Dye-doped Liquid Crystal Research Articles

Simple optical modeling of polarized fluorescent liquid crystal cells

We investigate the fluorescence emission properties of dye-doped nematic liquid crystal (LC) cells with planar and zigzag emissive layers. To explain the measured angular dependence of the emitted light (emission pattern), we introduce a simple optical model, in which both planar and zigzag emissive layers are approximated as a homogeneously oriented flat LC layer with an equivalent anisotropy. It was revealed that all the measured emission patterns under different LC molecular orientation conditions agree well with the optical model, whereas the conventional Lambertian-based distribution function does not coincide with the measured profile in homeotropically aligned LC cells.

Optically tunable/switchable omnidirectionally spherical microlaser based on a dye-doped cholesteric liquid crystal microdroplet with an azo-chiral dopant

This paper presents an optically wavelength-tunable and intensity-switchable dye-doped cholesteric liquid crystal (DDCLC) spherical microlaser with an azo-chiral dopant. Experimental results present that two functions of optical control - tunability of lasing wavelength and switchability of lasing intensity - can be obtained for this spherical microlaser at low and high intensity regimes of non-polarized UV irradiation, respectively. If the DDCLC microdroplet is subjected to weak UV irradiation, azo-chiral molecules may transform to the bent cis state at a low concentration rate. The effect can slightly decrease the local order of LCs and thus the helical twisting power of the CLC in the microdroplet. As a result, the CLC pitch may become slightly elongated, which will cause the gradual red-shift of both omnidirectional PBG and lasing emission of the DDCLC spherical microdroplet. In contrast, when the microdroplet is subjected to strong UV irradiation, numerous azo-chiral molecules may simultaneously change to bent cis-isomers to seriously disarrange the helical texture of the CLC, which will quickly deform the PBG and deactivate the lasing emission of the microdroplet. Prolonged irradiation of a blue beam after strong UV irradiation may cause the cis azo-chiral molecules quickly convert back rod-like trans-isomers, which may then regenerate the CLC Bragg onion and PBG structures and reactivate the lasing emission of the microdroplet. Optical control of the DDCLC spherical microlaser is realized on a scale of seconds and minutes when UV irradiation is strong and weak, respectively. The 3D DDCLC spherical microlaser is a highly promising controllable 3D micro-light source or microlaser (e.g., all-optical 3D single photon microlaser) for applications of 3D all-optical integrated photonics, laser displays, and biomedical imaging and therapy, and as a 3D UV microdosagemeter or microsensor.

Low threshold and coherent random lasing from dye-doped cholesteric liquid crystals using oriented cells

We demonstrate the realization of a coherent random laser in a strongly scattering medium, which contained a dispersive solution of Ag nanoparticles and laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) in cholesteric liquid crystals that was injected into an oriented cell. Multiple scattering of Ag nanoparticles and helical domains greatly enhanced by the oriented cell confinement effect was experimentally verified to account for the coherent lasing observed in our oriented system. Based on non-oriented cells and oriented cells, this paper extends the active and scattering media-based random lasers from the incoherent regime to the coherent regime.

Photopolymerisation kinetics and electro-optical properties in mixtures of dichroic dye-doped nematic liquid crystal and photocurable polymer

This work reports the effect of dye on the photopolymerisation and electro-optical properties of polymer dispersed liquid crystal (PDLC) composite films. Dichroic PDLC (DPDLC) films based on a photocurable polymer and nematic liquid crystal (LC) with an azo dichroic dye were prepared by photopolymerisation-induced phase separation method. Polarising optical microscopy has been used for monitoring the phase separation kinetics and two-phase morphology evolution in the DPDLC system. LC domains with radial structures during initial period of phase separation adopted a resultant morphology of bipolar configuration over the course of polymerisation. The phase separation and morphology of LC domains was found to be dependent on the amount of dye used. Moreover, the addition of small amount of dye reduced the switching voltage, and enhanced the contrast ratio with improved switching time in the PDLC films. It was shown that, under the application of an electric field, the molecular orientation and absorbance of dichroic dye can be controlled in DPDLC to induce non-linearity and colour contrast without the use of polarisers.

P.104: 2‐Face Viewable Display Using Dye‐Doped Liquid Crystal

AbstractWe present a liquid crystal display that allows two faces of the display viewed at opposite directions to show different image/video content simultaneously. This device is characterized by a dye‐doped liquid crystal, which can eliminate the need for polarizers and offer a very low crosstalk between two separated viewing angles.

