Azo-dye Doped Liquid Crystals Research Articles

Creation of topological charges by the spontaneous symmetry breaking phase transition in azo dye-doped nematic liquid crystals

Liquid crystals are anisotropic fluids with long-range directional order. They can be easily used to create topological defects, but creating a controlled topological defect is difficult and involves many tasks and complex processes. However, in our study, we were able to easily generate disclinations in the symmetry-breaking boundaries of an azo dye-doped nematic liquid crystal cell owing to photoisomerization and symmetry-breaking isotropic-nematic phase transition. The method proposed here marks the starting point for the easier control of topological defects in liquid crystals.

Competitive Dye Adsorption-Desorption on the Isotropic Surface at the Early Stage of the Photoexcitation of Azo Dye-Doped Liquid Crystals

The instantaneous response of liquid crystal (LC) molecules near the dye-adsorbed layer of azo dye-doped LCs (ADDLCs) in early-stage photoexcitation was observed through the pumped-attenuated total reflection (PATR) method. As the sensing depth of the attenuated total reflection (ATR) method is in the order of hundreds of nm, in situ observation using the PATR method directly inspected the restructuring of the adsorbed layer in the early-stage photoexcitation, which demonstrated the competition between adsorption and desorption on the isotropic surface without the interference of the elastic continuum in the bulk of a cell. Analyzing the frequency of the oscillation-like PATR signal, which reflects the evolution of the orientation of the LC molecules, revealed that, depending on the dose of the pump light, the structural evolution of the adsorbed layer is fast at the beginning of photoalignment in ADDLCs and slows down with the accumulation of the adsorbed layer. A suitable intensity of the pump light in the early stage of the photoalignment process can improve the reform of the adsorbed layer to strengthen its anchoring.

Fabrication, morphological investigation and spectral characterization of nano-encapsulated azo dye-doped nematic liquid crystals

Polymeric nanocapsules (NCs) containing dye-doped liquid crystal (DDLC) with an average diameter of 390 nm and good stability were successfully fabricated by the emulsification solvent diffusion (ESD) method. The surface morphology and flexibility of polymeric nanoshell were investigated by the field-emission scanning electron microscopy (FE-SEM) and the core-shell structure of the obtained spherical nanocapsules was shown by transmission electron microscopy (TEM) as well. Nanoencapsulation efficiencies (NEE) of the liquid crystal and azo disperse dye were also measured separately by using a comparative all spectroscopic analysis instead of weight-based methods. Thereafter, a polymeric thin-film consist of the fabricated DDLC nanocapsules was produced and then, the polarization rotation feature and spectral response of the obtained DDLC-NCs were investigated by polarized optical microscopy (POM) and UV/Vis spectroscopy, respectively.

Periodic oscillation of the optical transmittance in azo dye-doped liquid crystals between two crossed polarizers

The periodic oscillation of the optical transmittance in the nematic liquid crystals (5CB) doped with azobenzene compound (DR1) between two crossed polarizers has been demonstrated and investigated by monitoring the signal intensity variation of the transmitted light in different conditions of the light irradiation and environment temperature. It is found that the oscillation of the transmittance lasts more than 4 periods before the nematic–isotropic phase transition when the dye doped liquid crystal sample is heated from 20 to 40 °C. The oscillation is attributed to the continuous decrease of the birefringence of the dye doped liquid crystals with the temperature increasing. The oscillation curve shifts forward significantly when the sample is exposed to the pump light irradiation, which indicates that the pump light irradiation induces an additional decrease of the birefringence. A more accurate and comprehensive way for estimating the magnitude of the birefringence of the anisotropic materials is proposed based on the transmittance oscillation phenomenon. The switchable properties and the potential applications of the transmittance oscillation are discussed.

Electrically Tunable Fresnel Lens in Twisted-Nematic Liquid Crystals Fabricated by a Sagnac Interferometer.

This paper presents an electrically tunable Fresnel lens in a twisted nematic liquid crystal cell fabricated by using a Sagnac interferometer. When the Fresnel-patterned green beam, formed by the Sagnac interferometer, is irradiated on the azo-dye doped liquid crystal mixture, the azo-dye molecules undergo trans–cis photoisomerization and then generate the photo-alignment effect in the bright (odd) zones. The director of the liquid crystal molecules in the odd zones reorients the direction perpendicular to the polarization direction of the linearly polarized green beam. The various structures of liquid crystals in the odd and even zones will result in a phase difference and thus, a Fresnel lens can be generated. The experimental results show that the proposed Fresnel lens has a high diffraction efficiency of 31.5% under an applied alternating-currents (AC) voltage. The focal length of the Fresnel lens can also be tuned by thermally erasing the photo-alignment effect of the azo dyes and rewriting by a different Fresnel-like pattern.

Electrically Controlled Diffraction Grating in Azo Dye-Doped Liquid Crystals.

