Second harmonic generation in inclusion complexes

Second harmonic generation (S.H.G.) in inclusion complexes between dimethyl -cyclodextrin (dimethyl -CD) and nitroaniline derivatives occurs. This is due to the destruction of the centrosymmtric crystal structure in nitroaniline derivatives caused by forming inclusion complexes. S.H.G. intensity of dimethyl -CD complex with N-methyl-nitroaniline is 5.5 times as large as that of urea.     

Second harmonic generation in polymers containing dithiosquarylium dye

Polymer layers containing the dye 2,4-bis(1,3,3-trimethyl-2-indolinylidenemethyl)-1,4-dithiosquaraine display second-order nonlinear optical properties. Depending on conditions of preparation of the layers, the monomer form of the dye or the intermolecular charge transfer complex between two dye molecules are responsible for the second harmonic generation. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 86–91, January, 1999.     

Damping by bulk and shear viscosity of confined acoustic phonons for nanostructures in aqueous solution

A nanoparticle in aqueous solution is modeled as a homogeneous elastic isotropic continuum sphere in contact with an infinite viscous compressible Newtonian fluid. The frequencies and damping of the confined vibrational modes of the sphere are calculated for the material parameters of a CdSe nanoparticle in water and a poly(methyl methacrylate) nanosphere in water. Although the effects of viscosity are found to be negligible for macroscopic objects, for nanoscale objects, both the frequency and damping of the vibrational modes are significantly affected by the viscosity of the liquid. Furthermore, both shear viscosity and bulk viscosity play an important role. A model of the spherical satellite tobacco mosaic virus consisting of outer solid layers with a water core is also investigated, and the viscosity of the water core is found to significantly damp the free vibrational modes. The same approach can be applied for nonspherical geometries and also to viscoelastic nanoparticles.     

Second harmonic generation in ferroelectric liquid crystals and their mixtures

The second harmonic generation (SHG) in the ferroelectric liquid crystal (FLC) state has been studied as functions of electric field strength, rotating angle, temperature and molecular structure. It has been confirmed that a sharp angularly phase-matching curve of the SHG controlled by an electric field is observed even in the liquid crystal. The temperature dependences of the phase-matched SHG and Maker fringe in the ferroelectric phase have also been studied, and temperature dependences of non-linear optical coefficients obtained. The SHG in several kinds of FLC and dye doped FLC have also been measured, and the enhancement of SHG realized by means of doping the FLC with several kinds of dye.     

Second harmonic generation in ferroelectric liquid crystals and their mixtures

Abstract

The second harmonic generation (SHG) in the ferroelectric liquid crystal (FLC) state has been studied as functions of electric field strength, rotating angle, temperature and molecular structure. It has been confirmed that a sharp angularly phase-matching curve of the SHG controlled by an electric field is observed even in the liquid crystal. The temperature dependences of the phase-matched SHG and Maker fringe in the ferroelectric phase have also been studied, and temperature dependences of non-linear optical coefficients obtained. The SHG in several kinds of FLC and dye doped FLC have also been measured, and the enhancement of SHG realized by means of doping the FLC with several kinds of dye.     

Second harmonic generation and polar order in thin films of polyesters

A series of optically active main chain polyesters were synthesized by high temperature polycondensation from biphenolic azo chromophores with azobenzene-4,4'-dicarbonylchloride and 1,4:3,6-dianhydro-D-sorbitol ([!]_D²⁵=+42.5°). The second harmonic generation of the polyesters is studied in thin films. The polar order in films is determined from the UV-visible absorption spectra. The variation in SHG intensity with chiral monomer concentration is explained as a function of polar order. The polymers with chiral units showed good second harmonic efficiency. The intensity of second harmonic light (532 nm) increased with angle of incidence and indicated the polar order in the plane of the film. The SH signal also showed good temporal stability.     

