Orientational Optical Nonlinearity Research Articles

Anomalous resonance frequency shift in liquid crystal-loaded THz metamaterials

Abstract Babinet complementary patterns of a spectrally tunable metamaterial incorporating a nematic liquid crystal is normally assumed to exhibit the same tuning range. Here we show that for a hybrid, liquid crystal-loaded metamaterial, the sensitivity of its terahertz resonances to the variations of the refractive index differs substantially for the two complementary patterns. This is due to a mismatch between the alignment of the liquid crystal and the direction of the local electric field induced in the metamaterial patterns. Furthermore, and more intriguingly, our experimental data indicate that it is possible to shift the resonance of the positive metamaterial pattern beyond the limit imposed by the alignment mismatch. Our analysis suggests that the observed anomalous frequency shift results from the orientational optical nonlinearity of a nematic liquid crystal.

Sign-alternating optical reorientation in nematic liquid crystals with low-molar-mass and polymeric absorbing bis-azobenzene dopants

A comparative study of optical orientation in nematic liquid crystals (NLCs) doped with a low-molar-mass bis-azobenzene dye (monomer) and a comb-shaped polymer with side fragments similar in structure to the monomer has been carried out. Both types of bis-azobenzene dopants induced a sign-alternating nonlinearity in the NLC: the positive when the angle Ψ between the light field and the director is less than a certain critical value Ψc and the negative in the opposite case of Ψc < Ψ ≤ 90°. The transition from the monomer to polymer led to a decrease in the critical angle Ψc, i.e., to an expansion of the region of negative nonlinearity. At the same concentration of chromophores, an increase in both negative and positive nonlinearities occurs.The magnitudes of the optical-torque enhancement factors due to trans- and cis-isomers (ηT and ηC), and the ratios ηT/ηC for the low- and high-molar-mass dopants are compared.The results obtained show the possibilities of increasing the orientational optical nonlinearity when passing from low-molar bis-azobenzene dye dopants to the corresponding polymers and are important for elucidating the optimal architecture of absorbing additives.

Zernike filter based on orientational optical nonlinearity of liquid crystalline systems

Visualization of the phase object with the use of a phase-contrast scheme incorporating a nonlinear Zernike filter was carried out. A liquid crystal or liquid crystalline polymer doped with azobenzene dye inducing orientational optical nonlinearity was used as the filter. Inversion of the image contrast depending on the angle of incidence of the light beam on a nematic liquid crystal cell was implemented.

Dynamic and Permanent Gratings in Suspensions of Absorbing Nanocrystals in Organic Solvent

Both dynamic and permanent optical gratings were recorded in the suspension of absorbing pigment nanocrystallites in organic solvent. The dynamic gratings were recorded due to the transfer of the nanocrystallites in the temperature gradient induced by light interference pattern, and the permanent gratings were formed due to light-induced adsorption of the nanocrystalites on the surface of the cell substrate. Full Text: PDF References B.Y. Zel'dovich, N. V Tabiryan, "Orientational optical nonlinearity of liquid crystals", Sov. Phys. Uspekhi 28, 1059 (1985). CrossRef K. Thyagarajan, P. Lallemand, "Determination of the thermal diffusion ratio in a binary mixture by forced Rayleigh scattering", Opt. Commun. 26, 54 (1978). CrossRef F. Bloisi, L. Vicari, P. Cavaliere, S. Martellucci, J. Quartieri, P. Mormile, G. Pierattini, A. Felice, "Soret effect in forced Rayleigh scattering", Appl. Phys. B 106, 103 (1987). CrossRef W. Köhler, "Thermodiffusion in polymer solutions as observed by forced Rayleigh scattering", Chem. Phys. 98, 660 (1993). CrossRef J. Zamir Anvari, R. Karimzadeh, N. Mansour, "Thermo-optic properties and nonlinear responses of copper nanoparticles in polysiloxane oil", J. Opt. 12, 035212 (2010). CrossRef N.M. Rouhollah Karimzadeh, "Investigation of the Soret effect in copper nanoparticle colloids by self-phase modulation and Z-scan", J. Mod. Opt. 60, 163 (2013). CrossRef N. Tabiryan, W. Luo, "Soret feedback in thermal diffusion of suspensions", Phys. Rev. E 57, 4431 (1998). CrossRef A. Eremin, R. Stannarius, S. Klein, J. Heuer, R.M. Richardson, "Switching of Electrically Responsive, Light-Sensitive Colloidal Suspensions of Anisotropic Pigment Particles", Adv. Funct. Mater. 21, 556 (2011). CrossRef I.C. Khoo, "Nonlinear optics of liquid crystalline materials", Phys. Rep. 471, 221 (2009). CrossRef H. Löwen, "Anisotropic self-diffusion in colloidal nematic phases", Phys. Rev. E 59, 1989 (1999). CrossRef S. Kredentser, A. Eremin, P. Davidson, V. Reshetnyak, R. Stannarius,Y. Reznikov, submitted to Eur. Phys. J. E (2015). CrossRef

