Dye-doped Nematic Liquid Crystal Research Articles

Density of photon states in dye-doped chiral nematic liquid crystal cells in the presence of losses and gain

We calculate the density of photon states (DOS) of the normal modes in dye-doped chiral nematic liquid crystal (LC) cells in the presence of various loss mechanisms. Losses and gain are incorporated into the transmission characteristics through the introduction of a small imaginary part in the dielectric constant perpendicular and along the director, for which we assume no frequency dispersion. Theoretical results are presented on the DOS in the region of the photonic band gap for a range of values of the loss coefficient and different values of the optical anisotropy. The obtained values of the DOS at the photonic band gap edges predict a reversal of the dominant modes in the structure. Our results are found to be in good agreement with the experimentally obtained excitation thresholds in chiral nematic LC lasers. The behavior of the DOS is also discussed for amplifying LC cells providing additional insight to the lasing mechanism of these structures.

Surface morphology of doped nematic liquid crystals: role of dye concentration

We report on laser-induced ripple structures in dye-doped nematic liquid crystals for different dye concentrations varying between 0.5% and 2% wt. The surface morphology was studied by atomic force microscopy. For dye concentrations smaller than 0.56% the ripple structure was not obtained. The depth of the ripple structure depends on both the laser power and the dye concentration. Using the Berreman formula for periodic structure, the azimuthal anchoring energy was evaluated. An increase of the anchoring energy with the dye concentration and with the pump power was noted.

Dark Spatial Solitons in Liquid Crystals

We demonstrate the generation and propagation of dark spatial solitons in dye-doped nematic liquid crystals, exploiting a defocusing nonlinearity stemming from a reduced order parameter. Good agreement is obtained between the experimental measurements and a simplified model. Moreover, a probe beam is used to confirm the guiding properties of the soliton.

Two-wave mixing in chiral dye-doped nematic liquid crystals

Two-wave mixing in nematic liquid crystals doped with dyes and chiral agents is studied. The photo-induced spatial modulation of the chiral structure, together with the diffusion anisotropy of the mixture, determine a relatively fast response time, a spatial resolution of 1 μm, and a maximum gain for circularly polarized interacting beams. The gain is insensitive to changes in linear polarization, while it varies with the grating period and with the pump-to-signal intensity ratio.

Investigation of the tunable laser of one-dimensional photonic crystal with dye-doped nematic liquid crystal defect layer

We investigate the optical characteristics of the tunable laser of a one-dimensional photonic crystal (1D PC) containing Dye-doped nematic liquid crystal (NLC). The dielectric multilayer consisting of an alternating stack of SiO2 and TiO2 layers is used as the 1D PC whose defect layer is filled with the laser dye and NLC. The central wavelength of the stop band of the 1D PC is 650 nm. A second-harmonic light of a Q-switched Nd: YAG laser has a wavelength of 532 nm, and is used for excitation. With the temperature increasing from 25.5 ℃ to 48 ℃, the emission wavelength of the tunable laser is continuously shifted from 605.5 to 639.8 nm, the total wavelength shifts is 34.4 nm. With the external voltage increasing from 0 to 2.86 V, the emission wavelength of the tunable laser is shifted from 634.5 to 619.5 nm, the total wavelength shifts are 15 nm. The lasing threshold was about 12.3 J/pulse, and the linewidth of the emission peak was less than 1 nm.

LASER-INDUCED THERMOMECHANICAL EFFECTS IN NEMATIC LIQUID-CRYSTAL

As we know there are several mechanisms allowing us to convert absorbed energy by liquid crystal (LC) to the energy of reorientation of director. One of them is the third thermomechanical effect. Third thermomechanical effect induced by Gaussian beam was recently studied both theoretically and experimentally. It was shown that thermomechanical effects can decrease the threshold of Fréedericksz transition in dye-doped nematic liquid crystal (NLC). One of the big advantages of thermomechanical effect compared with other mechanisms (for instance giant optical nonlinearity (GON)), which are absent in the case of normal incidence of laser beam, is that it emerges at any angle of incidence of laser beam. Thermomechanical effects were also studied in NLC, containing azobenzene in their molecular structure. It was suggested that one of the mechanisms of optical nonlinearity observed in the experiment in such medium may be the thermomechanical effect.

Printed photonic arrays from self-organized chiral nematic liquid crystals

Active laser arrays, of arbitrary pattern, were created by inkjet deposition of self-assembled photonic structures in the form of dye-doped chiral nematic liquid crystals. The print process itself achieves the requisite alignment for photonic band-edge lasing action normal to the substrate; laser threshold and linewidth characteristics are presented.