Second harmonic generation in planar samples of ferroelectric liquid crystals

Non-linear optical properties of ferroelectric liquid crystals are often studied through second harmonic generation measurements on homeotropic cells. However, it is not always possible to prepare well aligned unwound samples in the homeotropic geometry, especially when dealing with photopolymers or tightly twisted SmC* materials. Here we analyse the problem of extracting the second order susceptibility coefficients dij from second harmonic generation measurements performed on planar samples. Several experimental configurations of polarizer, sample and analyser dispositions are proposed to deduce the complete dij tensor together with the dispersion of the refractive indices and the birefringence both at the fundamental and second harmonic frequencies. Measurements carried out on planar and homeotropic cells for a classical non-linear optical material (W314) are presented and the resulting dij coefficients are compared.     

Nonlinear optical polymers. Second harmonic generation in corona-poled thin films

The synthesis and second harmonic coefficients, d_3,1 and d_3,3 as well as the related susceptibilities χ⁽²⁾_zzz of five series of (NLO-dye methacrylate)-(methyl methacrylate) copolymers were investigated. The NLO-chromophores bound covalently to the polymer backbone were 5-(2,2-dicyanovinyl)-or 4-(2-cyano-2-methoxycarbonyl)vinyl-1-piperidino-2-thiophene (P₁ and P₂), 4-nitro-4′-alkoxy-stilbene (P₃), 4-nitro-3′-methoxy-4′-alkoxystilbene (P₄) and 4-nitro-4′-alkoxy-α-cyano stilbene (P₅). The second order nonlinear optical properties of corona-poled aligned thin polymer films, using a needle electrode in order to induce noncentrosymmetry, were evaluated. Nonlinear susceptibilities, χ⁽²⁾_zzz, of the films were derived from the analysis of full-angle Maker fringe patterns at 1064 nm, χ⁽²⁾_zzz values as high as 1.98×10⁻⁷ esu for P₂ copolymers and of 1.19×10⁻⁷ esu for P₃ copolymers could be achieved.     

Second harmonic generation induced by charge-transfer complexes in a centrosymmetric medium

We consider the second harmonic generation (SHG) caused by impurities embedded in a nonlinear (χ⁽³⁾) optical medium which itself does not display χ⁽²⁾ optical response. In the case when the impurity molecules (charge-transfer (CT) complexes) possess an optical transition accompanied by charge transfer, we demonstrate that the excitation of impurities with a pump field may result in a contribution of the host medium to SHG. Depending on the presence or absence of the angular alignment of the excited static CT dipoles, there may be coherent SHG or weak incoherent SH scattering.     

Second harmonic generation and optical anisotropy of a spin-cast polymer film

The nonlinear susceptibilities (χ⁽²⁾₃₃, χ⁽²⁾₃₁), and the refractive index anisotropy, of a main chain accordion polymer, BCSC (see the text for the complete chemical formula), have been measured by the techniques of second harmonic generation (SHG) and waveguiding. A large birefringence, which is induced in the BCSC film by the spin-coating process, is almost removed by electric field poling. Our results demonstrate the fundamental difference between the birefringence associated with quadrupolar orientational order and the dipolar order associated with SHG. © 1993 John Wiley & Sons, Inc.     

Luminescence in colloidal Mn-doped semiconductor nanocrystals

Recent advances in nanocrystal doping chemistries have substantially broadened the variety of photophysical properties that can be observed in colloidal Mn²⁺-doped semiconductor nanocrystals. A brief overview is provided, focusing on Mn²⁺-doped II–VI semiconductor nanocrystals prepared by direct chemical synthesis and capped with coordinating surface ligands. These Mn²⁺-doped semiconductor nanocrystals are organized into three major groups according to the location of various Mn²⁺-related excited states relative to the energy gap of the host semiconductor nanocrystals. The positioning of these excited states gives rise to three distinct relaxation scenarios following photoexcitation. A brief outlook on future research directions is provided.     

