Dye-doped Nematic Liquid Crystal Research Articles

Nonlinear continuous-wave optical propagation in nematic liquid crystals: Interplay between reorientational and thermal effects.

We investigate nonlinear optical propagation of continuous-wave (CW) beams in bulk nematic liquid crystals. We thoroughly analyze the competing roles of reorientational and thermal nonlinearity with reference to self-focusing/defocusing and, eventually, the formation of nonlinear diffraction-free wavepackets, the so-called spatial optical solitons. To this extent we refer to dye-doped nematic liquid crystals in planar cells excited by a single CW beam in the highly nonlocal limit. To adjust the relative weight between the two nonlinear responses, we employ two distinct wavelengths, inside and outside the absorption band of the dye, respectively. Different concentrations of the dye are considered in order to enhance the thermal effect. The theoretical analysis is complemented by numerical simulations in the highly nonlocal approximation based on a semi-analytic approach. Theoretical results are finally compared to experimental results in the Nematic Liquid Crystals (NLC) 4-trans-4'-n-hexylcyclohexylisothiocyanatobenzene (6CHBT) doped with Sudan Blue dye.

Light propagation in a photonic crystal fiber infiltrated with mesogenic azobenzene dyes

In this paper, light propagation in an isotropic photonic crystal fiber as well in a silica-glass microcapillary infiltrated with a mesogenic azobenzene dye has been investigated. It appeared that light spectrum guided inside the photonic crystal fiber infiltrated with the investigated azobenzene dye depends on the illuminating wavelength of the absorption band and on linear polarization. Also, alignment of the mesogenic azobenzene dye molecules inside silica glass microcapillaries and photonic crystal fibers has been investigated. Results obtained may lead to a new design of optically tunable photonic devices. Full Text: PDF ReferencesP. Russell. St. J. "Photonic-Crystal Fibers", J. Lightwave Technol. 24, 4729 (2006). CrossRef T. Larsen, A. Bjarklev, D. Hermann, J. Broeng, "Optical devices based on liquid crystal photonic bandgap fibres", Opt. Exp. 11, 2589 (2003). CrossRef D. C. Zografopoulos, A. Asquini, E. E. Kriezis, A. d'Alessandro, R. Beccherelli, "Guided-wave liquid-crystal photonics", Lab Chip, 12, 3598 (2012). CrossRef F. Du, Y-Q. Lu, S-T. Wu, "Electrically tunable liquid-crystal photonic crystal fiber", Appl. Phys. Lett 85, 2181 (2004) CrossRef D. C. Zografopoulos, E. E. Kriezis, "Tunable Polarization Properties of Hybrid-Guiding Liquid-Crystal Photonic Crystal Fibers", J. Lightwave Technol. 27 (6), 773 (2009) CrossRef S. Ertman, M. Tefelska, M. Chychłowski, A. Rodriquez, D. Pysz, R. Buczyński, E. Nowinowski-Kruszelnicki, R. Dąbrowski, T. R. Woliński. "Index Guiding Photonic Liquid Crystal Fibers for Practical Applications", J. Lightwave Technol. 30, 1208 (2012). CrossRef D. Noordegraaf, L. Scolari, J. Laegsgaard, L. Rindorf, T. T. Alkeskjold, "Electrically and mechanically induced long period gratings in liquid crystal photonic bandgap fibers", Opt. Expr. 15, 7901 (2007) CrossRef M. M. Tefelska, M. S. Chychlowski, T. R. Wolinski, R. Dabrowski, W. Rejmer, E. Nowinowski-Kruszelnicki, P. Mergo, "Photonic Band Gap Fibers with Novel Chiral Nematic and Low-Birefringence Nematic Liquid Crystals", Mol. Cryst. Liq. Cryst. 558(1), 184 (2012). CrossRef S. Mathews, Y. Semenova, G. Farrell, "Electronic tunability of ferroelectric liquid crystal infiltrated photonic crystal fibre", Electronics Letters, 45(12), 617 (2009). CrossRef V. Chigrinov, H-S Kwok, H. Takada, H. Takatsu, "Photo-aligning by azo-dyes: Physics and applications", Liquid Crystals Today, 14:4, 1-15, (2005) CrossRef A. Siarkowska, M. Jóźwik, S. Ertman, T.R. Woliński, V.G. Chigrinov, "Photo-alignment of liquid crystals in micro capillaries with point-by-point irradiation", Opto-Electon. Rev. 22, 178 (2014); CrossRef D. Budaszewski, A. K. Srivastava, A. M. W. Tam, T. R. Woliński, V. G. Chigrinov, H-S. Kwok, "Photo-aligned ferroelectric liquid crystals in microchannels", Opt. Lett. 39, 16 (2014) CrossRef J-H Liou, T-H. Chang, T. Lin, Ch-P. Yu, "Reversible photo-induced long-period fiber gratings in photonic liquid crystal fibers", Opt. Expr. 19, (7), 6756, (2011) CrossRef T. T. Alkeskjold, J. Laegsgaard, A. Bjarklev, D. S. Hermann, J. Broeng, J. Li, S-T. Wu, "All-optical modulation in dye-doped nematic liquid crystal photonic bandgap fibers", Opt. Exp, 12 (24), 5857 (2004) CrossRef K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki, K. Aoki, "Reversible change in alignment mode of nematic liquid crystals regulated photochemically by command surfaces modified with an azobenzene monolayer", Langmuir, 4, 1214 (1988) CrossRef http://www.beamco.com/Azobenzene-liquid-crystals DirectLink K. A. Rutkowska, K. Orzechowski, M. Sierakowski, "Wedge-cell technique as a simple and effective method for chromatic dispersion determination of liquid crystals", Phot. Lett, Poland, 8(2), 51 (2016). CrossRef L. Deng, H.-K. Liu, "Nonlinear optical limiting of the azo dye methyl-red doped nematic liquid crystalline films", Opt. Eng. 42, 2936-2941 (2003). CrossRef J. Si, J. Qiu, J. Guo, M. Wang, K. Hirao, "Photoinduced birefringence of azodye-doped materials by a femtosecond laser", Appl. Opt., 42, 7170-7173 (2008). CrossRef

