Dye-doped Liquid Crystal Films Research Articles

Colored PDLC Films with Wide Gamut Range

Due to the discoloration properties under different applied voltages, dye-doped polymer-dispersed liquid crystal (PDLC) films are widely used as camouflage nets and invisibility cloaks. However, the range of the discoloration has an intuitive effect on their applications. In this work, we studied the gamut range of PDLC film doped with dyes of red, green, blue, and yellow, with the concentration corresponding to the minimum haze of these dyes. The influence of the applied voltage on the color range of single-layer and double-layer films with different backgrounds was studied. The relationship of the voltage with the color was set from 0 V to 60 V at steps of 5 V, to characterize the discoloration of the PDLC films. The results showed that the films could cover 42.48% of the sRGB gamut and even exceed the range.

Fluorescent Dye-Doped Brightening Polymer-Stabilized Bistable Cholesteric Liquid Crystal Films.

Brightening polymer-stabilized bistable cholesteric liquid crystal (PSBCLC) films with doped fluorescent dyes were prepared using the polymerization-induced phase separation (PIPS) method. The transmittance performance behavior of these films in both states (focal conic and planar) and absorbance change in multiple dye concentrations were studied using a UV/VIS/NIR spectrophotometer. The change occurring in dye dispersion morphology with different concentrations was obtained by means of the polarizing optical microscope. The maximum fluorescence intensity of different dye-doped PSBCLC films was measured using a fluorescence spectrophotometer. Moreover, the contrast ratios and driving voltages of these films were calculated and recorded to demonstrate film performance. Finally, the optimal concentration of dye-doped PSBCLC films with a high contrast ratio and a relatively low drive voltage was found. This is expected to have great potential applications in cholesteric liquid crystal reflective displays.

Single-, Dual-, Triple, and Quadruple-Wavelength Surface-Emitting Lasing in Blue-Phase Liquid Crystal.

Soft organic lasers with multiwavelength output and high spectral purity are of crucial importance for versatile photonic devices, owing to their monochromaticity, coherence, and high intensity.However, there remain challenges for the achievement ofsurface-emitting multiwavelength lasing in soft photonic crystals, and the relative mechanisms need to be investigated. Herein, single-, dual-, triple-, and quadruple-wavelength lasers are successfully achieved in dye-doped blue-phase liquid crystal (BPLC) film. The number and wavelength of the lasing peaks can be manipulated by tuning the center of the bandgap, the order parameter of the laser dye, the quality of the resonance cavity, and even the pump energy. For single-wavelength lasing, a lasing peak withan ultranarrow linewidth of 0.04nm (Q-factor of 13454) is achieved. Multiwavelength lasing is attained based on the following aspects: i) the narrow bandgaps of the BPLCs withfull width at half maximum of 14-20nm; ii) a laser dye with high gain over a wide wavelength band, having a low-order parameter in the liquid crystal matrix; iii) appropriate relative positions between the reflection and fluorescence peaks; and iv) the highly ordered crystal lattice of BPLC film. The proposed single-to-quadruple-wavelength surface-emitting lasers can be employed as coherent light sources for next-generation optical devices.

Enhancing morphological, electro-optical and dielectric properties of polymer-dispersed liquid crystal by doping of disperse Orange 25 dye in LC E7

ABSTRACT In this work, we have investigated morphological, electro-optical and dielectric properties of disperse orange 25 dye-doped polymer-dispersed liquid crystal (DPDLC) films. The DPDLC film composed of monomer Norland optical adhesive (NOA 65), nematic liquid crystal (E7), and disperse orange 25 dye was prepared through the polymerisation-induced phase separation (PIPS) method. The polarising optical microscope (POM) has been utilised to observe the effect of dye concentration and voltage on liquid crystal (LC) droplet morphology. The electro-optical properties of such DPDLC films were studied at a fixed frequency of 200 Hz and different voltage and temperature conditions (0–100 V, 25–50°C). The change in absorbance value with varying dye concentration and the electric field was monitored through a UV-Vis spectrophotometer. Dielectric relaxation characteristics of DPDLC films were examined using a precision impedance analyser in the frequency range 20 Hz–20 MHz and temperature range 25–60°C. Debye and Cole-Cole fitting techniques were used to describe the dielectric properties of DPDLC films.

Security use of bilayer dichroic films made of liquid crystal polymer networks

ABSTRACT We report on bilayer dichroic dye-doped liquid crystal (BDLC) films for patternable color codes, which can be used in anti-counterfeiting applications such as security codes and, unlike the conventional single-layer LC color films, show unique color changes depending on the direction of the polarization of the incident light that passes through them. This is a facile way to enhance the security level of security codes. BDLC films are fabricated by laminating two single-layer LC polymer network films with crossed optical axes. We believe that BDLC films can be used as a new platform for a security-enhanced anti-counterfeiting code that can replace complex chiral materials or expensive plasmonic particles.

