Dye-doped Nematic Liquid Crystal Research Articles

Modeling of laser generation in a Fabry–Pérot-Tamm structure with a nematic liquid crystal layer

In the presented work, the possibility of controlling laser generation using a nematic liquid crystal (NLC) in a hybrid layered structure consisting of a thin metal layer (Ag), a layer of NLC doped with a light-absorbing dye, and a distributed Bragg reflector (DBR) with a rectangular refractive index profile is theoretically studied. Spectral dependencies of the reflection, transmission, and absorption coefficients of light as well as the localization coefficient of the light field in NLC within the photonic bandgap of the DBR are obtained. Narrow dips in the reflection coefficient and peaks in the transmission coefficient are achieved due to the excitation of plasmons at the Ag-NLC interface. The dependence of the spectral position and magnitude of the plasmonic dips/peaks and the enhancement of the light field in the NLC medium on the thickness and orientation of the NLC layer as well as the impact of a light-absorbing dye doping are investigated. Theoretical calculations of the temporal dependencies of luminescence pulses for pumping pulses of different power settings (below, above, and at the threshold of laser generation) and different values of light absorption in the dye-doped NLC medium are performed, taking into account the peculiarities of the optical properties of the dye-doped NLC.

Physical Unclonable Functions Employing Circularly Polarized Light Emission from Nematic Liquid Crystal Ordering Directed by Helical Nanofilaments.

This study proposes the use of physical unclonable functions employing circularly polarized light emission (CPLE) from nematic liquid crystal (NLC) ordering directed by helical nanofilaments in a mixed system composed of a calamitic NLC mixture and a bent-core molecule. To achieve this, an intrinsically nonemissive NLC is blended with a high concentration of a luminescent rod-like dye, which is miscible up to 10 wt % in the calamitic NLC without a significant decrease in the degree of alignment. The luminescence dissymmetry factor of CPLEs in the mixed system strongly depends on the degree of alignment of the dye-doped NLCs. Furthermore, the mixed system prepared in this study exhibits two randomly generated chiral domains with CPLEs of opposite signs. These chiral domains are characterized not only by their CPLE performances but also by their ability to generate random patterns up to several millimeters, making them promising candidates for high-performance secure authentication applications.

Modeling and optimization strategies for dye-doped super twisted nematic liquid crystal light valves

This study provides a comprehensive investigation of dye-doped super twisted nematic liquid crystal (DDSTNLC) with the aim of uncovering its potential applications. Using design of experiments (DoE) techniques, we elucidated the relationships between physical variables and constructed a transmission model. We then used Monte Carlo simulations to demonstrate the potential applications of DDSTNLC in various optical performances, including high light transmittance and contrast ratio. The investigation combining the modeling and DoE paves the way for advancing progress in the development of DDSTNLC-based light valves.

Lasing behavior of a nematic liquid crystal microdroplet depending on the irradiation position

This paper reports the random lasing and whispering gallery mode (WGM) lasing in dye-doped nematic liquid crystal (NLC) droplets. We discuss the emission behavior of the droplets in detail. Their lasing behavior depends on irradiation position and droplet diameter. Moreover, the two types of lasers likely share energy. Naturally, stopping one increases the efficiency of the other. Besides, the coexistence of the two types of lasers in an NLC droplet offers a controllable dye laser emitting two lights in different wavelength ranges.

Effect of dye on frequency and bias voltage-dependent electrical parameters of a nematic liquid crystal

The dielectric characteristics of pure and dye-doped (aniline blue dye) nematic liquid crystals at room temperature by altering the bias voltage and frequency have been reported in the present work. The HP-4194A impedance gain phase analyser was used to assist with the measurements of the dielectric properties. It has been discovered that the dielectric constant and dielectric loss both drop when dye is doped in liquid crystal. When bias voltage is increased, it has also been found that the dielectric anisotropy grows quickly up to 6.0 V bias voltage, after which its increase steadily.

Optical nonlinearity of a dual-frequency nematic liquid crystal via temperature-mediated mapping of dielectric anisotropy.

Liquid crystals are of great interest in the field of nonlinear optics due to their efficient response to low intensity light fields. Here we present a new, to the best of our knowledge, mechanism of a nonlinear optical response which is observed for a dye-doped dual-frequency nematic liquid crystal. The local increase in temperature caused by the absorption of light beam in the liquid crystal medium leads to spatial variation and inversion of the sign of the dielectric anisotropy. When an alternating current electric field with a frequency close to the cross-over frequency is applied to the liquid crystal cell, the planar orientation sustains at the beam periphery, but elastic deformation occurs in the irradiation region. In the case of a dye dopant with negative absorption dichroism, a first-order orientational transition with large bistability region is obtained.