Linear dichroism and order parameters of nematics doped with azo dyes

ABSTRACTWe investigated solvent-solute effects using dye-doped nematic liquid crystals (NLCs) where three Sudan dyes of different concentrations were used as solutes doped into high- and low-polar NLCs as anisotropic solvents. The polarized spectroscopic absorption, dichroic ratio (R), and order parameter (S) were measured, which showed that the rod-like Sudan black B had a higher S and R due to a higher anisotropic solvent-solute effect produced by transition moment and global charge transfer along the molecular axis induced by high polar NLC. The order parameters measured by the Vuks method also showed a higher solvent-solute effect for the high-polar NLC.

Manipulation of Polarized Random Lasers from Dye-Doped Twisted Nematic Liquid Crystals Within Wedge Cells

The characteristics of polarized random lasers (RLs), made from dye-doped twisted nematic liquid crystals (DD-TNLCs) in a wedge cell, have been investigated. Regardless of the multiple light scattering within disordered LCs inside the cell, the output polarization is compulsorily determined by the rubbing direction of PI on glass plate. Moreover, the output intensity of the RLs can be manipulated by the orientation of the linear pump polarization relative to the rubbing direction of polyimide on the incident glass plane. Through thermal control, the resonance characteristics of the RLs can be altered to generate the highest slope efficiency and lowest lasing threshold at 25 °C. The measured results indicate abundant and intriguing optical and physical properties of the DD-TNLC laser.

All-optical guided-wave random laser in nematic liquid crystals.

Spatial solitons can affect and enhance random lasing in optically-pumped dyedoped nematic liquid crystals. Upon launching two collinear beams in the sample, the first to pump the fluorescent guest molecules and the second to induce a reorientational soliton, strikingly the second beam not only guides the emitted photons in the soliton waveguide, but also enhances the lasing efficiency and modulates its spectral width. By altering the scattering paths of the emitted photons, the soliton also contributes to the selection of the lasing modes, as further confirmed by the observed kinks in the input/output characteristics. These experimental results demonstrate that random lasing can be efficiently controlled by a light beam which does not interact with the gain molecules, opening a route towards light-controlled random lasers.

Study on the Polarization of Random Lasers from Dye-Doped Nematic Liquid Crystals

Random lasers from dye-doped nematic liquid crystal (DDNLC) cells with different rubbing methods were observed due to different random ring cavities that were formed. Through constructing cells with different rubbing methods on the forward and backward surfaces of light-emitting sides, we can get two random laser beams with different polarization directions from one DDNLC cell at the same time, and the polarization direction is along the rubbing direction of the light-emitting sides. Additionally, the influence of external electric field on the polarization degree of random lasers was also studied.

Widely tunable photonic bandgap and lasing emission in enantiomorphic cholesteric liquid crystal templates

Widely tunable reflection and lasing emission in a dye-doped nematic liquid crystal refilled gradient-pitched enantiomorphic cholesteric liquid crystal polymer template.

Soliton-assisted random lasing in optically-pumped liquid crystals

We demonstrate a guided-wave random laser configuration by exploiting the coexistence of optical gain and light self-localization in a reorientational nonlinear medium. A spatial soliton launched by a near-infrared beam in dye-doped nematic liquid crystals enhances and confines stimulated emission of visible light in the optically-pumped gain-medium, yielding random lasing with enhanced features.