Symmetric Continuously Tunable Photonic Band Gaps in Blue-Phase Liquid Crystals Switched by an Alternating Current Field.

Symmetric continuously tunable three-dimensional (3D) liquid photonic crystals have been investigated using self-organized blue-phase liquid crystal films. The photonic band gap in the overall visible spectrum can be tuned continuously, reversibly, and rapidly as the applied electric field changes. After driven by the applied field, four-time enhancement of the reflectivity results in more vivid reflection colors. A lasing emission of tuning working wavelength has been demonstrated by using the dye-doped blue-phase liquid crystal film. With the advantages of fast response speed, no alignment layer, large-scale electrically shift of the photonic band gap, and macro optical isotropy, this self-assembled soft material has many potential applications in high-performance reflective full-color display, 3D tunable lasers, and nonlinear optics.

Low threshold polymerised cholesteric liquid crystal film lasers with red, green and blue colour

ABSTRACTWe demonstrate dye-doped low threshold polymerised cholesteric liquid crystal (PCLC) film lasers with red, green and blue (RGB) colour. The polymer network structure of lasers, which were fabricated by washing-out/refilling method, enhanced the stability of the laser thus avoiding the external interference like temperature and electromagnetic field and prevented the efficiency of laser dye from decreasing during polymerisation. The proposed RGB PCLC film lasers have application potential in such as coherent light source, multi-wavelength biomedical light source, white laser and other photonic applications.

Electro-Optical Characteristics and Curvature Resistance of Dye-Doped Liquid Crystal Gel Films for Stretchable Displays

Electro-Optical Characteristics and Curvature Resistance of Dye-Doped Liquid Crystal Gel Films for Stretchable Displays

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.

Study of laser action in dye-doped polymer dispersed cholesteric liquid crystal film

In this work, laser dye DCM, nematic liquid crystal TEB30A, chiral dopant S-811 and PVA are used to prepare polymer dispersed cholesteric liquid crystal (LC) films by the method of microcapsule. Lasing mechanism and temperature-tunable characteristics of the films are investigated. The diameters of large droplets in the films are about 80 μm and LC molecules are planar-textured in the droplets when viewed between crossed polarisers through a polarization microscope. Pumped by the 532 nm Nd:YAG laser, narrow linewidths in laser emissions are obtained at 634.5 nm and 680.2 nm, and the linewidths are of 0.25 nm and 0.29 nm, respectively. The device is then compared with a dye-doped cholesteric liquid crystal laser. The wavelength of output laser will blueshift when the temperature of the device is increased, and a 22.8 nm tunable lasing, from 666.7 nm to 643.9 nm, is obtained. Analysis concludes that the lasing mechanism of the film is band-edge lasing, and the emission wavelengths are corresponding to the two edges of the photonic band gap.

The complete morphological, electro-optical and dielectric study of dichroic dye-doped polymer-dispersed liquid crystal

Polymerisation-induced phase separation method was used to prepare dye-doped polymer-dispersed liquid crystal (DPDLC) films consisting of nematic liquid crystal, photo-curable pre-polymer and a dichroic dye. The change occurring on droplets morphology with the change in dye concentration and voltage was investigated by means of polarising optical microscope and scanning electron microscope. Electro-optic behaviour of these composite films in the presence of an externally applied AC electric field (0–100 V, 200 Hz) and temperature range 25–50°C was evaluated using He–Ne laser. The maximum absorbance of dye and effect of electric field on the absorbance of dye was studied using UV-Vis spectrophotometer. Dielectric relaxation spectroscopy was carried out in the frequency range 20 Hz–20 MHz and over the temperature range 25°C–90°C. Dielectric parameters such as distribution parameter, relaxation frequency, relaxation strength and relaxation time of these DPDLC films were calculated, and the modelling of experimental data was done using Debye and Cole–Cole method.

Switchable holographic image splitter fabricated with dye-doped liquid crystals

Two electrically controllable holographic polarization gratings are spatially multiplexed to serve as an image splitter for further stereogram application. An external AC voltage is used to modulate the diffraction efficiency of the fabricated holographic image splitter for the switching of a 3D stereogram and a 2D plane image. The polarization grating was generated by two writing beams with orthogonal circular-polarization state in dye-doped liquid crystal films. With proper experimental arrangements, a quasi single order diffraction was obtained and the other non-zero order diffractions were suppressed in this presented polarization grating. This characteristic leads to a good image contrast ratio for a stereogram. Finally, a switchable stereogram system using circular-polarization beam as backlighting was demonstrated successfully.