Amorphous silicon-liquid crystals device for photonic applications

We demonstrate that an hydrogenated amorphous silicon (a:Si-H)-liquid crystals hybrid device could be used for the recording of high resolution (0.8-2 µm) dynamic holograms. A maximum diffraction efficiency of 3.3% was obtained at low power (1.5 mW) He-Ne laser. The nonlinear refractive index change at 0.6 W/cm2 is n2~1x10−2 cm2/W, although small compared to that obtained in dye-doped liquid crystal, is equal to the reported in pure liquid crystal although with much higher power density (~50 W/cm2). The device operates in the red to near-infrared part of the spectrum which makes it attractive due to its potential applications in telecommunications and military applications.

Polarization and polarization control of random lasers from dye-doped nematic liquid crystals

A polarimetric study of random laser (RL) emitted from dye-doped nematic liquid crystals (NLCs) is presented. We observed linearly polarized light, the orientation of which is in proximity to the bisection between the polarization direction at the maximal scattering in NLCs and the nematic director. Any arbitrary linear polarization of RLs can be obtained by rotating the NLC sample. The efficiency and output uniformity over the complete direction angle of 2π can be optimized by choosing a proper pump polarization.

Optical bistability in a silicon nitride microring resonator with azo dye-doped liquid crystal as cladding material

This investigation reports observations of optical bistability in a silicon nitride (SiN) micro-ring resonator with azo dye-doped liquid crystal cladding. The refractive index of the cladding can be changed by switching the liquid crystal between nematic (NLC) and photo-induced isotropic (PHI) states by. Both the NLC and the PHI states can be maintained for many hours, and can be rapidly switched from one state to the other by photo-induced isomerization using 532 nm and 408 nm addressing light, respectively. The proposed device exhibits optical bistable switching of the resonance wavelength without sustained use of a power source. It has a 1.9 nm maximum spectral shift with a Q-factor of over 10000. The hybrid SiN- LC micro-ring resonator possesses easy switching, long memory, and low power consumption. It therefore has the potential to be used in signal processing elements and switching elements in optically integrated circuits.

A study of lasing wavelength by DOS in the temperature-tunable cholesteric liquid crystal lasers

For the dye-doped cholesteric liquid crystal (CLC) laser system, we established the relationship between the density of optical state (DOS) and temperature based on the transmission coefficient of the CLC, and calculated the change of DOS distribution with the temperature that results in tunable lasing wavelength. The simulation was further verified by the experiment and it was found that the laser emission wavelength from the dye-doped CLC can be tuned from 662 to 622nm by the temperature, which is in good agreement with our proposed theory.

Wave Mixing in Dye-Doped Chiral Nematics

Two-wave mixing (TWM) in nematic liquid crystals (LC) doped with azo-dyes and chiral agents that induce an helical structure is studied. Photo-induced changes of the cholesteric liquid crystal reflective band-gap are observed. The photo-induced spatial modulation of the chiral structure shows relatively fast response time and large nonlinearity. We have characterized the dependency of the nonlinear gain function on the different experimental parameters. It is also shown that the group velocities of light pulses can be decelerated dramatically in dye-doped cholesteric liquid crystal through TWM performed with a continuous pump and a time modulated signal. The highly dispersive properties of the medium can be exploited in interferometric sensors.

Low-voltage-modulated laser based on dye-doped polymer stabilized cholesteric liquid crystal

An electrically-modulated laser was fabricated based on cholesteric liquid crystal photonic band gap. To lower the modulation voltage, the nematic liquid crystals with high dielectric constant was selected, and thus, the emission energy can be modulated with a voltage of less than 10 V. Polymer stabilization was carried out to obtain a stable and switchable helical liquid crystal system. It is noteworthy that visible light initiation system was adopted to prevent the deformation of photonic band gap. The monomer concentration effects on lasing performances were studied and discussed. The results indicate that the laser emission threshold is decreased and the response time is shortened with the increasing of monomer, while the hysteresis may be enhanced. This study provides some new insights into the fabrication, materials improvements and performances of dye-doped liquid crystal laser, and such electrically modulated laser will be a prospective candidate in the laser displays, micro-light-source or the other optical systems.

First-order optical Freedericksz transition in a dye-doped nematic liquid crystal

A first-order optical Freedericksz transition in a nematic liquid crystal doped by a small quantity of a dichroic dye with negative dichroism is observed. The initial homeotropic state corresponds to maximum absorption for the linearly polarized incident beam, while reorientation reduces absorption increasing the optical torque: this process produces the positive feedback leading to the observed bistable behavior.

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.

