V. Ostroverkhov

Electrically controlled surface diffraction gratings in nematic liquid crystals

Photorefractive diffraction gratings were studied in cells of homeotropically aligned pentyl-cyanobiphenyl liquid crystal. These holographic gratings were induced by the simultaneous and nonsimultaneous application of dc and coherent optical electric fields. The observed behavior was consistent with a predominantly surface-mediated photorefractive effect. Beam coupling was observed in all cases and led to a model involving screened and unscreened interfacial trapped charges driving a modulation of the easy axis. Holographic gratings could be switched on and off by the application of a small voltage.

["Λ-like chromophores for chiral non-linear optical materials"]

Abstract The first hyperpolarizability of several Λ -like chromophores including Malachite Green and Brilliant Green were measured by means of Kleinman-disallowed hyper-Rayleigh (harmonic light) scattering (KD-HRS). Such chromophores are of interest as components of a new class of chiral and axial macroscopic materials. Light scattering measurements were carried out in both the non-resonant and the anomalous dispersion regime in order to compare experimental results with a two-level model that indicates that B-symmetry excited states will contribute. Large hyperpolarizabilities were observed in all cases and evidence that the lowest-lying excited state has B-symmetry was found in some molecules.

Optimization of the molecular hyperpolarizability for second harmonic generation in chiral media

Molecular properties leading to second harmonic generation in chiral media in the electric dipole approximation for the cases of axial and biaxial symmetry are described. The components of the hyperpolarizability tensor that transform like a second-rank pseudotensor (La 2) and a third-rank tensor (La 3) contribute. The sum-over-states quantum formula for the hyperpolarizability is used to illuminate the molecular features necessary for optimizing the secondrank pseudotensor for dipolar molecules including orthogonal moments and high frequency. The example of C2v, appropriate for K-shaped molecules, is examined in more detail. Results of the measurements of these components in representative molecules using hyper-Rayleigh scattering are presented. Two compounds in which the delocalized pisystem is two-dimensional, a camphorquinone derivative and crystal violet are found to exhibit sizable La 2 and La 3 components. ” 2000 Elsevier Science B.V. All rights reserved.

Prospects for chiral nonlinear optical media

This paper describes the development and optimization of chiral, nonpolar media with large second-order nonlinear optical responses. We employ molecular engineering, quantum-mechanical sum-over-states theory, and measurements of molecular hyperpolarizability by means of Kleinman-disallowed hyper-Rayleigh scattering in order to understand molecular properties. Then we analyze the appropriate arrangement of the chromophores that produce an optimum axial nonlinear optical medium. Chromophores with large Kleinman disallowed traceless symmetric second-rank tensor hyperpolarizabilities /spl beta/ can be aligned so as to result in large susceptibilities /spl chi//sup (2)/ in structures that lack polar order. We found that /spl Lambda/-shaped chromophores with C/sub 2v/ or similar symmetry are good candidates for these materials, as they can exhibit large second-rank components of the hyperpolarizability tensor. A wide variety of techniques can be used to fabricate bulk materials belonging to the chiral nonpolar symmetry groups D/sub /spl infin// and D/sub 2/. The microscopic chromophore alignment schemes that optimize the nonlinear optical response in such materials are deduced from general symmetry consideration for both molecules and bulk. We also speculate on the possible application of such materials as high-bandwidth spatial light modulators.

Second-harmonic generation in nonpolar chiral materials: relationship between molecular andmacroscopic properties

An oriented gas model for the second-order nonlinear optical response for optical second-harmonic generation in axially aligned nonlinear optical chromophores in chiral media is presented. Design criteria for the alignment of chromophores possessing Kleinman-disallowed traceless symmetric second-rank tensor hyperpolarizabilities β resulting in large nonlinear optical susceptibilities X(2) are enumerated. These chromophores could be oriented in high-density, highly ordered structures due to the absence of dipolar interactions resulting in exceptionally large X(2) values. The alignment of chromophores that have C2v or Dn (n greater than 2) symmetry in two chiral nonpolar symmetry groups for the medium, D∞ and D2, are considered. Criteria for response optimization and several physical examples are described. Additionally, certain nonpolar symmetry groups for the medium, of which the most easily realizable is probably D2, also admit Kleinman-allowed octupolar susceptibilities for which the optimum chromophore alignment is discussed.

Optimization of /spl Lambda/-shaped molecules for second harmonic generation in media with nonpolar alignment

Summary form only given. Considerable effort has been directed towards creating organic nonlinear materials for second harmonic generation electro-optic control that would implement the non-vector irreducible components of the first hyperpolarizability tensor. In particular, the second-rank tensor component can give rise to macroscopic nonlinearity /spl chi//sup (2)/ in systems having chiral nonpolar symmetry of the bulk, such as D/sub /spl infin// or D/sub 2/. These types of alignment can be achieved in uniaxially or biaxially stretched chiral polymers, or in various uniaxial (N, SmA) and biaxial (SmC) liquid crystal phases. To make an efficient frequency conversion material, one has to optimize both the molecular response of the active chromophore and the macroscopic alignment scheme.

Optimization of the nonlinear optical response in chiral media

This paper describes the development and optimization of chiral, non-polar media with large second-order nonlinear optical responses. We employ molecular engineering, quantum- mechanical sum-over-states theory, and measurements of molecular hyperpolarizability by means of Kleinman-disallowed hyper-Rayleigh scattering in order to understand molecular properties. Then we analyze the appropriate arrangement of the chromophores that produce an optimum axial nonlinear optical medium. Chromophores with large Kleinman disallowed traceless symmetric second rank tensor hyper-polarizabilities (beta) can be aligned so as to result in large susceptibilities, (chi) (2), in structures that lack polar order. We found that (Lambda) -shaped chromophores with C2v or similar symmetry are good candidates for these materials as they can exhibit large second-rank components of the hyperpolarizability tensor. A wide variety of techniques can be used to fabricate bulk materials belonging to the chiral non-polar symmetry groups, D(infinity ) and D2. The microscopic chromophore alignment schemes that optimize the NLO response in such materials are deduced from general symmetry consideration for both molecules and bulk.

Quadrupolar molecular nonlinear optics

Summary form only given. Measurement and exploitation of the various rotational invariants of the first hyperpolarisability tensor have been the subject of recent study. In particular, the quadrupolar and octupolar invariants may be of interest as new approaches to attaining noncentrosymmetric materials for application in second order nonlinear optics. In this work, we will examine the structure-property relationships for the J=2, or quadrupolar rotational invariant.