Unique spatial continuously tunable cone laser based on a dye-doped cholesteric liquid crystal with a birefringence gradient

The present study develops and investigates for the first time a unique spatial continuously tunable cone laser based on a dye-doped cholesteric liquid crystal (DDCLC) film with an LC-birefringence (Δn) gradient. A continuous Δn variation can be generated in a cell by diffusion and self-organization of CLC after four DDCLC mixtures with a discrete variation of Δn are successively injected into an empty cell. Not only the CLC photonic structure but also the lasing wavelength and the cone angle of the obtained conically symmetric emitted lasing ring can be tuned continuously by continuously changing the pumped position of the cell with an Δn gradient. The continuous tunabilities in the lasing wavelength and the corresponding emitted cone angle of the lasing ring are 605.8 → 568.1 nm and 29° → 50°, respectively, within a spatial interval of about 33 mm in the cell.

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.

Novel dye-doped cholesteric liquid crystal cone lasers with various birefringences and associated tunabilities of lasing feature and performance

This study is the first to investigate novel cone lasers and the tunabilities of their lasing feature and performance based on dye-doped cholesteric liquid crystal (DDCLC) films with various LC birefringences (Δn). A unique conically-symmetric lasing ring with a low energy threshold occurs at a specific nonzero oblique angle (θ(ring)). The low energy threshold is comparable to those for common lasing signals occurring simultaneously at the short- and long-wavelength edges (SWE and LWE) of the CLC reflection band (CLCRB) for 0°. The lasing ring is induced by the enhancement in the density of photonic state for the fluorescence with a wavelength of λ(ring) based on an edge-overlapping effect, in which λ(ring) is just located at an edge-overlapping spectral position of the SWE of the CLCRB for 0° and the LWE of the CLCRB for θ(ring). The lasing feature (i.e., the lasing wavelengths of the three lasing signals and the emitted angle of the lasing ring) are tuned by varying Δn. The simulated relationship of an oblique angle with Δn, in which the SWE of the CLCRB for that oblique angle just overlaps the LWE of the CLCRB for 0°, can be obtained by calculating the dispersion relation of a planar CLC structure with various values of Δn based on Berreman's 4 × 4 matrix approach. The result of the calculation is highly consistent with the experimental data for the dependence of θ(ring) on Δn. Furthermore, the dependence of lasing performance (energy threshold and relative slope efficiency) on Δn for the three lasing signals is also measured, which findings can be used to qualitatively identify positive interaction or competition among the three lasing signals.

Dye adsorption induced nonvolatile reading of erasable polarization holograms in liquid crystal films

A nonvolatile reading of erasable polarization holograms in dye-doped liquid-crystal films using the same writing wavelength is presented. The recorded hologram can be easily erased with the illumination of one p-polarized wave; nevertheless, it becomes nonvolatile when reading the hologram with one s-polarized wave. We have proven the nonvolatile reading property is induced by dye adsorption on the substrate of the sample.

The influence of the easy axis on laser-induced ripple structures in dye-doped liquid crystal films

We have investigated the role of the easy axis in the photoalignment effect induced by an Ar+ linearly polarized laser beam on a dye-doped nematic liquid crystal cell in a pump–probe experiment. The induced permanent structure in the irradiated zones was studied using a polarizing optical microscope, and the laser-induced surface morphology was studied by atomic force microscopy. The depth of the ripple structure depends on both the pump power and the irradiated surfaces. The azimuthal anchoring energy provided by the ripple structure was evaluated and it was concluded that it varies with the cell's symmetry.

Effect of recording-beam ratio on diffraction efficiency of polarization holographic gratings in dye-doped liquid-crystal films

The +1 order diffraction of polarization holographic gratings generated by two linear orthogonally polarized writing beams in dye-doped liquid-crystal films were increased by using two recording beams with unequal intensity. The maximum diffraction efficiency of the polarization holograms in our experimental sample can reach 23% when the intensity ratio of two recording beams is 7:1, and this value is almost triple that when the intensity ratio of two recording beams is 1:1.

Band-tunable color cone lasing emission based on dye-doped cholesteric liquid crystals with various pitches and a pitch gradient

This study elucidates, for the first time, a novel band-tunable color cone lasing emission (CCLE) based on dye-doped cholesteric liquid crystal (DDCLC) films with various pitches. For several CLC cells with different pitches it was shown experimentally that the lasing band on the CCLE can be tuned among various color regions measured within different angular ranges. Some important features of the tunable CCLE are also identified and discussed. A spatially band-tunable color cone laser, based on a single DDCLC with a gradient pitch, is developed as a real application.