Dye-doped Liquid Crystal Research Articles

Multi-wavelength laser from dye-doped cholesteric polymer films

A multi-wavelength laser is demonstrated using a dye-doped cholesteric polymer film whose reflection bandwidth is broadened with several oscillations. Due to the abrupt change of the density of state between oscillation peak and valley, each oscillation functions as a photonic band gap for generating a laser wavelength under the excitation of a pumping laser. As a result, a multiple wavelength laser is generated. Results indicate that the dye-doped cholesteric liquid crystal polymer film is a good candidate for fabricating broadband lasers such as white light lasers. Potential applications include experimental testing of laser materials, identification markers, information displays, and inertial confinement laser fusion.

All-optical switching of diffraction gratings infiltrated with dye-doped liquid crystals

We report the realization and the characterization of an all-optical switching device based on a transmission grating recorded in a polymeric substrate infiltrated with a methyl red-doped liquid crystal. The properties of this highly nonlinear mixture are exploited to modulate the diffraction of the grating by a pump beam when a static electric field is applied. The behavior of the device is in agreement with the existing model for methyl red-doped liquid crystals.

All-optically controllable random laser based on a dye-doped liquid crystal added with a photoisomerizable dye

This study investigates, for the first time, an all-optically controllable random laser based on a dye-doped liquid crystal (DDLC) cell added with a photoisomerizable dye. Experimental results indicate that the lasing intensity of this random laser can be all-optically controlled to decrease and increase sequentially with a two-step exposure of one UV and then one green beam. All-optically reversible controllability of the random lasing emission is attributed to the isothermal nematic(N)-->isotropic(I) and I-->N phase transitions for LCs due to the UV-beam-induced trans-->cis and green-beam-induced cis-->trans back isomerizations of the photoisomerizable dye, respectively. The former and the latter can decrease and increase the spatial fluctuations of the order and thus of the dielectric tensor of LCs, respectively, subsequently increasing and decreasing the diffusion constant (or transport mean free path), respectively, and thus decaying and rising the scattering strength for the fluorescence photons in their recurrent multi-scattering process, respectively. The consequent decrease and increase of the lasing intensity for the random laser and thus the rise and descent of its energy threshold are generated, respectively.

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.

Continuous spatial tuning of laser emissions with tuning resolution less than 1 nm in a wedge cell of dye-doped cholesteric liquid crystals

We report the design and fabrication of a wedge structured CLC film incorporating a spatial gradient of a chiral dopant concentration. A continuous spatial laser tuning in the broad visible spectral range with tuning resolution less than 1 nm is demonstrated, which renders a CLC-based micron-sized laser an important continuously tunable laser device.

Nonlinear reflection from liquid crystal elastomers

We have studied light induced deformations in dye doped liquid crystal elastomers. At high intensities, the incident light causes the elastomer to deform, and the reflected light from the deformed elastomer surface produces an intensity dependent diffraction pattern in the far field. We discuss the experimental observations, and relate these to the processes leading to photoinduced surface relief. Full Text: PDF

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.

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.

Polarization‐independent and electrically tunable liquid‐crystal Fresnel lenses based on photoalignment in dye‐doped liquid crystals

Abstract— This work demonstrates polarization‐independent and electrically tunable liquid‐crystal (LC) Fresnel lenses based on photoalignment in dye‐doped liquid crystals (DDLCs). The LC alignments in the adjacent odd and even zones of the LC Fresnel lens are orthogonally hybrid alignments. Those are generated by surface treatment of homeotropic and homogeneous alignment layers, and the dye‐adsorption layer onto the UV‐cured surface‐relief Fresnel zone plate. The maximum focusing efficiency (∼34.14%) of the fabricated LC Fresnel lens by applying a suitable AC voltage is close to the maximal theoretical focusing efficiency of a binary phase LC Fresnel lens (∼40.5%). Additionally, the focusing efficiency is polarization‐independent and electrically tunable.

