Designing novel chalcone single crystals with ultrafast nonlinear optical responses and large multi-photon absorption coefficients

A chalcone single crystal, 1-(4-chlorophenyl)-3-(4-methoxyphenyl)prop-2-en-1-one that is transparent over the visible to infrared region is introduced as a new potential material to third-order nonlinear optical applications. The crystal exhibits ultrafast optical response (≤90 fs) and large optical nonlinearity in the wavelength range 800–1200 nm. A very large effective two-photon absorption coefficient β_eff exceeding 120 cm/GW can be obtained with this chalcone crystal, at a low intensity threshold of 41 MW/cm². The mechanism of nonlinear absorption at different levels of intensity has been discussed. The crystal shows no damage against the laser pulse intensity as high as 8 GW/cm². We discuss the molecular and crystal designing of chalcones with large and ultrafast optical nonlinearity combined with low optical cut-off (<450 nm).     

Dependence of viscoelastic parameters of nematic liquid crystals on pretilt angle and temperature

The viscoelastic parameters of nematic liquid crystal (LC) E7 in both splay and twist relaxation modes are investigated as functions of pretilt angle and temperature by the technique of dynamic laser light scattering. The results show that the elastic constants of the liquid crystal in the two modes not only depend on temperature, but also depend on pretilt angle. There is a critical pretilt angle (β_C) at which nematic LC begins exhibiting elastic property, and beyond which the elastic constants increase with pretilt angle, then keep constant after exceeding another angle (β_S, with β_S > β_C). This phenomenon is observed to be universal for different nematic LC’s, and significant not only in the understanding of the molecular mechanism of exhibiting viscoelastic properties in nematic LC, but also for their practical purpose.     

Theoretical study of reorientation and torque of liquid crystal molecules under influence of external electric field and experimentally generation of spatial optical soliton beam and getting a sharp switching in chiral nematic liquid crystal

Liquid crystals (LC) are anisotropic materials which experience a torque if an electric field is present. This field can be due to an external voltage or to the presence of a light beam. Reorientation due to light leads to non-linear behavior in the optical behavior. Due to this kind of nonlinearity therefore it is possible to generate optical spatial soliton beam in LC by bias voltage or without it and interestingly chiral nematic liquid crystals has a opportunity to generate spatial optical solitons without the need for a bias voltage. In this paper we also demonstrate that a sharp switching of the helix structure occurs when the spatial soliton is launched in the middle of two regions where soliton generation is favorable. Due to the optical nonlinearity, the helical structure becomes asymmetric and a sharp switching in one direction can be obtained. Moreover, in this paper, the torque and reorientation of the liquid crystal and the change in angular momentum of the light are discussed.     

Identification of optical nonlinearities of dye-doped nematic and polymer-dispersed liquid crystals using Z-scan technique

This study investigates the nonlinear optical properties of azo-dye-doped nematic and polymer-dispersed liquid crystal (ADDPDLC) films with nano-sized LC droplets using the Z-scan technique, which is a simple but powerful technique for measuring the optical Kerr constants of materials. The results indicate that the optical Kerr constant (n₂) of the azo-dye-doped nematic LC (ADDLC) film is large because of the photoisomerization effect and the thermal effect. Therefore, the optical Kerr constant of this material can be modulated by varying the temperature of the sample and the direction of polarization of incident laser. The range of n₂ modulated is from −5.26 × 10⁻³ to 1.62 × 10⁻³ cm²/W. The optical Kerr constants of ADDPDLC films at various temperatures are also measured. The experimental results reveal that liquid crystals in the ADDPDLC film strengthen the nonlinearity. The n₂ of the ADDPDLC film is maximal at ∼35 °C, because of the decrease in the clearing temperature of the ADDPDLC films. The clearing temperatures of the liquid crystals (E7), and the ADDPDLC film used in this work were found to be 61 °C and 43 °C, respectively.     