The influence of Ag nanoparticles on random laser from dye-doped nematic liquid crystals

The threshold energy and electric field response characteristic of random laser have been investigated in dye-doped nematic liquid crystal (DDNLC) with the addition of different concentrations of Ag nanoparticles (NPs). Due to the localized surface plasmon resonance (LSPR) induced by Ag NPs, random laser from DDNLC with Ag NP doping had a lower threshold energy. From another point of view, nematic liquid crystals (LCs) in a DDNLC cell with the addition of Ag NPs could be more easily influenced by the electric field, which allowed the random laser to be controlled at a lower applied voltage. The turn-off time and turn-on time of random laser also decreased in the DDNLC cells with increasing the concentration of Ag NPs. This is due to the enhancement of the electro-optical characteristic of LC and the restoring force imparted by the locally ordered LCs induced by the Ag NPs, respectively.

Electrically controllable plasmonic enhanced coherent random lasing from dye-doped nematic liquid crystals containing Au nanoparticles.

An electrically controllable plasmonic enhanced coherent random lasing from the dye-doped nematic liquid crystal containing Au nanoparticles is demonstrated. To achieve the optimal control of the RL properties, the polarization of the pump light should be parallel to the rubbing direction of the cells. The lasing output intensity is direction-dependent and the substantial output distributes in an angle range of 0°~30° deviating from the direction of the pump stripe. The coherent feedback associated with the coherent random lasing mainly originates from the cooperative effect of the enhanced localized electric field in the vicinity of Au nanoparticles and the multiple scattering caused by the fluctuations of the liquid crystal director and local dielectric tensor.

Tuning the lasing wavelength of dye-doped chiral nematic liquid crystal by fluid flow

ABSTRACTThe influence of hydrodynamic flow on the lasing wavelength and stability of the laser emission average power of a dye-doped chiral liquid crystalline (DDCLC) structure is studied. Multiple lasing peaks at equidistant wavelengths are observed, which are originating from multiple domains that are induced by the hydrodynamic flow of the DDCLC. A simultaneous blue shift over 4 nm for all lasing peaks is observed after increasing the flow velocity to 0.3 mm/s for cells with a thickness of 10 μm. It is observed that the CLC flow dramatically increases the stability of the laser emission compared to conventional cells because dye bleaching and LC molecular reorientation due to pumping beam is reduced.

Wavelength-tuneable laser emission from a dye-doped achiral nematic liquid crystal dispersed into a chiral polymer scaffold

ABSTRACTWe compare the emission characteristics of a thin-film liquid crystal (LC) laser created using a polymer-stabilized, dye-doped chiral nematic LC to that of an LC laser that was fabricated using an achiral, dye-doped nematic refilled into a chiral polymer scaffold that was templated from the same chiral nematic host. Both lasers exhibit wavelength tuning upon the application of an external electric field. However, for the templated sample, tuning is found to occur across a broader wavelength-range for the same electric field amplitude. We discuss the benefits of the templated approach and how it can be used to circumvent dye bleaching that may occur during photo-polymerisation.

Electro-optical Kerr effect of azo-dye-doped liquid crystals using nulled intensity method

ABSTRACTThe electro-optical Kerr effect and pre-transition behavior have been investigated for different concentration of dimethyl-aminoazobenzene dye-doped nematic liquid crystal by using the nulled intensity method. Measurement results have showed that the molecular reorientation of liquid crystals induced by photoisomerization of azo dyes come about to increase an amount of Kerr constant at low percentages of dye concentration. Additionally, the results of pre-transition temperature which were obtained by extrapolating the invers Kerr constant as a function of temperature have been found to be in good agreement with predictions of the Landau-De Gennes model.

Nematicon-driven injection of amplified spontaneous emission into an optical fiber

We investigate experimentally the interaction between amplified spontaneous emission (ASE) and a soliton, which are both generated in a dye-doped nematic liquid crystal (LC) cell. A light beam is injected through an optical fiber slid into the cell to form a soliton beam. ASE is then automatically collected by this self-induced waveguide and efficiently coupled into the same optical fiber, in the backward direction. We demonstrate that the presence of the soliton improves the ASE collection by one order of magnitude. We also show that the ASE is highly polarized in the plane of the LC cell and that the ASE spectrum depends on the pump stripe orientation with respect to the LC director. The origin of the spectral anisotropy of the gain curves is determined with the help of femtosecond pump-probe spectroscopy.

Study of low-threshold and high-intensity random lasing in dye doped liquid crystals

Random lasers in dye-doped nematic liquid crystal (DDNLC) cells with different structures are studied. By choosing the cell gap and the cells rubbing methods, the DDNLC random laser obtains lower energy threshold. The DDNLC random laser energy threshold can also be decreased with an Al mirror as the external feedback from 4.2 μJ/pulse to 1.5 μJ/pulse. It is worth mentioning that the random laser shifts red and intensity increases with the Al mirror. The study of the DDNLC random laser with cell structure is aimed to obtain a low power consumption laser at a lower cost.