Second harmonic generation in polymers containing a new azo chromophore based on phenylnitrobenzoxazole

The synthesis of the new chromophores 2‐{4‐[4‐(N,N‐dihydroxyethylamino)‐phenylazo]‐phenyl}‐6‐nitrobenzoxazole ( 1 ) and 4‐[4‐(N,N‐dihydroxyethylamino)phenylazo]‐benzoic acid ( 2 ) is described. Three homopolymers obtained by the polycondensation of 1 with propyloxy‐terephthalic acid and 1 and 2 with 2,4‐tolylenediisocianate as well as two copolymers obtained by polycondensation of appropriate mixtures of chromophores 1 and 2 with 2,4‐tolylene‐diisocianate were prepared and characterized. All as‐prepared polymers but one (homopolyurethane of 1 ) were amorphous and exhibited good optical clarity, good thermal stability above the glass transition, and solubility in organic solvents. The second‐order nonlinear optical behavior of the amorphous polymers was investigated on spin‐coated corona‐poled films through second harmonic generation (SHG) experiments at 1064 nm. At this wavelength, resonance‐enhanced values of the SHG coefficient d₃₃ were found in the range between 6 and 34 pm/V where higher values were observed for polymers with a higher molar content of 1 . © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1468–1475, 2002     

Quantum confinement in silver selenide semiconductor nanocrystals

We prepare Ag(2)Se nanocrystals with average diameters between 2.7 and 10.4 nm that exhibit narrow optical absorption features in the near to mid infrared. We demonstrate that these features are broadly tunable due to quantum confinement. They provide the longest wavelength absorption peaks (6.5 μm) yet reported for colloidal nanocrystals.     

Second harmonic generation measurements in aligned samples of liquid crystals composed of bent-core molecules

The second-order susceptibility tensor of three liquid crystals composed of banana-shaped molecules has been fully determined. The measurements were carried out in single domain samples, aligned by means of an electric field parallel to the cell surface. Special care was taken with the appropriate determination of the index mismatch between the fundamental and second harmonic waves. This was achieved by using samples of different thickness. This parameter is very important for obtaining reliable values of the second order susceptibility tensor components, and is seldom directly measured. The suitability of the experimental procedure was checked by comparing the results obtained for the reference compound P-8-O-PIMB with previously reported measurements for this material. The d_ij coefficients obtained in this work are in the range 1–4?pm?V^?1 and are compared with other values obtained in previous reports.     

Pump–probe second harmonic generation study of ultrafast spin dynamics in semiconductor multilayers

We report the first application of pump–probe second harmonic generation (SHG) measurements to characterize optically induced magnetization in non‐magnetic multilayer semiconductors. In the experiment, spin‐polarized electrons are excited selectively by a pump beam in the GaAs layer of GaAs/GaSb/InAs structures. However, the resulting net magnetization manifests itself through the induced SHG probe signal from the GaSb/InAs interface, thus indicating a spin‐polarized electron transport across the heterostructure. We find that the magnetization dynamics are governed by an interplay between the spin density evolution at the interfaces and the spin relaxation. Copyright © 2003 John Wiley & Sons, Ltd.     

Second harmonic generation measurements in aligned samples of liquid crystals composed of bent-core molecules

The second-order susceptibility tensor of three liquid crystals composed of banana-shaped molecules has been fully determined. The measurements were carried out in single domain samples, aligned by means of an electric field parallel to the cell surface. Special care was taken with the appropriate determination of the index mismatch between the fundamental and second harmonic waves. This was achieved by using samples of different thickness. This parameter is very important for obtaining reliable values of the second order susceptibility tensor components, and is seldom directly measured. The suitability of the experimental procedure was checked by comparing the results obtained for the reference compound P-8-O-PIMB with previously reported measurements for this material. The d_ij coefficients obtained in this work are in the range 1-4 pm V^-1 and are compared with other values obtained in previous reports.     

Nonlinear optics in quantum confined semiconductor particles

The field of nonlinear optics is dominated by the search for materials which show a large nonlinear optical response. Physical confinement of charge carriers, whether in 1, 2 or 3 dimensions, can greatly enhance the nonlinear optical behaviour. We have studied the nonlinear optical properties of small particles, in the size range 100-10Å which provide 3-D charge confinement. The larger particles are too big to show significant quantum confinement effects. Smaller particles show distinct confinement effects and a large optical nonlinearity.