Nonlinearity Of Nematic Liquid Crystals Research Articles

Enhanced optical nonlinearity in nematic liquid crystal on doping with CdSe quantum dot

Third order nonlinear optical properties of CdSe quantum dot doped liquid crystal was studied in both isotropic and nematic state. CdSe quantum dots were synthesized by kinetic growth method. The optical and morphological analysis provided confirmation of the quantum dot formation. Nonlinear optical properties of the samples were explored by a single beam Z scan technique using Q-switched Nd:YAG laser, delivering pulses at 532 nm and 7 ns pulse width. Nonlinear optical absorption was found to be enhanced significantly on increasing the concentration of quantum dots in the liquid crystal. Janossy effect and charge transfer mechanism play a crucial role in the observed enhancement. The obtained values of the nonlinear refractive index, n2 and the third order optical susceptibility, χ(3) were of the order of 10−10 m2/W and 10−11 esu, respectively. The optical limiting thresholds of the studied systems are found to be 1.57 J/cm2 (isotropic) and 2.1 J/cm2 (nematic). The nonlinear optical characteristics of the CdSe-liquid crystal blend validate its application as an optical limiter.

Curved optical solitons subject to transverse acceleration in reorientational soft matter

We demonstrate that optical spatial solitons with non-rectilinear trajectories can be made to propagate in a uniaxial dielectric with a transversely modulated orientation of the optic axis. Exploiting the reorientational nonlinearity of nematic liquid crystals and imposing a linear variation of the background alignment of the molecular director, we observe solitons whose trajectories have either a monotonic or a non-monotonic curvature in the observation plane of propagation, depending on either the synergistic or counteracting roles of wavefront distortion and birefringent walk-off, respectively. The observed effect is well modelled in the weakly nonlinear regime using momentum conservation of the self-collimated beams in the presence of the spatial nonlocality of the medium response. Since reorientational solitons can act as passive waveguides for other weak optical signals, these results introduce a wealth of possibilities for all-optical signal routing and light-induced photonic interconnects.

High optical nonlinearity of nematic liquid crystal doped with graphen oxide

Colloidal suspensions of nematic liquid crystal (NLC) doped with graphene oxide (GO) were prepared and their third-order nonlinear optical (NLO) response χ(3), nonlinear refractive index (NLR), and nonlinear absorption coefficient (NLA) were investigated using Z-scan technique employing He: Ne laser at 632.8nm. GO doped system exhibits a significantly different nonlinear optical response compared to pure NLC and the highest nonlinearity was observed at the system including 0.75% GO.The results of GO doped NLC show that the external field changes the NL parameter are dependent upon the electrical field direction since it affects the director axis, n→, compared to their initial orientation. The external field is applied perpendicular to the cell surface. The χ(3) change value was measured as the applied electrical filed varies. The mentioned studies were carried out without and under the electric field effect using different AC voltages. The value of NLO, NLA and χ(3) in present of electric field were more significant compare without of electric field. Additionally, the optical limiting (OL) behavior of colloidal suspensions was investigated. The obtained results provide new useful information about the NLO properties of NLC's nanocomposite in particular to design optical limiting devices and also other photonic applications.

Bistable Beam Propagation in Liquid Crystals

Light-controlling-light is one of the most advanced paradigms in optical signal processing, including light-induced waveguides as well as all-optical switching and routing. Other fundamental aspects of all-optical processing are optical memories and sequential elements, which require responses depending on the evolution history of the system, such as the hysteresis stemming from optical multistability. Hereby, we report on optical bistability and hysteresis in cavityless geometries and in the presence of self-localized beams, i.e., spatial optical solitons, exploiting the nonlocal reorientational nonlinearity of nematic liquid crystals (NLC). When the optic axis of NLC is initially orthogonal to the applied electric field, the molecular dipoles start to rotate above a threshold named after the Freedericksz transition, the latter being usually of the second order. Here, we show that such transition become of the first order via the intrinsic feedback provided by self-focusing, in turn leading to the appearance of a hysteresis loop between diffracting and self-confined beams. We report on hysteresis of the beam size versus input power, as well as hysteresis versus applied voltage at a fixed beam power. Our findings introduce a novel kind of cavity-less optical bistability with propagating light beams and disclose a novel approach to information storage based on light self-localization.