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.

Controllable Liquid Crystal Micro Tube Laser

This study demonstrates controllable random lasing emissions in a dye-doped nematic liquid crystal (DDNLC)-infiltrated microcapillary. The emission wavelength of the micro tube laser can be adjusted by changing the focusing position of the pumped pulses on the center or the periphery of the liquid crystal region of the microcapillary. In addition, with doping azo-dyes in the DDNLC of the micro tube laser, optical controllability of the lasing intensity and wavelength can be further achieved. The controllable micro tube laser may find highly widespread photonic applications in multicolor emitting sources, and vibration and UV sensors.

Random lasing and mode tunability by extraordinary strong and controllable scattering during the nematic-isotropic phase transition

Random lasing emission from the lateral end face of dye doped nematic liquid crystal (DDLC) cells was investigated during the nematic-isotropic phase transition process and abundant lasing behaviors were observed. There was a temperature range of ~10 °C for the phase transition process of DDLCs, in which anisotropic light scattering was investigated. Light scattering of DDLCs was greatly enhanced during the phase transition process and the enhancement of scattering was strongly dependent on the temperature. And a peak scattering coefficient of more than 1000 cm−1 was achieved at a certain temperature, which we called the peak-scattering temperature. Random lasing induced by resonant feedback was observed near the peak-scattering temperature under the appropriate pump energies. In this situation, broad emissive spectra originating from non-resonant feedback or fluorescence were not evident. When the temperature departed from the peak-scattering value and the pump energy was fixed, the hybrid mode caused by both resonant and non-resonant feedback was achieved. And the proportion of the two feedbacks induced lasing modes could be tuned by temperature and pump energy. All the above random lasing phenomena were observed from 6-µm-thick DDLC cells. The strong scattering in the phase transition process was mainly induced by the inhomogeneity of DDLC due to coexistence of both nematic and isotropic phase. The rich lasing behaviors could be used to control lasing modes by temperature or energy of pump pulse.

Electrically tunable polarization of random lasing from dye-doped nematic liquid crystals.

Tunable polarizing direction of random lasing emission by an applied electric field which radiated from the lateral end face of homogeneously aligned, dye-doped nematic liquid crystal (NLC) cell was demonstrated for the first time, to the best of our knowledge. The lasing emission was partially polarized in the direction along the director of the NLC without the applied electric field. By tuning the applied electric field, the NLC director could be rotated to arbitrary direction from homogeneous to homeotropic alignment, resulting in the polarizing direction of lasing emission to any direction from parallel to perpendicular to the substrate surface in the end face.

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.

Optothermal vortex-solitons in liquid crystals.

We report on vortex-solitons generated in dye-doped nematic liquid crystals by a purely optothermal nonlocal nonlinearity. This response not only supports stable doughnut-shaped ordinary-wave beams with orbital angular momentum, but also provides self-confined solitary waves with excellent trajectory and profile stability over time. Using an interferometric technique, we also investigate the role of nonlocal nonlinearity in the non-illuminated axial region.

Pump-Controlled Plasmonic Random Lasers from Dye-Doped Nematic Liquid Crystals with TiN Nanoparticles in Non-Oriented Cells

Manipulation of the performance of the random lasers from dye-doped nematic liquid crystals with TiN nanoparticles in non-oriented cells is studied. The experimental results show that the introduction of TiN nanoparticles into dye-doped nematic liquid crystals significantly reduces the threshold of random lasing due to the localized surface plasmon resonance of TiN nanoparticles. The emission spectrum of random lasers can be controlled by the shape of the pump spot. The threshold of random lasers increases with the decrease of the length of pump stripe. In order to obtain the emission spectrum with fine discrete sharp peaks, the narrow pump stripe is more effective than the circular pump spot. When the pump area is more like a circle, the emission spectrum is more like an amplified spontaneous emission. The underlying mechanisms of these phenomena are discussed in detail. This study provides a promising platform for designing the high-quality and low-threshold random lasers which can be controlled by the shape of the pump spot.

Random lasing in dye-doped chiral nematic liquid crystal

Random laser action is observed in the PM597 dye-doped chiral nematic liquid crystal (DDCLC) cells. A focal conic texture of chiral nematic liquid crystal (CLC) with many micro-domains is achieved in the cell without rubbing treatment. An Nd:YAG pulsed solid-state laser with a wavelength of 532 nm is employed as a pump source. Discrete and sharp random lasers with a line-width of 0.3 nm and wavelength range of 570–590 nm are obtained. The pump threshold energy of a random laser significantly depends on the cell thickness. The DDCLC cell with a thickness of 80 μm shows the lowest threshold value Eth of 7.6μJ, whereas the 10 μm DDCLC cell presents the highest Eth of 19.6μJ. The lower Eth is attributed to a denser micro-domain distribution that provides a stronger photon scattering at a certain pump depth. The random laser emission is shut down when the temperature reaches 61 °C.