Light propagation in periodic photonic structures formed by photo-orientation and photo-polymerization of nematic liquid crystals

Propagation of linearly polarized light beams in a nematic liquid crystal cell with distinguished regions of different molecular orientation has been analyzed. Specifically, combination of the planar/homogenic and homeotropic alignment, forming thus spatially limited regions characterized by a different LC molecular orientation, has been tested, as achieved by means of the photo-orientation and photo-polymerization processes, independently. An influence of molecular orientation on the light beam propagation has been checked for different directions of the linear polarization. Thanks to the molecular reorientation induced by the low frequency external electric field and also to the reorientational nonlinearity taking place in NLCs, propagation direction of the light beam can be additionally controlled by the electric bias and/or optical power, respectively. Proposed structural solutions and techniques, related to the photo-orientation and photo-polymerization processes described in this communication, give rise to the novel LC geometries and structures. The latter act as promising candidates for new practical photonic applications as they are expected to be of a particular importance for integrated optic elements and devices.     

Electro-Optical and Thermophysical Characterization of Poly (Tripropylene Glycol Di-Acrylate)/Liquid Crystal Composite Materials Prepared by Polymerization Induced Phase Separation

Abstract

This work examines the development and characterization of tripropylene glycol di-acrylate/liquid crystal E7 (TPGDA/LC E7) PDLCs composite materials (polymer-dispersed-liquid-crystals). These systems were produced by UV irradiation photopolymerization (PIPS) of a mixture of the monomer tripropylene glycol di-acrylate (TPGDA) and the liquid crystal E7 (LC E7, a mixture of three cyano-biphenyl and one cyano-terphenyl LCs), in the presence of 2?wt% (of the acrylate/E7 mixture) of a photoinitiator. Electro-optical, thermal and optical characterization was used to understand the effect of the LC concentration on the electro-optical and thermo-physical properties of these materials. Polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) studies were performed to observe the system morphology and to determine the transition temperatures of these materials, both as a function of their composition. The findings showed a slight variation of the nematic-isotropic transition temperature, T_NI, of the LC E7 and of the glass transition temperature, T_g, of the TPGDA polymeric matrix as a function of the mass percentage of the LC E7. A very good electro-optical response for the composition 30/70?wt % TPGDA/LC E7 was obtained.     

Increasing rewriting speed of optical rewritable e-paper by selecting proper liquid crystals

Effect of interaction between liquid crystal (LC) and photoalignment material on speed of optical rewriting process is investigated. The theoretical analysis shows that smaller frank elastic constant K₂₂ of liquid crystal corresponds to larger twist angle, which gives rise to larger rewriting speed. Six different LC cells with the same boundary conditions (one substrate is covered with rubbed polyimide (PI) and other with photo sensitive rewritable sulfuric dye 1(SD1)) are tested experimentally under the same illumination intensity (450 nm, 80 mW/cm²). The results demonstrate that with suitable liquid crystal, LC optical rewriting speed for e-paper application can be obviously improved. For two well known LC materials E7 (K₂₂ is larger) and 5CB (K₂₂ is smaller), they require 11 s and 6 s corresponding to change alignment direction for generating image information.     

采用液晶空间光调制器进行激光光束的空间整形

提出采用液晶空间光调制器进行激光束的实时,可调控光束空间整形的新方法,研究了液晶空间光调制器的光学调制特性,用液晶空间光调制器衬时产生的软边切趾光阑与空间滤波器结合,有效地对任意光束进行空间整形,获得光束填充因子高,近“平顶”光强的光束近场分布。     

Propagation properties of anomalous hollow beam in uniaxial crystals orthogonal to the optical axis

The analytical formulae for anomalous hollow beam propagating in uniaxial crystals orthogonal to the optical axis are derived. The numerical results show that the anomalous hollow beam spreads at different rates in the directions along and orthogonal to the optical axis. The beam spreads more rapidly in the direction along the optical axis than orthogonal to the optical axis in positive crystal (n_e/n_o>1), and the beam spreads more rapidly in the direction orthogonal to the optical axis than along the optical axis in negative crystal (n_e/n_o<1).     