Gravitational and thermocapillary mechanisms of hydrodynamic motions in liquid crystals induced by laser with a spatially periodic intensity structure

A possibility of excitation of hydrodynamic convective motions of the Rayleigh-Benard and Marangoni type in isotropic liquids and nematic liquid crystals upon absorption of light with a spatially periodic intensity distribution is demonstrated theoretically and experimentally. The opportunity of control and the stability of convective motions are studied. Benard cells become unstable when the light intensity is high. These instabilities are of the thermal origin because the Prandtl number for the medium under study is considerably larger than unity. The competition between the gravitational and thermocapillary mechanisms of photohydrodynamic reorientation of nematic liquid crystal director is also studied. The reorientation of the molecules due to the thermocapillary mechanism causes an optical nonlinearity which is three orders of magnitude stronger than the well-known direct orientational optical nonlinearity.

Laser-induced orientational nonlinear phenomena and new vortex structures

Theoretical study of the third-type thermomechanical effect in a twist cell of nematic liquid crystal is presented. The solution of director equation is obtained and optical nonlinearity is evaluated. It is proved that the nonlinearity is of the same order as in case of the direct orientational optical nonlinearity. Possibility of observing thermomechanical effect in non-uniform vortex structures is also discussed.

Some Light-Induced Orientational Effects in Nematic Liquid Crystals

Some new light-induced orientational-hydrodynamical nonlinear optical effects in nematic liquid crystals (NLC) are presented. Among them jump-like light-induced hydrodynamic reorientation caused by direct volume expansion, thermomechanical oscillations in hybrid NLC and thermomechanical mechanism of orientational optical nonlinearity in twist aligned NLC as well as in azobenzene liquid crystals. It is also shown that the orientational optical nonlinearity of revealed thermomechanical effect may be as strong as well-known giant optical nonlinearity. Besides, at normal incidence of laser the giant optical nonlinearity does not exist, but thermomechanical effect exists at any angle of incidence of laser radiation and the nonlinearity of thermomechanical effect remains in the same order.

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.

Optical Director Reorientation in NLCs Doped with Light-Absorbing Codendrimers of Different Generations

The orientational optical nonlinearity induced in NLCs by light-absorbing carbosilane codendrimers with statistical distribution of terminal aliphatic and azobenzene fragments has been studied comparatively for different (from the second to the fourth) codendrimer generations. It was found that all the codendrimers induce the negative nonlinearity (director rotates away from the light field thus decreasing the refractive index of the extraordinary wave). The orientational response increases with the generation number. The optical bistability is manifested for NLCs doped with the codendrimers of the second and third generations.

