The current status of organic low-molecular weight and polymeric materials for third-order nonlinear optics is reviewed. The importance of organic materials lies in their promise of large nonlinear optical figure of merit, high optical damage thresholds, ultrafast optical responses, architectural flexibility, and ease of fabrication. Organic materials exhibiting interesting third-order nonlinear optical properties are discussed to illustrate the importance of structure–property correlations. Results on emerging organic materials that include liquids, dyes, fullerenes, charge-transfer complexes, π-conjugated polymers, dye-grafted polymers, organometallic compounds, composites, and liquid crystals are presented. Organic nonlinear optical materials seem promising for a wide range of applications and their potential for integrated optics should be further explored.

Organic Materials for Third‐Order Nonlinear Optics

Liquid crystals are nowadays widely used in all types of display applications. However their unique electro-optic properties also make them a suitable material for nondisplay applications. We will focus on the use of liquid crystals in different photonic components: optical filters and switches, beam-steering devices, spatial light modulators, integrated devices based on optical waveguiding, lasers, and optical nonlinear components. Both the basic operating principles as well as the recent state-of-the art are discussed.

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Liquid-crystal photonic applications

A new organic stilbazolium derivative crystal 2-[2-(3-hydroxy-4-methoxy-phenyl)-vinyl]-1-methyl-pyridinium naphthalene-2-sulfonate dehydrate (C25H23NO5S·2H2O) (VSNS) was synthesized successfully. Single crystals were grown in a mixed solvent of methanol–acetonitrile (1 : 1) using a slow evaporation method at room temperature. Solubility of the synthesized VSNS material was experimentally determined for various temperatures using a methanol–acetonitrile mixed solvent. A single crystal X-ray diffraction study confirmed the crystal structure and morphology of VSNS. The crystalline nature of the title material was analyzed by powder X-ray diffraction analysis, and the presence of expected functional groups and the molecular structure of VSNS was identified by FT-IR and 1H NMR spectroscopic studies. Optical absorption was recorded using UV-Vis-NIR spectral analysis, and linear optical constants such as the absorption coefficient, band gap, extinction coefficient, refractive index and reflectance were calculated. The luminescence property of the crystal grown showed green emission radiation. The thermal stability of the crystal was analyzed by TG–DTA studies, and the hardness, Meyer index, yield strength, and elastic stiffness constant were estimated using a Vickers microhardness tester. Layer growth pattern was observed in chemical etching studies using a Carl Zeiss optical microscope at 50× magnification. Laser damage threshold energy was measured using an Nd:YAG laser (1064 nm). Variation of the dielectric response of the grown crystal was studied at room temperature. The third-order nonlinear optical property of VSNS was investigated in detail using a Z-scan technique with He–Ne laser at 632.8 nm. The second-order molecular hyperpolarizability γ of the crystal grown was 7.986 × 10−34 esu. This encouraging result of the Z-scan studies suggests that the VSNS crystal is a candidate material for photonics devices, optical switches, and optical power limiting applications.

Investigation of synthesis, crystal structure and third-order NLO properties of a new stilbazolium derivative crystal: a promising material for nonlinear optical devices

Optical imaging through turbid media is demonstrated using a degenerate four-wave mixing correlation time gate. An apparatus and method for detecting ballistic and/or snake light while rejecting unwanted diffusive light for imaging structures within highly scattering media are described. Degenerate four-wave mixing (DFWM) of a doubled YAG laser in rhodamine 590 is used to provide an ultrafast correlation time gate to discriminate against light that has undergone multiple scattering and therefore has lost memory of the structures within the scattering medium. Images have been obtained of a test cross-hair pattern through highly turbid suspensions of whole milk in water that are opaque to the naked eye, which demonstrates the utility of DFWM for imaging through turbid media. Use of DFWM as an ultrafast time gate for the detection of ballistic and/or snake light in optical mammography is discussed.

Optical imaging through turbid media with a degenerate four-wave mixing correlation time gate

This article contains recent progress and including the author's work in the field of solvent effects on the absorbance and fluorescence spectra. UV-Vis absorption or/and emission (fluorescence) band positions, intensity and shape of solvent-sensitive molecules show changes in organic solvents of varying polarities. A general overview of solvent effects on the electronic absorption and fluorescence spectra is presented. In order to describe the behavior in different solvents were selected the examples of compounds with varying architectures. The solvent effects on the intramolecular charge transfer bands are discussed using the possible correlations (with advantages and disadvantages) between adsorption/fluorescence maxima and various solvent parameters.

Solvent effects on the electronic absorption and fluorescence spectra

In this paper we report the dc Kerr constant and the pretransitional temperature of a polar nematic liquid crystal 4-(trans-4'-4'-n-pentylcyclohexy)-benzonitrile (PCH-5). The observed second-order pretransitional temperature ${\mathit{T}}^{\mathrm{*}}$ from our measurements is found to be 1.2 \ifmmode^\circ\else\textdegree\fi{}C below the first-order nematic to isotropic transition temperature. The dependence of the Kerr constant on (T-${\mathit{T}}^{\mathrm{*}}$${)}^{\mathrm{\ensuremath{-}}1}$ is found to be in good agreement with the predictions of the Landau--de Gennes model. The highest value of the Kerr constant observed at 632.8 nm is found to be 9.737\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ m ${\mathrm{V}}^{\mathrm{\ensuremath{-}}2}$, corresponding to a temperature of 55.3 \ifmmode^\circ\else\textdegree\fi{}C.

Kerr-effect investigations in a nematic liquid crystal.

Measurements are reported on the DC Kerr response and the pre-transitional behaviour of a negative nematic liquid crystal-p-ethoxy-benzylidine-p-butylaniline (EBBA)-at temperatures above the nematic-isotropic transition temperature. The observed second-order pre-transitional temperature T* from the measurements is found to be within 1 degrees C below the first-order nematic-isotropic transition temperature. The observed dependence of the Kerr constant on (T-T*)-1 is found to be in good agreement with the predictions of the Landau-de Gennes model. The pre-transitional temperatures have also been investigated in p-ethoxy benzylidine p-butyl aniline-p-methoxy benzylidene p-n-butyl aniline (EBBA-MBBA) liquid crystal mixtures.

https://iopscience.iop.org/article/10.1088/0022-3727/25/8/012/pdf

Transient Kerr response in a nematic liquid crystal

Electro-optic Kerr effect studies in liquids and binary liquid mixtures.

Measurements are reported for the d.c. Kerr response and the pretransitional behaviour of a new liquid crystal, 4-cyanophenyl 4-pentylbenzoate at and above the nematic-isotropic phase transition temperature. The real parts of the third order nonlinear susceptibilities chi(3) at and above the nematic-isotropic transition temperatures are determined for the new liquid crystal from the electro-optic Kerr effect (EOKE) experiments. The variation of the Kerr constant with temperature is discussed on the basis of the Landau-de Gennes model of the nematic-isotropic phase transition. The susceptibility values are also compared with that of the well known liquid crystal 4'-pentyl-4-cyanobiphenyl (5CB). The effect of the linking group of the liquid crystal on the suceptibility value is also discussed.

T. A. Prasada Rao

Papers

Kerr-effect investigations in a nematic liquid crystal.

Transient Kerr response in a nematic liquid crystal

Electro-optic Kerr effect studies in liquids and binary liquid mixtures.

Kerr effect investigations in a liquid crystal

Transient kerr effect in nematic liquid crystal mixtures