Determination of polar anchoring energy of dye-doped liquid crystals by measuring capacitance

Polar anchoring energy of a dye-doped liquid crystal (DDLC) cell is determined based on capacity measurements. Experimental results indicate that under the illumination of a pump beam, the polar anchoring energy (7.90×10−6 J/m2) includes the adsorption-induced anchoring energy (5.74×10−6 J/m2) and the preexisting-dye-induced anchoring energy (2.16×10−6 J/m2). The latter is attributed to the torque, which is induced by the dye molecules on the boundary surface even before illumination. After the illumination, the adsorption-induced anchoring energy remains and the preexisting-dye-induced anchoring energy disappears, with the former revealing that weak anchoring affects the decay time and threshold voltage of the DDLC cell. Moreover, plotting the polar angle distributions in the DDLC cell reveals its correlation with variations in the capacitance of the DDLC cell.

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.

Low threshold lasing from dye-doped cholesteric liquid crystal multi-layered structures

We fabricated novel hybrid structures composed of a dye-doped low-molecular-weight cholesteric liquid crystal sandwiched by multi-layered polymer cholesteric liquid crystal films and evaluated their lasing characteristics. Lasing was observed with an extremely reduced threshold (12 nJ/pulse) by a factor of 10 compared with that in a simple dye-doped low-molecular-weight cholesteric liquid crystal cell. Lasing characteristics experimentally obtained were discussed by comparing them with the simulated photonic density of states spectra.

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.

Self-Turning Self-Confined Light Beams in Guest-Host Media

We demonstrate power-dependent steering of self-confined light beams propagating in the nonparaxial regime in a guest-host nonlinear medium. Exploiting the Janossy enhancement in dye-doped liquid crystals with reorientational response, we observe sub-mW spatial soliton self-steering as large as 39 degrees, accompanied by trapping of the light spontaneously emitted by the excited dye. The self-turning is modeled in terms of an effective transverse force stemming from the longitudinal electric field in the tightly confined beams, with good agreement between data and simulations.

Towards an aligned luminophore solar concentrator

Luminescent solar concentrators promise to reduce the cost of solar energy, but are hindered by a number of losses. Escape of luminescence through the large waveguide-air interfaces can be attenuated through alignment of the optical transition dipole of the luminophore along the waveguide surface normal, directing the maximum possible proportion of luminescence into waveguide modes. We demonstrate such alignment using a guest-host dye-doped liquid crystal sandwiched between conductive glass slides. Application of a potential while illuminating through a narrow edge caused a drop in the intensity of luminescence escaping the large surfaces, and an increase in the intensity of light escaping the narrow edges of the system. This is explained in terms of alignment of the transition dipoles of the dye. We discuss implementation in a luminescent solar concentrator.

Linear polarization rotators based on dye-doped liquid crystal cells

Linear polarization rotator is fabricated by a single-side homogenously aligned dye-doped liquid crystal cell and linear variable neutral density filter (LVNDF). When a pump beam passing through the transmittance-linear region of LVNDF irradiates on the untreated surface, the surface LC director in the irradiation region is photoaligned into a continuous twist from 0° to 90°. Consequently, the bulk director gradually transits from a homogeneous to twist orientation. This device is capable of rotating the polarization of an input linearly polarized light depending on the beam position, exhibiting a large continuous twist region (5.6 mm) and high contrast ratio (∼1000:1).

Dielectric relaxation dynamics in a dual-frequency nematic liquid crystal doped with C.I. Acid Red 2

The dielectric relaxation dynamics of a dual-frequency liquid crystal-dye composite were studied and the effect of the photoinduced isomerization of the azo dye on the dielectric spectrum of the cell under illumination by an argon-ion laser operating at wavelength of 514.5 nm and irradiance of 550 mW cm −2. It was found that the illumination process has a profound influence on the dielectric spectrum of the dye-doped dual-frequency liquid crystal in the low-frequency range.

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.

Photorefractive Holograms in Dye-doped Nematic Liquid Crystals for Three-dimensional Displays

Photorefractive Holograms in Dye-doped Nematic Liquid Crystals for Three-dimensional Displays