Thermomechanical mechanism of orientational optical nonlinearity in twisted nematic liquid crystals

In a nonuniform nematic liquid crystal director reorientation may take place as a result of a temperature gradient. We formulated a two-dimensional model of the temperature gradient caused by the light absorption of a Gaussian laser beam impinging normally on the liquid crystal cell. Within this model, we calculated the director reorientation and the subsequent optical nonlinearity of the liquid crystal.

Light-induced thermomechanical effect as a new mechanism of orientational optical nonlinearity in liquid crystals

A laser-induced thermomechanical effect in twist-aligned nematic liquid crystals has been revealed and experimentally studied. It has been proved that the nature of this phenomenon is orientational rather than diffraction. It has been shown that the orientational optical nonlinearity resulting from this effect can be as strong as the well-known giant optical nonlinearity. Moreover, giant optical nonlinearity does not exist at normal incidence of the laser beam, whereas thermomechanical nonlinearity remains of the same order of magnitude at any angle of incidence of the beam.

Light-Induced Director Reorientation in Nematic Liquid Crystals Doped with Azobenzene-Containing Macromolecules of Different Architecture

The results of the studies of the orientational optical nonlinearity of a nematic host doped with high-molar-mass compounds of different molecular architecture such as a comb-like polymer and a dendrimer containing azobenzene chromophores are presented. The nonlinearity induced by these compounds was found to be higher than that induced by the low-molar-mass dyes similar in structure to the light-absorbing constituents of the polymer and dendrimer. Possible reasons for the nonlinearity enhancement for macromolecular dopants are discussed.

Possible limitations of the classical model of orientational optical nonlinearity in nematics

Abstract Orientational nonlinearity is the major mechanism of nonlinear optical phenomena observed in liquidcrystalline phase while it does not appear to such extent in any other materials. It is caused by distortion of initial molecular arrangement of an anisotropic medium induced by optical field. Deformation of the anisotropic structure means spatial changes of refractive index of the medium. This effect has been studied in earnest since the 1980s as its application became more apparent. In this paper, some results of experimental examination of molecular reorientation in nematics by optical field are presented, which are not explained in frame of existing Oseen-Frank model and Erickson-Leslie continuous theory. Possible reasons of this discordance are considered and a way of explanation is suggested.

Orientational optical nonlinearity induced by comb-shaped polymers in a nematic liquid crystal

The effect of optical orientation in nematic liquid crystals containing small additions of high-molecular compounds, i.e., comb-shaped polymers with light-absorbing azobenzene side fragments, was studied. The effects of light-induced reorientation of the director of nematic liquid crystals caused by light absorption of polymers and a low-molecular compound with a structure similar to side fragments of the polymers were compared in detail. An explanation was proposed for large values of the orientational nonlinearity induced by polymers.

Enhancement factor formulation of doped nematic liquid crystals in the presence of photoisomerization

The mechanism of the orientational optical nonlinearity enhancement in the azo-dye doped liquid crystals is studied. The expressions of the enhancement factors based on the multiple-level transition model of azo-compound are derived. Our results show that, the simpler the model, the fewer the number of items contained in the expression of the enhancement factor. Using the enhancement factor expression obtained by the four-level model to describe the influences of the photoisomerization of azo-dyes on the enhancement of orientational optical nonlinearity is better than using that acquired by the three-level model. Besides, the expression of the enhancement factors in the case that the cis molecules do not absorb light is also obtained from the enhancement factor expression obtained by the four-level model. Except for a minor difference, the enhancement factor expression obtained in this case is the same as what was obtained for the anthraquinone-dye doped liquid crystals by Marrucci et al in 1997. This result also shows that under a given simplified condition, our theory is consistent with the last theory.