Magnetically controllable random laser in ferromagnetic nematic liquid crystals.

This paper first reports random laser action in dye-doped ferromagnetic nematic liquid crystals, which act as a randomly distributed cavity. The random laser intensity of the ferromagnetic nematic liquid crystals can be controlled by a weak magnetic field (∼1 mT). Moreover, the magnetic switching of random laser is attributed to the direction and polarization dependent emission of light in the ferromagnetic nematic liquid crystals in an external magnetic field.

Absorption-mediated stabilization of nonlinear propagation of vortex beams in nematic liquid crystals

We theoretically investigate the effect of linear absorption on the stability of nonlinear vortex beams in dye-doped nematic liquid crystals. We show that the combined effects of intensity attenuation and thermal defocusing provide a stabilization mechanism which significantly extends the stable propagation distance of the vortex beam for first- and second-order topological charges.

Dynamic Grating Recording by a Light-Induced Modification of the Order Parameter in Dye-Doped Chiral Nematic Liquid Crystals

The dynamic grating recording in dye-doped chiral nematic liquid crystals is studied. It is shown that the mechanism responsible for the grating recording can be ascribed to a photoinduced modification of the order parameter of a liquid crystal within the range of optical intensities, for which the orientational nonlinearity remains quenched by the structural order of the chiral structure. The two-wave mixing dynamic behavior is analyzed for different intensities of the recording beams, by comparing the self-diffraction regime with the probe diffraction. This allows us to distinguish a particular mechanism of optical nonlinearity. The photo-induced modulation of the order parameter and the respective changes of medium’s refractive indices determine the relatively fast response times, local nonlinear response, and quite high diffraction efficiency within an extremely wide intensity range (more than three orders of magnitude) The chiral helical structure hinders the nematic director reorientation, prevents the appearance of surface effects, and is very favorable for the manipulation of a nonlinear polarization. Such new mechanism could also be extended to the recording of arbitrary phase profiles as requested in several applications for the manipulation of a light-beam.

The photoelectric properties characteristics of dye-doped nematic liquid crystal microcapsules with different structural composition

In this study, we promote a new dye-doped nematic liquid crystal (DDNLC) microcapsules system sandwiched between two ITO substrates for making an electrochromic fabric coating with both low driving voltage and wide versatility. The effects of the dye dosage, particle size and shell thickness of microcapsules on the electro-optical properties of DDNLC microcapsules were systematically investigated. Results show that optical appearance of the optimized DDNLC microcapsules can be reversibly changed between transparent and colored states by switching on and off the direct-current power. Driving voltage, response time and delay time were close related to the particle size of DDNLC core in microcapsules, and an alignment model was used to calculate the geometric construction of the microcapsules. The proposed experimental results provide reliable simulations of the electro-optical properties of DDNLC microcapsules which showing great potential applications in a wide range of uses, such as electronic paper, military camouflage coating or intelligent flexible display fabric.

Mode coupling by scattering in chiral nematic liquid crystal ring lasing.

Lasing in dye-doped chiral nematic liquid crystal can be realized with low pump energy and relatively high efficiency, thanks to the high reflectivity of the periodic structure. When the helical axis is oriented perpendicular to the substrates, the main lasing peak is normal to the substrates. In some cases, ring lasing of a particular wavelength is observed into an emission cone with axial symmetry. In this paper we explain how scattering of light in the liquid crystal layer leads to optical coupling between normal modes and inclined modes. Based on a numerical model that takes into account spontaneous emission, gain and scattering we show that scattering leads to emission characteristics that are similar to experimental results.

Thermal and electrical tuning of whispering gallery modes in liquid crystal-filled hollow glass microsphere

This paper demonstrates a thermally and electrically tunable hollow glass microsphere resonant cavity filled with dye-doped nematic liquid crystal. We controlled the orientation of in-sphere liquid crystal and encapsulated it with polydimethylsiloxane film. The typical whispering gallery modes (WGMs) lasing spectra were obtained under a pulsed laser pump and the effects of the temperature and electric field on the peaks wavelengths were studied. The WGM spectra present a blueshift significantly with increasing temperature and electric field due to the decrease in the effective refractive index.