Optically Controlled Light Propagation in Dye-Doped Nematic Liquid Crystals with Homogeneous Alignment

Light propagation in an optical waveguide fabricated by employing a dye-doped liquid crystal (DDLC) was observed. The propagation of a light signal in the waveguide was varied by irradiation with a control light whose wavelength was in the absorption band of the DDLC. By considering the photothermal effect of the DDLC, which enables the change of the refractive index due to temperature variation based on the absorption of light, we qualitatively explained the observed light propagation and demonstrated manipulation of the propagation.

Applied electric field effect on light-scattering birefringence of dye-doped liquid crystal molecule and consistent neural network empirical physical formula construction for scattering intensities

In this paper, we realized two objectives. Firstly, birefringence of azo and anthraquinone dye-doped nematic liquid crystal (NLC) molecules was investigated by applied electric field dependent laser scattering intensities. The birefringence was essentially calculated from ordinary and extraordinary ray phase difference, which is determined from the measured intensities corresponding to parallel and perpendicular orientations of analyzer to polarizer. The birefringence was found to be dependent on both applied voltage and the kind of the doping dye. As the second objective, by nonlinear universal function approximator layered feedforward neural network (LFNN), we constructed explicit form of empirical physical formulas (EPFs) for experimentally measured dye-doped NLC nonlinear scattering intensities. Excellent LFNN test set predictions over yet-to-be measured experimental data proved that the constructed LFNN-EPFs estimated the measured intensities consistently. The correlation coefficients assessing the goodness of predictions were about r = 0.998for all cases. The LFNN-EPFs also extracted the intensity dependency on the kind of dye used. When theoretical and LFNN-EPFs intensities are compared, we conclude that given certain experimental conditions, theoretical and LFNN-EPFs predictions are in excellent agreement. In this sense, we can say that the physical laws embedded in the birefringence scattering data can be consistently extracted by LFNN. Therefore, judging from the consistent extraction of the molecular dependencies of pure and doped NLC intensities, we predict that the LFNN-EPFs can help to identify unknown molecular structural parameters in liquid crystal extracts. More concretely, by suitable mathematical operations such as differentiation, integration, minimization on these intensity LFNN-EPFs, some useful information into the charge distributions of the LC molecules can be gained.

TRANSVERSE SELF-ACCELERATION OF NEMATICONS: CAN A SELF-CONFINED BEAM CHANGE ITS OWN PATH?

We report on power-dependent self-transverse acceleration of optical spatial solitons in two settings: pure nematic liquid crystals, where the self-steering is a consequence of all-optical reorientation through nonlinear changes in birefringent walk-off, and dye-doped nematic liquid crystals, where the guest-host interaction amplifies reorientation and enhances the transverse force acting on the soliton, thus changing its trajectory.

Effects of carbon nanotubes on electro-optical properties of dye-doped nematic liquid crystal

Electro-optical effects of planar nematic liquid crystal cells doped with dye and carbon nanotubes (CNTs) were investigated by utilizing the absorption property of the dye. Voltage-dependent transmittance and hysteresis measurements under applied ac and dc fields were presented. Cells doped with both Methyl Red and CNTs showed increased conductivity and capacitance, slower ac-rise switching time and dc-backflow on turn-off. The cells exhibited little hysteresis but those containing CNTs reduced residual dc by about 50%. First-order probe diffraction of permanent gratings revealed a correlation of ac-field-enhanced diffraction efficiency and Freedericksz threshold voltage.

Investigation of dielectric properties and diffraction efficiency enhancements caused by photothermal effect in DR9 dye-doped nematic liquid crystal

In this work, we studied dielectric properties and laser-induced refractive index changes originating from photothermal effects of liquid crystal material doped with Disperse Red 9 (DR9) dye. Dye concentration is arranged to be between percentages changing from 0.2 wt.% to 1 wt.% in E63 nematic liquid crystal. Nonlinear optical properties such as diffraction efficiency ( η) and refractive index modulation (Δ n) were investigated by diffraction grating measurements. It was found the diffraction efficiency of pure E63 nematic liquid crystal is 1%. As the doping amount of DR9 dye in nematic LC is increased, diffraction efficiency took higher values and the maximum diffraction efficiency of 10% was gained with E63 doped with 0.8 wt.%DR9 dye. Moreover, dielectric permittivity and dielectric anisotropy values of the samples were investigated in the frequency range of 100 Hz–10 MHz by using dielectric spectroscopy technique. It was observed that dielectric constant values of the liquid crystal material are strongly affected by doping with dye.

The molecular ordering phenomenon in dye-doped nematic liquid crystals

The experimental results of this work point out the role of the guest dye molecules in the molecular ordering of nematic liquid crystals. We have discussed the changes in the energies of interactions between rod-like nematic molecules and anthraquinone dye by considering the presence of steric and dipole–dipole interactions in the dye-doped system. The concentration of the dye plays an important role in the determination of molecular ordering in such dye-doped systems. Below a certain concentration of dye (known as the critical concentration), where the interaction between the dye molecules can be neglected, the addition of dye molecules introduces some disorder into the system in the form of domain formation. Above this critical concentration, this disorder is small.