The Dependence of Orientational Optical Nonlinearity in Dye-Doped Liquid-Crystal Films on the Polarization Direction of the Recording Beams

We study, both experimentally and theoretically, the dependence of the absorbance and the two-beam coupling diffraction efficiency of dye-doped liquid-crystal films on the angle between the director vector of the liquid-crystal molecules and the polarization direction of the recording beams, which is shifted continuously from parallel to antiparallel corresponding to the director vector of the liquid-crystal molecules. The sample used in our experiment is the nematic liquid-crystal 5CB-doped with Disperse Red 1 (DR1). Also, there is no electronic field applied to it. The diffraction efficiency of the probe beam increases with the increase of the recording light absorption. Coupled-wave equations are developed to describe the laser-induced refractive index change, grating formation, and coherent wave-mixing effect. The analytical solutions of the recorded holographic grating and the diffraction efficiency of the probe beam are obtained for the case of taking losses into account. The results show that the diffraction efficiency can be improved by choosing suitable material parameters and recording parameters

Polarization characteristics of two-wave self-diffraction in media with a nonlinear response of the Kerr type

Characteristics of transient two-wave self-diffraction, due to the polarization of the interacting waves, are considered for media with an orientational optical nonlinearity. It is shown that the efficiency of energy exchange is maximal between the waves with mutually orthogonal circular polarizations. On the other hand, in the case of mutually orthogonal linear polarizations the transport of energy is in the "anomalous" direction, i.e., to the stronger wave.

Orientational optical nonlinearity of liquid crystals

The orientational optical nonlinearity of the mesophases of liquid crystals (LCs) is attracting ever more attention, owing to the high values of the constants characterizing it, owing to the many interesting specific features that LCs introduce even into traditional nonlinear optical effects, and owing to the potentialities of studying the physical properties of LCs. This article presents the state of the theory and experiment of orientational interaction of light waves with LCs. Especial attention is paid to the physical mechanisms of interaction?the light-induced Freedericksz effect, recording of director gratings, and the action of surface light waves on the orientation of the director. Varied manifestations of the effects of orientational self-action and interaction of light waves are discussed: self-focusing of light, nonlinear optical activity, mutual focusing, stimulated light scattering, and wave-front conjugation. Optical nonlinearities specific for the mesophase of a liquid crystal and associated with absorption of light quanta are also discussed. They include photoconformational nonlinearity, thermal-orientational nonlinearity, and liquid-crystal light valves. Especial attention is paid to the methodological problem of deriving the Euler?Lagrange?Rayleigh variational equations and to the correct choice of the free energy for the system LC + electromagnetic field. Section 8 traces the history of the studies of orientational optical nonlinearity of LCs and reviews the studies whose results are not directly reflected in the main text, and also reviews the studies on practical applications of light-induced orientational effects.

Effects of hydrodynamic motion on the establishment of orientational optical nonlinearity in a liquid crystal

Extensive research is on hand on orientational optical nonlinearity in the mesophase of nematic liquid crystal (NLC), but in all studies on the dynamics of orientation effects it has been usual to employ a dissipative function in the simple form R =0.5y1(~n/3t) 2, where n is the NLC director. This does not incorporate the effects of hydrodynamic motion in the NLC on the establishment of the optical nonlinearity, although the motion of the liquid itself may accompany the director rotation, as occurs for example in SB deformation [I] (see also review [2]), in which the effect is important and the viscosity YI should be replaced by some effective value y ef, which incorporates the effects of the inverse NLC flow, i.e., the changes in velocity V of the NLC during direct reorientation. For small SB deformations in static fields, the effective viscosity y1 ef takes the form [i] ef 2 ef o T t X y : G 3 / ~ b , Y:B = y:--a~/~qc, where the subscripts S and B relate correspondingly to the S and B effects, ~c and ~b are the Mesovich viscosity coefficients, y~ =~a -~2; and ui are the Lesley coefficients. Here we examine the effects of the reverse flow on the settling time for the giant optical nonlinearity and the light-induced Fredericks transition.

The Orientational Optical Nonlinearity of Planarly Aligned Long-Pitch Cholesterics

Abstract Optical nonlinearity of self-focusing type is investigated both theoretically and experimentally, which is due to reorientation of the cholesteric director by the light field, the effective nonlinear susceptability decreasing abruptly with the shortening of the pitch P.