Random lasing in unbounded dye-doped nematic liquid crystals

ABSTRACTRandom lasing in dye-doped nematic liquid crystals (NLCs) contained in a quartz cuvette was studied. With the pumping energy rising, random lasing with nonresonant feedback and resonant feedback emerge successively with the lasing threshold of 20 and 40 μJ, respectively. The lasing properties including fluctuation of the emission spectrum, divergence angle of emission, independence of the emission on the polarisation of the pump beam and the depolarisation of emission are distinct from those of dye-doped nematics confined in the sandwich structure cell or the capillary tube. The distinctions are associated with the thermal fluctuation of dielectric tensor since there is negligible surface anchoring for the nematics in the pump volume. This indicates that the anchoring state of NLCs is an important factor determining the random lasing properties.

Low-voltage and wide-band tuning of lasing in a dye-doped liquid-crystal sandwich structure.

This paper reports the electrical tuning of a lasing in a liquid crystal (LC) sandwich structure. A dye-doped nematic LC (NLC) layer is sandwiched between two CLC layers to act as a phase retarder with the CLC layers acting as cavity mirrors, for the selective reflection of light in the photonic band with the same sense of helix handedness as that of the CLC layers. The transmittance spectrum of the sandwich cell provides a large range of modulation due to the wavelength dependent nature of phase retardation between the optical eigenmodes in the NLC layer. Lasing occurs at wavelengths corresponding to the maximum transmittance within the reflection band of the CLC layers. The application of voltage to the NLC layer makes it possible to shift the wavelengths of maximum transmittance, thereby tuning the wavelength of lasing. In these experiments, an applied voltage of 1.25 V was sufficient to shift the lasing peak wavelength by approximately 47 nm.

Behaviors of random laser in dye-doped nematic liquid crystals

Random lasing in the nematic liquid crystals (NLCs) with a high doping concentration of the laser dye was observed and characterized. With increasing the pump energy after the occurrence of the random laser (RL), the RL intensity first increases gradually to a maximum, then drops sharply to zero, accompanied by the gradual enhancement of scattering manifested by the growth of far-field diffraction rings of the transmitted pump beam in number. The threshold energy per unit pump area, slope efficiency, and maximal output intensity of the NLC RL depend heavily and nonmonotonically on the pump angle. A model involving the pump pulse induced molecular reorientation in NLCs leading to the pump angle dependent enhancement of scattering is proposed to explain the pump angle dependent properties of RLs.

Manipulation of Random Lasing Action From Dye-Doped Liquid Crystals Infilling Two-Dimensional Confinement Single Core Capillary

The experimental detail of a random lasing action from dye-doped nematic liquid crystals (LCs) inside single-core capillaries with a core diameter below 50 μm was investigated. The resonant characteristics, including the number of emission spikes, the full width at half maximum (FWHM), and the estimated Q-factor, were shown to depend on the core diameter of a capillary. In contrast with a capillary with a larger core diameter and having various emission spikes, only three emission spikes were excited from a capillary with a 10-μm core diameter, owing to the smaller effective area of the pump beam. However, the decrease in the amplitude of emission spikes and the broadening of the linewidth accompanying the higher lasing threshold from a capillary with a 50-μm core diameter are attributed to the decrease in the pump fluence and the increase in the scattering loss, respectively. In this paper, a random laser (RL) with the shortest FWHM emission peak of about 0.47 nm and the highest Q-factor of about 1268 was generated from the capillary with a 20-μm core diameter. By means of temperature adjustment, the emission spectra of the RL that is related to the birefringence and alignment ordering of the LC molecules inside a capillary with a 20-μm core diameter can be effectively altered. Our experiments show that the RL, revealing adjustable output emission spectra, can be a promising device in using remote sensing applications.

Lasing in a nematic liquid crystal cell with an interdigitated electrode system

Waveguide lasing in a layer of a dye-doped nematic liquid crystal has been observed. The liquid-crystal layer was sandwiched between a quartz substrate and a glass cover plate on whose surface was deposited an interdigitated electrode system. This system had a period of and played a dual role, namely, it created a spatial periodicity of the waveguide medium refractive index (thus creating distributed feedback) and served as a diffraction grating coupling out a part of waveguide radiation into the glass cover plate. The distributed feedback ensured lasing in the 18th diffraction order for the TE modes and in the 19th order for the TM modes of the waveguide. The generated radiation was observed at the exit from the glass plate end face at the angles to the waveguide plane of for TM modes and for TE modes. The intensity and position of the TE emission line showed no regular dependence on the voltage on the electrodes. In the case of TM radiation, an increase in the voltage led to a short-wavelength shift of the laser line and to a decrease in its intensity.