Light-scattering determination of visco-elastic and electro-optic parameters of azo and anthraquinone dye-doped liquid crystal molecules and consistent neural network empirical physical formula construction for scattering intensities

In this paper, we achieved two aims. Firstly, laser light-scattering intensities in methyl red (MR) azo and disperse red (DR) anthraquinone dye-doped nematic liquid crystal (NLC) molecules were measured versus scattering angle and applied bias voltage. The following three NLC molecular structure parameters were determined: the visco-elastic constant ratios K 11/ K 22 and K 33/ K 22 by data-regression and Freedericksz voltages from the graphs drawn. All these NLC parameters were found to be dependent on the kind of the dye used. As the second aim, by nonlinear universal function approximator layered feedforward neural network (LFNN), we constructed explicit form of empirical physical formulas (EPFs) for theoretically unknown nonlinear azo and anthraquinone dye-doped NLC scattering intensity functions. Excellent LFNN test set (i.e. yet-to-be measured experimental data) predictions prove that the constructed LFNN-EPFs estimate unknown intensity functions consistently. The LFNN-EPFs, too, confirmed the dependency on the kind of dye used. In conclusion, physical laws embedded in the scattering data can be consistently extracted by LFNN. One significant potential application in molecular nonlinear optics domain is that these LFNN-EPFs, by various mathematical tools such as differentiation, integration, and minimization, can be used to obtain further NLC scattering intensity knowledge related molecular structural parameters. Such knowledge in turn may prove useful in developing new optical materials.

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.

Microseconds–Nanoseconds All-Optical Switching of Visible-Near Infrared (0.5 µm–1.55 µm) Lasers with Dye-Doped Nematic Liquid Crystals

Laser induced disorder is an efficient and very fast mechanism to induce changes in the birefringence of nematic liquid crystals, which in turn enable many nonlinear optical processes. Using a 90-degrees twist alignment nematic liquid crystal doped with suitable dye to impart the required photonic absorption and order parameter modulation, we have demonstrated ultrafast all-optical shuttle operation for lasers spanning the visible to near-infrared spectral region (532 nm; 488 nm; 750 nm; 1060 nm; 1550 nm). With increasing intensity, the on-time of the switch decreases from microseconds to the nanoseconds regime, in such a manner that the transmitted light energy/intensity is clamped to below the eye- or sensor-safe levels.

Electro-Optical Properties of Dye-Doped Nematic Liquid Crystals

The electro-optical properties of dye-doped nematic liquid crystals have been investigated in this work. The absorption and transmittance spectrum of the azo-dye doped nematic liquid crystal are measured. The band gaps of the samples are calculated and the effect of concentration on band gap is studied. The results indicate that energy band gap decreases with the increase in dye concentration.

Heat-driven and electric-field-driven bistable devices using dye-doped nematic liquid crystals

We have demonstrated memory and rewritable bistable devices based on an anchoring transition of a nematic liquid crystal on a perfluoropolymer surface. Spontaneous orientation changes between planar and homeotropic occur on cooling and heating with a large temperature hysteresis. Photo (heat) addressing is possible from homeotropic to planar using dye-doped samples. For a coumarin dye, photoaddressed images are preserved even after heating up the sample to the isotropic temperature, whereas, for a 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye the images can be erased by decreasing the temperature out of the thermal hysteresis. Orientation switching also occurs by applying an electric field with a response time of several milliseconds depending on the field strength.

Solitary wave propagation and steering through light-induced refractive potentials

The steering of a self-guided beam in a dye-doped nematic liquid crystal caused by an external illumination (control beam) that induces changes in the refractive index of the medium is theoretically analyzed. The interaction between the control beam and the dye molecules modifies the anchoring of the nematic molecules, so changing the director orientation in the bulk of the medium. Beam evolution is investigated by use of a modulation theory approach. It is found that the beam trajectory is independent of the beam profile, as long as this profile is self-similar. Solutions obtained from the modulation theory approach are in excellent agreement with numerical solutions.

Alignment-to-polarization projection in dye-doped nematic liquid crystal microlasers

We report a dye-doped nematic liquid crystal microlaser that allows the two-dimensional alignment of the liquid crystal to be projected directly on the output polarization of the laser beam. The laser cavity is composed of a pair of dielectric multilayers sandwiching a dye-doped nematic liquid crystal with patterned alignment, and exploits the fact that the resonance modes in such systems are split into two orthogonally polarized modes experiencing either the extraordinary or ordinary refractive index of the liquid crystal. Azimuthally polarized lasing is demonstrated using a concentrically aligned liquid crystal layer.