Nonlinear Optics of Nematic and Blue Phase Liquid Crystals

We present a critical reexamination of individual electronic and collective optical nonlinearities of nematic liquid crystals (NLC) in the context of all-optical transmission switching of femtoseconds – CW lasers, and report recent observations of similar optical nonlinearities in Blue-phase liquid crystals (BPLC) and their utilization in feasibility demonstrations of polarization-free nonlinear grating diffraction and transmission switching of CW as well as pulsed lasers.

High Optical Nonlinearity of Nematic Liquid Crystals Doped with Gold Nanoparticles

Tetraoctylammonium bromide (TOAB)-coated gold nanoparticles dispersed in nematic liquid crystals lead to an enhancement of the nonlinear index coefficient, n2. A small amount of particles is sufficient to increase n2 and two-beam coupling gain by more than an order of magnitude while maintaining high optical quality and the ability of the suspensions to withstand strong applied electric fields. However, TOAB plays a crucial role in increasing the pretilt angle and reducing the anchoring strength, which leads to a stronger modulation of both the reorientation and the refractive index of liquid crystal suspensions.

Orientational optical nonlinearity of nematic liquid crystals induced by high-molecular-mass azo-containing compounds

Processes of light-induced reorientation of nematic liquid-crystalline molecules induced by the addition of low concentrations (0.1–2.0 wt %) of comb-shaped polymers and carbosilane dendrimers containing azobenzene fragments are studied. When the molecular structure of the above compounds becomes more complicated, the induced orientational nonlinearity increases. The introduction of 2G and 3G dendrimers into a nematic has for the first time made it possible to visualize and study a purely optical first-order Freedericksz transition in the field of a linearly polarized wave.

Nematicons Interaction in Chiral Nematic Liquid Crystals

Recently, it has been shown experimentally that due to the reorientational nonlinearity in nematic liquid crystals (NLCs) it is possible to generate self-trapped non-diffractive light beams for relatively low power. Such spatial solitary waves in NLCs are called nematicons. Very interesting and promising properties have nematicons in chiral nematic liquid crystals (ChNLCs), where the incident light propagates perpendicular to the helical axis. In this work we report on the experimental studies of the existence of nematicons in such ChNLCs cells. We show that it is possible to utilize multi-layers for propagation of independent and interacting nematicons.

Optical bistability of the director field of the dendrimer-doped nematic liquid crystal

It was found that the second-generation carbosilane dendrimers with statistically distributed terminal aliphatic and azobenzene fragments induce negative orientational nonlinearity in nematic liquid crystals (NLC) (the NLC director turns perpendicular to the light field, decreasing the refractive index of an extraordinary wave). The light-induced Freedericksz transition in planar oriented NLCs is the first-order orientational phase transition characterized by a wide bistability region.

NONLINEAR AND ELECTRO-OPTICS OF NANO-DISPERSED NEMATIC LIQUID CRYSTALS WITH TUNABLE NEGATIVE-, ZERO-, AND POSITIVE INDICES

We present a review of recent progress in the studies of the nonlinear- and electro-optics of liquid crystals, particularly in their meta-material forms. An analytical expression for the "ultimate" optical nonlinearity of nematic liquid crystals is obtained, and several routes to realizing such optical nonlinearities are discussed. We also describe two approaches for realizing tunable or reconfigurable negative-zero-positive index materials: (1) planar nano-structured frequency selective surfaces [FSS] containing nematic liquid crystals; (2) core-shell nano-spheres randomly distributed in bulk nematic liquid crystal matrix. Such metamaterials can be designed for applications in the visible-infrared, as well as Terahertz and microwave regimes. These liquid crystalline meta-materials are capable of supra-nonlinearities characterized by refractive index changing coefficients of over 1 cm2/watt and microseconds response times.

Tunable all-optical switching in periodic structures with liquid-crystal defects

We suggest that tunable orientational nonlinearity of nematic liquid crystals can be employed for all-optical switching in periodic photonic structures with liquid-crystal defects. We consider a one-dimensional periodic structure of Si layers with a local defect created by infiltrating a liquid crystal into a pore, and demonstrate, by solving numerically a system of coupled nonlinear equations for the nematic director and the propagating electric field, that the light-induced Freedericksz transition can lead to a sharp switching and diode operation in the photonic devices.