Optical absorption spectrum of Ni2+ ion doped in synthetic lecontite

The optical absorption spectrum of Ni²⁺ ion doped in lecontite (sodium ammonium sulphate dihydrate) single crystal has been studied at room and liquid air temperatures. All the bands could be assigned assumingO _h symmetry for the Ni²⁺ ion in the crystal. The splitting of³ T _1g (F) band at liquid air temperature has been attributed to spin-orbit interaction. The crystal field and spin-orbit parameters derived areD _q=1000 cm^?1;B=740 cm^?1;C/B=4.27 and ζ=600 cm^?1. All the bands observed show a blue shift when the crystal was cooled to liquid air temperature.     

Third order nonlinear optical properties of potassium dichromate single crystals by Z-scan technique

Single crystal of potassium dichromate (KDC) has been grown from aqueous solution by slow evaporation technique. The lattice parameters of the grown crystal were determined by X-ray diffraction analysis. The optical absorption studies reveal that the crystal has UV cut-off wavelength around 240 nm. Thermo gravimetric and differential thermal (TGA/DTA) studies revealed that the crystal thermally stable up to 397.1 °C. The mechanical strength of the grown crystal was carried out by Vickers micro hardness test. The crystal perfection was confirmed by etching studies. Third order nonlinear optical studies was performed using by single beam Z-scan technique using continuous Nd:YAG laser. Closed aperture Z-scan studies reveal the negative nonlinearity in the crystals and open aperture Z-scan reveals the saturation absorption. Also various parameters such as nonlinear refractive index n₂, absorption co-efficient β and nonlinear optical susceptibility χ⁽³⁾ were calculated for the grown crystal.     

Fast and Accurate Phase Retardation Measurement of 0° Twisted Nematic Liquid Crystal Panels Based on the Principle of Homodyne Receiving

We describe herein a method to measure quickly (in less than 2 min) and accurately the phase retardation between the extraordinary and ordinary axis of homogeneous or 0° twisted nematic liquid crystal panels. Owing to the homodyne detection scheme, the setup allows us to probe liquid crystal (LC) panels with an optical power down to a fraction of 1 nW and, hence makes it possible to focus the light beam to a spot of only a few wavelengths in diameter without heating or inducing observable variations in the liquid crystal. The dependence of the phase retardation on the LC driver frequency and the temperature are presented and compared with the theoretical prediction.     

Electric-field-induced unwinding of ferroelectric helix in thin smectic C* layers with soft and rigid anchoring of molecules

The unwinding of a helical structure in thin films of a ferroelectric smectic liquid crystal (LC) by an external electric field has been theoretically studied using a discrete model in which every LC layer is characterized by a two-dimensional vector ξ _i (describing the orientation of molecules) and by the polarization P _i . It is established that the unwinding of the LC helix in thin films significantly differs from the well-known behavior of thick samples. In particular, discrete intermediate states (differing by an integer or half-integer number of turns) are formed in thin films for both weak and strong anchoring of molecules to a substrate surface. The physical factor responsible for this behavior is the presence of near-surface regions with thicknesses below the helix pitch and the corresponding uncompensated polarization.     

The orientational mechanism of nonlinearity and the self-focusing of He-Ne laser radiation in nematic liquid crystal mesophase (theory and experiment)

A giant optical nonlinearity of self-focusing type in the oriented mesophase of nematic liquid crystals (NLC) due to the director reorientation under the action of a light wave field is predicted. Self-focusing of He-Ne laser radiation with power ～10^?2 W and power density ～50 W/cm² in a planar oriented 60 μm thick NLC layer has been carried out experimentally. The measured value of the nonlinearity effective constant ?₂ = 0.07 cm³/erg corresponds to theoretical predictions, and turns out to be larger than the CS₂ nonlinearity by ? 10⁹ times.     

Absorption spectra of vanadyl ion doped in MgNH4PO4·6H2O (struvite) crystal

Results of Electron Paramagnetic Resonance (EPR) and optical absorption studies of VO²⁺ ion doped in struvite at room liquid nitrogen temperatures are reported. Three preferential V=O bond directions in the crystal have been identified. The optical and EPR data have shown the formation of NH₄(PO₄VO(H₂O)₅ complex in the crystal as a result of VO²⁺ doping. Correlating the optical and EPR data the molecular orbital coefficients are also obtained and discussed.     