This research applies the non-linear effect of azo dye-doped liquid crystal materials to develop a small, simple, and adjustable beam-splitting component with grating-like electrodes. Due to the dielectric anisotropy and optical birefringence of nematic liquid crystals, the director of the liquid crystal molecules can be reoriented by applying external electric fields, causing a periodic distribution of refractive indices and resulting in a diffraction phenomenon when a linearly polarized light is introduced. The study also discusses the difference in the refractive index (Δn), the concentration of azo dye, and the rising constant depending on the diffraction signals. The experimental results show that first-order diffraction efficiency can reach ~18% with 0.5 wt % azo dye (DR-1) doped in the nematic liquid crystals.

Polarisation-free and high-resolution holographic grating recording and optical phase conjugation with azo-dye doped blue-phase liquid crystals

ABSTRACTWe present the results of a detailed study of the mechanism and dynamics of holographic Bragg grating formation in azo-dye doped blue-phase liquid crystals. The principal mechanism leading to refractive index modulation is lattice distortion caused by photo-activated trans–cis isomerism of the azo-dye dopant. High diffraction efficiency bulk (Bragg) gratings with grating spacing as small as 1 μm can be written with low optical power and do not require specific optical field polarisation. Furthermore, the written grating can be prolonged by uniform illumination with another laser, instead of being erased. Azo-doped blue-phase liquid crystals thus present themselves as highly promising photosensitive materials for high-resolution holographic recording and image processing application. An experimental demonstration of holographic image reconstruction via optical phase conjugation with excellent aberration correction capability is presented.

Photo-physical and structural studies of some synthesized arylazoquinoline dyes

This study presents the spectral and structure characteristics of seven azoquinoline dyes with different substituents and their new methylated counterparts for the first time, where some compounds are newly synthesized. The solvatochromic, tautomeric, halochromic, and dichroic behavior of the compounds were studied by electronic spectroscopy in various media. The different types of media were ordinary, multifunctional, and ordered liquids. The experiments were extended to include under acidic or basic conditions. The orientational behavior of the azo dye-doped liquid crystals was studied, and it was established that the azo form is the main species in high polar anisotropic media. The multi-parameter polarity scales were used to correlate the spectral data. Influence of acid and base on the absorption spectra of the dyes was also examined. Ionization constants for these dyes were determined in ethanol-water media. As a result, at the high dye concentrations, the intermolecular hydrogen bonding is more stable than the intra-molecular hydrogen bond, and therefore, the azo form is the main species in concentrated solutions. In order to provide more details, time-dependent density functional theory (TD-DFT) calculations were carried out for the representative models.

High efficiency holographic Bragg grating with optically prolonged memory.

In this paper, we show that photosensitive azo-dye doped Blue-phase liquid crystals (BPLC) formed by natural molecular self-assembly are capable of high diffraction efficiency holographic recording with memory that can be prolonged from few seconds to several minutes by uniform illumination with the reference beam. Operating in the Bragg regime, we have observed 50 times improvement in the grating diffraction efficiency and shorter recording time compared to previous investigations. The enabling mechanism is BPLC’s lattice distortion and index modulation caused by the action of light on the azo-dopant; upon photo-excitation, the azo-molecules undergo transformation from the oblong-shaped Trans-state to the bent-shaped Cis-state, imparting disorder and also cause the surrounding BPLC molecules to undergo coupled flow & reorientation leading to lattice distortion and index modulation. We also showed that the same mechanism at work here that facilitates lattice distortion can be used to frustrate free relaxation of the lattice distortion, thereby prolonging the lifetime of the written grating, provided the reference beam is kept on after recording. Due to the ease in BPLC fabrication and the availability of azo-dopants with photosensitivity throughout the entire visible spectrum, one can optimize the controlling material and optical parameters to obtain even better performance.

P-84: An Updatable Holographic 3D Display with Accommodation Based on Photorefractive Doped Liquid Crystals

We report an updatable holographic 3D display based on photo-refractive azo-dye-doped liquid crystals (ADDLCs). The display is capable of recording and displaying 3D images with all depth cues, including accommodation. A holographic 3D image of 4K resolution is demonstrated in this paper.

Effect of surface condition on liquid crystal photoalignment by light-induced azo dye adsorption phenomena

ABSTRACTThe effect of surface condition on the photoalignment of light-induced methyl red (MR) adsorption phenomena on various substrates based on azo dye-doped liquid crystals is investigated. The results show that the polar surface energy determines the initiation of MR adsorption phenomena. The surface polarity energy of different substrates, which are modulated by oxygen plasma treatment (OPT), is further examined using the sessile drop method. The results also indicate that the uniformity and efficiency of MR adsorption can be enhanced by OPT. Light intensity influences the MR adsorption rate. The uniformity of MR adsorption onto various substrates is analysed via polarised optical microscopy.