Signal processing by opto-optical interactions between self-localized and free propagating beams in liquid crystals

The reorientational nonlinearity of nematic liquid crystals enables a self-localized spatial soliton and its waveguide to be deflected or destroyed by a control beam propagating across the cell. We demonstrate a simple all-optical readdressing scheme by exploiting the lens-like perturbation induced by an external beam on both a nematicon and a copolarized guided signal of different wavelength. Angular steering as large as 2.2deg was obtained for control powers as low as 32mW in the near infrared.

Interplay between nonlocality and nonlinearity in nematic liquid crystals

Spatial nonlocality has a crucial role in the optical response of bulk liquid crystals. With specific reference to one-dimensional modulation instability, we demonstrate the tuning of transverse nonlocality in the nonlinear response of nematics.

Solitary waves in liquid crystalline waveguides.

Recently, it has been shown experimentally that the nonlinearity in nematic liquid crystals can govern spatial solitons in both waveguide and bulk geometry. Such solitons require a few milliwatts of light power and can be controlled by the state of light polarization or by an external electrical field. In this paper a detailed theoretical analysis of optical solitary waves in nematic liquid crystal waveguides is presented. The self-focusing is induced by reorientation nonlinearity in the homeotropically aligned nematic layer. This configuration corresponds to the experimental setup in which we previously observed such solitary waves. The theoretical results presented in this paper correlate exactly with the experiments.

Recent Studies of Optical Limiting, Image Processing and Near-Infrared Nonlinear Optics with Nematic Liquid Crystals

The mechanisms for recently observed optically induced refractive index changes in nematic liquid crystals, and their use in novel optical limiting and image processing are reviewed, along with an assessment of their potentials for applications. We have also studied the optical nonlinearities of nematic liquid crystals in the near IR communication spectral region [1.55 µm]. Phase modulation of several π's can be generated with mW laser power in micron thick films.

Nonlinear Optical Effects in Nematic Liquid-Crystal Films in the 1.55 µm Spectral Region

We have studied the optical nonlinearities of nematic liquid crystals in the near IR communication spectral region [1.55 µm]. The origins of the refractive index changes are thermal indexing effect and director axis reorientations. Very large refractive index and phase modulation of several π's can be generated with mW laser power in micron thick dye-doped nematic liquid crystal films.

A REVIEW OF NONLINEAR OPTICAL PROPERTIES OF NEMATIC LIQUID CRYSTALS

We present a review of the electronic and non-electronic optical nonlinearities of nematic liquid crystals (NLC), and recently observed nonlinear optical processes. In both the ordered and isotropic phases, NLCs exhibit large optical nonlinearities with response times ranging from picoseconds to milliseconds. The fundamental principles underlying these nonlinearities, their dynamics and practical application potential are discussed. Comparisons are made with other nonlinear optical materials currently being investigated.

STUDIES OF SOME PECULIARITIES OF NONLINEAR DYNAMICS OF FOUR-WAVE MIXING OF LIGHT WAVES PROPAGATING FORWARD IN LIQUID CRYSTAL ON THERMAL NONLINEARITY

Evolution of four waves propagating forward in nematic liquid crystal is studied using Hamiltonian representation of initial equations describing the process. Here we extend theory of the four-wave mixing on thermal nonlinearity of nematic liquid crystal. All types of analytical solutions are found for these equations. The possibilities of observing of optical switching in this mixing are discussed.

Grating Orientational Nonlinearity: Utilization in Optical Feedback Arrangements

Abstract Operation of passive ring mirror and double-conjugation scheme utilizing orientational nonlinearity of nematic liquid crystal is investigated theoretically and experimentally.

Measurements of Third Order Optical Nonlinearities of Nematic Liquid Crystals

Abstract In order to better understand the physical mechanisms responsible for the large observed optical nonlinearities of nematic liquid crystals, we have carried out nonlinear absorption and nonlinear refraction measurements on the pure liquid crystals 5CB and 8CB, and the liquid crystal mixture E7 using picosecond, nanosecond and millisecond pulses. We have used the recently developed Z-scan method1–2, a sensitive single-beam technique which allows the determination of the sign and magnitude of the nonlinear refraction and the magnitude of the nonlinear absorption for aligned samples3. We have performed these measurements on aligned samples at wavelengths of 514nm and 532nm. In addition, we have studied the temperature dependence of the nonlinear refractive indices. Possible mechanisms responsible for the observed nonlinearities are discussed.