Light-controlled helical pitch and dynamic gratings

A chiral nematic liquid crystal (LC) with a photo-sensitive chiral agent is employed for a light-controlled modulation of optical activity and used as a model substance for a dynamic diffraction grating recording. The described liquid crystalline system has shown a strong nonlinear response with effective parameter of cubic nonlinearity being much greater of that characteristic of the orientational nonlinearity of LC. A simple mathematical model of light diffraction on the grating of modulated optical activity was developed. Calculated values of intensities and polarisation states of diffracting beams have shown very good agreement with the experimental data. LC systems with light-controlled chirality could be promising media for nonlinear optical applications or all-optical switching devices.     

Entropy-driven aggregation of adhesion sites of supported membranes

A large number of interesting phenomena related to the insertion of colloidal particles in liquid crystals (LC) have recently been reported. Here, we investigate effects caused by the addition of spherically shaped ferroelectric nanoparticles to a nematic liquid crystal. Using molecular dynamics (MD) simulations, the density of LC molecules, the orientational order parameter, and the polar and azimuthal angle profiles are calculated as functions of the distance to the center of the immersed nanoparticle for different temperatures of the system. We observe that the assembly of ferroelectric nanoparticles enhances the nematic order in the LC medium changing many properties of its host above the nematic-isotropic transition temperature T ^* _NI .     

Nearly-analogue blazed phase grating using high birefringence liquid crystal

Diffraction of liquid crystal gratings has been thoroughly studied for many applications such as diffraction optics, optical processing, and spectral analysis. In pure optical processing one varies the direction of propagation of light beam without any mechanical adjustment. In this work we propose a beam steering device using highly birefringent liquid crystal material. Using a highly birefringent material one can reduce the LC layer thickness needed to achieve 2π of phase modulation and thus reduce the fringing effect caused by deformation of the electric field at the edge of the pixel. Here, we present 1.5-μm thick, high-resolution diffraction grating with non-detectable fringing.     

The effects of external applied voltage on the nonlinear refractive index and the birefringence of a dye-doped nematic mixture

The nonlinear refractive index, n₂, and the birefringence, Δn, of a nematic liquid crystal mixture (denoted as 1294-1b) doped with a anthraquinone derivative (denoted as AQ) dye were measured as a function of an external ac applied voltage. The self-phased modulation effect was used to measure the nonlinear refractive index of homeotropical-aligned samples. The change of the dyed liquid crystal nonlinearity, which is the direct impact of the applied voltage on the dye molecules orientation, was not observed for the nonlinear response of the pure liquid crystal. Planar (homogeneous) aligned samples were used for birefringence measurements by taking into account the absorption coefficients of the dye. The birefringence decreased to a limiting value in the high voltage region for both pure and dye-doped samples, although in the presence of the absorbing dye the birefringence of the liquid crystal was not approximately changed.The polarized absorption spectra of the dye in nematic mixture were recorded in parallel-aligned liquid crystal cell and its dichroic ratio R and the order parameter S_d were obtained. The electro-optical effect of the guest-host system was also using polarized spectroscopic method.     

Mechanism of phase conjugation via stimulated Brillouin scattering in narrow band gap semiconductors

We develop a theoretical model to study optical phase conjugation via stimulated Brillouin scattering (OPC-SBS) in narrow band gap transversely magnetized semiconductors. Threshold value of pump electric field and reflectivity of the image radiation for the onset of OPC-SBS are estimated. The analysis is applied to both cases viz. centrosymmetric (CS) and non-centrosymmetric (NCS) crystals. Numerical estimates made for n-type InSb crystal at liquid nitrogen temperature duly irradiated by nanosecond pulsed 10.6 μm CO₂ laser shows that high OPC-SBS reflectivity (90%) can be achieved in NCS crystals at moderate pump electric fields if the crystal is used as an optical waveguide with relatively large interaction length (L = 5 mm) which proves its potential in practical applications such as fabrication of phase conjugate mirrors.