Irreversible phase and anchoring transitions of chiral azodye-doped nematic liquid crystal triggered by photostimulation

Reported herein are the simultaneous phase and anchoring transitions of the chiral azodye-doped liquid crystal (LC) mixture. The transitions are influenced by photostimulation and are stable against light and thermal treatments. The photochromic trans state of the azobenzene molecule used in the study is mesogenic with a chiral nature. The photoinstigated cis form of the molecule has a much-enhanced dipole moment with a broken chromophore symmetry. Due to the high dipole–dipole interaction, the cis-state molecules stack together and form nano-sized aggregates. Subsequently, the cis aggregates get phase-separated, resulting in the chiral- to achiral-phase transition of the host LC. The phase-separated cis aggregates diffuse to the interface between indium-tin-oxide and LC and modify the surface, causing planar to homeotropic anchoring transition of the host LC. The stability and irreversibility of the transition, and the fact that there is no need for pre-treatments for LC alignment, provide a fascinating opportunity for LC device applications.

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.

Optical Memory Based on Azo-Dye-Doped Nematic Liquid Crystals

In this work the dynamic of formation of permanent holographic grating have been investigated. Also the effect of cell thickness and intensity of writing beams on the grating permanency in Azo Dye Doped Liquid Crystals (ADDLC) is studied. Homogeneously aligned pure 1294-1b liquid crystal and doped with Methyl Red (with 1% wt) are used to achieve this purpose. The results are compared with literatures, and showing the optimized properties for constructing an optical memory with high diffraction efficiency and long duration.

All-Optical and Thermal Tuning of a Bragg Grating Based on Photosensitive Composite Structures Containing Liquid Crystals

We report on the fabrication and characterization of a tunable filtering effect, observed in a waveguide grating made of alternated strips of photocurable polymer and a mixture of azo-dye doped liquid crystal. The grating is sandwiched between two borosilicate glasses, one of which includes an ion-exchanged channel waveguide, which confines the optical signal to be filtered. We analyze the effects of a low power visible, pump light beam that hits the sample and temperature effects. Both effects are investigated experimentally. The application of the pump light allowing the filtered wavelength to be tuned over a 6.6 nm range, while a temperature variation of 4°C, implies a tuning of 14 nm.

Azo dye adsorption effect induced by elliptically polarized light in azo dye-doped liquid crystals

This study elucidates the azo dye adsorption effect induced by elliptically polarized light in azo dye-doped liquid crystals (ADDLCs). Experimental results reveal that the light-induced molecular reorientation that is caused by the adsorbed azo dyes declines as the absolute value of the ellipticity of the pumping beam, having a proper selectively fixed light intensity or a proper selectively fixed light component along the direction of major axis, increases. The long axes of the adsorbed dyes are found to be independent of the sign of the light ellipticity, but they do depend on the direction of the major or minor axis of the elliptically polarized light. Notably, the tilt angle in LC alignment is not observed in this experiment. Additionally, neither twist angles nor tilt angles can be induced using a circularly polarized pumping beam.

Modeling the mechanisms of the photomechanical response of a nematic liquid crystal elastomer

Recent studies of azo-dye doped liquid crystal elastomers show a strong photomechanical response. We report on models that predict experimental results that suggest photothermal heating is the dominant mechanism in a planar constrained geometry. We compare our models with experiments to determine key material parameters, which are used to predict the dynamical response as a function of intensity. We show that a local strain from photothermal heating and a nonlocal strain from thermal diffusion is responsible for the observed length changes over time. This work both elucidates the fundamental mechanisms and provides input for the design of photomechanical optical devices, which have been shown to have the appropriate properties for making smart materials.

Photoinduced Polarization-Independent Refractive Index Modulation Using Azo-Dye Doped Liquid Crystal

We demonstrate polarization-independent refractive index modulation using an azo-dye doped liquid crystal (azo-LC). The azo-LC is aligned homeotropically and irradiated by ultraviolet (UV) light resulting in optically induced nematic-isotropic phase transition. This homeotropic-isotropic transition causes a refractive index modulation, which is independent of polarization for light travelling perpendicularly to the azo-LC layer. In order to evaluate the amount of the modulation, we fabricated an etalon which contains a homeotropically aligned azo-LC layer between the mirrors, and observed the polarization-independent refractive index modulation by 0.03.

Biphotonic-induced reorientation inversion in azo-dye-doped liquid crystal films

The reorientation inversion of liquid crystal molecules in azo-dye-doped liquid crystal (ADDLC) films irradiated by biphotonic (green and red light) lasers is studied using the self-phase-modulation method. The results show that the induced change of refractive index (∆n) in an ADDLC cell illuminated with a fixed intensity of green light increases with the red-light intensity. Notably, the value of ∆n is initially negative, and becomes positive when the intensity of the red laser is increasing. It is due to the fact that an extra positive torque is exerted on the LCs, which is produced by cis-isomers upon absorbing red light to compensate the negative torque induced by the excitation of the green light.

Optically controllable and focus-tunable Fresnel lens in azo-dye-doped liquid crystals using a Sagnac interferometer

This study demonstrates a tunable Fresnel lens in an azo-dye-doped liquid crystal (ADDLC) film using an interference technique. One Fresnel-patterned green beam using a Sagnac interferometer irradiated the UV-illuminated ADDLC cell, yielding a concentric zone plate distribution with homeotropic and isotropic structures in bright and dark regions of the green interference pattern. The proposed Fresnel lens is polarization independent, focus tunable, and the focusing efficiency of the device can be optically controlled.