Random lasing from dye-doped chiral nematic liquid crystals in oriented and non-oriented cells

These are the experimental results describing random lasing in dye-doped chiral nematic liquid crystals. A novel random lasing emission is studied in this article based on the helical domains of dye-doped chiral nematic liquid crystals in oriented and non-oriented cells. Under frequency doubled 532 nm Nd:YAG (yttrium aluminum garnet) laser-pumped optical excitation, we carefully observed and analysed random lasing from dye-doped chiral nematic liquid crystals with wavelength ranges from 600 nm to 620 nm. In addition, the line-width of multi-mode peaks is less than 0.2 nm. The difference between the two random lasing behaviours in the oriented and non-oriented cells arises from the fact that random lasing appearing in the oriented cell results from stronger multiple scattering of light generated by the spiral domains of the liquid crystal molecules. Furthermore, chiral nematic liquid crystal micro-domains with different orientations can induce variation of the diffusion constant, thereby resulting in a decrease or increase in the lasing intensity of the random lasers, and an increase or decrease in their energy thresholds. In addition, a detailed comparison of the two experimental results is also presented in the article, showing the dependence of the lasing threshold and the number of lasing modes on the transport mean free path, the excited area, and the sample size. This process allows us to obtain a random laser by changing the structure of the sample, realising tunable random lasers at low cost.

Numerical simulation of stimulated emission and lasing in dye doped cholesteric liquid crystal films

Dye-doped chiral-nematic liquid crystal lasers have great potential as small size, low-cost, widely tunable lasers. We present a numerical model for stimulated emission and lasing in liquid crystal films based on thin film optics. The gain threshold is modelled and the results are confirmed experimentally. The effect of the orientation of dye molecules and the matching of the photonic bandgap to the dye spectrum on the threshold for lasing is discussed.

Laser-driven microflow-induced bistable orientation of a nematic liquid crystal in perfluoropolymer-treated unrubbed cells

We demonstrate laser-driven microflow-induced orientational change (homeotropic to planar) in a dye-doped nematic liquid crystal. The homeotropic to planar director alignment is achieved in unrubbed cells in the thermal hysteresis range of a discontinuous anchoring reorientation transition due to the local heating by light absorption in dye-doped sample. Various bistable patterns were recorded in the cell by a programmable laser tweezers. The width of the patterns depend on the scanning speed of the tightly focussed laser beam and the minimum width obtained is approximately equal to 0.57μm which is about 35 times smaller than the earlier report in the rubbed cells. We show that the motion of the microbeam spot causes local flow as a result the liquid crystal director is aligned along that direction.

Tunable Microlenses Optically Recorded in Dye-Doped Liquid Crystals

We report on optically recorded microlenses in conventional liquid crystal cells doped with the azo-dye methyl-red. The focal length can be tuned electrically with low external voltage and changed in a wide range with just a small variation of the applied dc field. No patterned electrodes, built-in polymeric lens or patterned molecular reorientation are required. The effect underlying the proposed fabrication method is connected to the surface induced nonlinear effect (SINE) observed a few years ago in dye doped liquid crystals.

Tunable capability of dye-doped cholesteric liquid crystal lasers

Based on the fact that helical pitch and average refractive index of a cholesteric liquid crystal is electric-tunable and temperature-tunable, we have designed dye-doped cholesteric liquid crystal (CLC) lasers. First, the effects of the concentration of chiral and the temperature on helical pitch are investigated. Next, we gain the relationship of tunable laser with temperature and electric field. When the temperature is changed from 23 to 35℃, the wavelength shift of the lasing peak vasies from 618.90 to 594.76 nm, nearly 24.14 nm in total; when the electric field is changed from 0 to 9 V, the wavelength shift of the lasing peak ehanges from 617.40 to 608.11 nm, nearly 9.29 nm in total.

Angular dependences of the luminescence and density of photon states in a chiral liquid crystal

Luminescence spectra of a laser dye-doped chiral liquid crystal have been studied in a wide range of angles (up to 60°) to the axis of its helical structure using a semicylindrical quartz prism, which made it possible to observe the shift and evolution of the photonic band gap in response to changes in angle. Using measured spectra and numerical simulation, we calculated the spectral distributions of the density of photon states in such a cholesteric crystal for polarised and unpolarised light, which characterise its structure as that of a chiral one-dimensional photonic crystal.

Investigation of the laser action in a dye-doped cholesteric liquid crystal wedge cell

The wedge cell of cholesteric liquid crystal device is designed and fabricated, and doping PM580 (exciton) in the cholesteric liquid crystal, the laser emission action is studied. There appear a series of parallel wedge edges of dislocation lines and different shapes of domain in the wedge cell. The planar state of alignment is confirmed. A second harmonic Nd:YAG 532 nm laser is adopted as a pump light beam, and then one-dimensional tunable laser radiation is obtained and its wavelength interval is approximately 1 nm and tuning range is about 17 nm. In the processs of interaction between the torques of LC and the surface anchoring of direction film, the pitch elongation takes place. The positon of photonic band gap is changed, so the laser radiation wavelength can be tuned in the band edge.