensity can be optimized to obtain a minimum transmittance. The experimental results can be described by a theory based on Beer's law. Besides, the experimental results show that the curing temperature is an important factor that affects the nematic liquid crystal droplet size in the UV curing process.