Optical Second Harmonic Generation Measurements Research Articles

Compression-shear-induced tilt azimuthal orientation of amphiphilic monolayers at the air-water interface: a C(infinity)-->C2v transition in the flow of a two-dimensional hexatic structure.

Compression-shear-induced tilt azimuthal orientation of amphiphilic monolayer in tilting phases (L2 and L'2) at the air-water interface is analyzed as dynamical equilibrium of the elastic distortion of orthogonally hexagonal structure of the molecules under compression-induced shear flow. It is shown theoretically that the compression can induce molecular tilts lying along and/or against the flow direction. All these tilts makeup the initial random tilt azimuth of the molecular tails along a uniform direction. At a threshold compression speed, it causes a C(infinity)-->C(2v)-symmetry transition at the air-water interface. With Maxwell displacement current and optical second-harmonic generation measurements, the above theoretical results are verified experimentally in a monolayer of 4-heptyloxy-4(')-cyanobiphenyl.

Structural Features of a Clay Film Hybridized with a Zwitterionic Molecule as Analyzed by Second-Harmonic Generation Behavior

AbstractHybrid Langmuir-Blodgett (LB) films of a single clay layer and a donor-(π-electron system)-acceptor (Dδ-π-Aδ−) zwitterionic molecule (ind-TCNQ), prepared from 1,3,3-trimethyl-2-methyleneindoline (donor) and 7,7,8,8-tetracyanoquinodimethane (acceptor) were prepared. The characteristics of the electrostatic interaction between the clay layer and the ind-TCNQ molecules were investigated. The amount of ind-TCNQ molecules spread onto the air-water interface was increased systematically so that mono- and multi-layers of the molecules could be formed on the clay layer. The hybrid LB films, which were deposited onto a solid substrate, were characterized by means of UV-vis absorption spectroscopy and optical second-harmonic generation measurement. The amount of ind-TCNQ molecules adsorbed on the clay layer increased with an increase in the amount of ind-TCNQ molecules spread onto the air-water interface. On the other hand, the SHG intensities of the films did not change with the change in amount of ind-TCNQ spread. The results indicate that the electrostatic interaction between the clay layer and the ind-TCNQ molecules operates only in the region where the ind-TCNQ molecules are in direct contact with the clay layer.

Synthesized rare-earth doped oxide glasses for nonlinear optics

Photostimulated nonlinear optical effects in synthesized PbO–Ga2O3–Bi2O3–CdO (PGBC) glasses doped by Yb3+, Er3+, and Dy3+ rare-earth (RE3+) ions were discovered. Temperature-dependent measurements of optical photoinduced second-harmonic generation (PISHG) and two-photon absorption were performed in the infrared (IR) range. CO pulse laser (λ=5.5 μm, energy power density up to 3.8 GW/cm2 per pulse) was applied as a source of IR-photoinducing and probing (fundamental) light. Absolute values of the PISHG were more than 22% higher than corresponding values obtained for other glasses: As2Te3–CaCl2–PbCl2 [1] or Sb2Se3–BaCl2–PbCl2 [2] type. The investigated PGBC system possesses a shorter time response (about 18 ps), compared with other IR nonlinear optical glasses. We have also established that all nonlinear optical susceptibilities are dependent on the type of RE3+ ion. A maximal value of the PISHG is achieved for the glasses doped by Yb3+. The PISHG values increase significantly below 25 K. We have carried out ab initio molecular dynamics and quantum chemical simulations in order to evaluate the possible contribution of electron–phonon anharmonic interactions in the observed phenomena. We have modeled the influence of the external CO photoinduced beam through the photoinduced anharmonic electron-phonon interactions. A decrease of the delaying time response is achieved. To obtain independent confirmation of the observed dependencies, we have carried out measurements of the ε2(E) during the external IR pumping. We have also compared the measured and theoretically calculated dependencies of the IR-induced effects.

Surface Versus Bulk Anisotropy of Photo-Sensitive Poly(vinyl cinnamate) Alignmet Layers

Reaction kinetics and orientational anisotropy of poly(vinyl cinnamate) (PVCN) alignment layers photo-co-polymerized with linearly polarized UV light were probed by measurements of linear dichroism (LD) and surface optical second-harmonic generation (SSHG). The results show that the bulk and the surface properties of PVCN films are very similar and suggest that in PVCN there are no specific surface photochemical processes, which could presumably reduce the anchoring of liquid crystals on PVCN substrates. The azimuthal surface anchoring energy coefficient of nematic liquid crystal 5CB in contact with our substrates was of the order of 10−6 J/m 2 and strongly depended on the procedure used to measure it. This can be explained by a presence of various memory effects on the LC/PVCN interface, which strongly affect the alignment ability of the PVCN layers.

Ferroelectric phases in a chiral bent-core smectic liquid crystal: dielectric and optical second-harmonic generation measurements

We report unambiguous evidence for ferroelectricity in a chiral bent-core liquid crystal. This evidence includes (i) a single current peak in the polarization reversal current, (ii) a strong dielectric response that is suppressed by applying a bias electric field, and (iii) strong optical second-harmonic generation in the absence of electric field even at normal incidence. We propose a phase structure that is anticlinic in tilt and ferroelectric in polar order, i.e., smectic-C(A)P(*)(F).

Interaction and diffusion of potassium onCr2O3(0001)/Cr(110)

The interaction of potassium atoms on top of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}(0001)/\mathrm{C}\mathrm{r}(110)$ has been studied using work-function (\ensuremath{\Delta}\ensuremath{\Phi}), temperature programmed desorption (TPD), and optical second-harmonic generation (SHG) measurements. Potassium grows via the completion of a first layer, followed by a second layer in the form of two-dimensional (2D) islands, and at higher coverage 3D clusters are formed. This growth model is supported by and consistent with the results obtained from all three methods. Work-function data suggest that annealing at temperatures above 350 K results in the formation of a surface potassium oxide compound, provided the potassium coverage is higher than 0.5 monolayers (ML). Diffusion of alkali-metal atoms on an oxide surface is reported here over distances of several micrometers. This was measured using optical SH diffraction from coverage gratings that were generated by laser-induced thermal desorption. The activation energy for surface diffusion of potassium on ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}(0001)/\mathrm{C}\mathrm{r}(110)$ has been determined to be $11\ifmmode\pm\else\textpm\fi{}0.5\mathrm{k}\mathrm{c}\mathrm{a}\mathrm{l}/\mathrm{m}\mathrm{o}\mathrm{l}$ with a preexponential factor ${D}_{0}{=10}^{5}{\mathrm{cm}}^{2}/\mathrm{s}\mathrm{e}\mathrm{c}$ in the coverage range of 1.5--2.5 ML, dropping to 9 kcal/mol and ${D}_{0}=3\ifmmode\times\else\texttimes\fi{}{10}^{3}{\mathrm{cm}}^{2}/\mathrm{s}\mathrm{e}\mathrm{c}$ at a coverage of 3.0 ML. These results are consistent with the diffusion of atoms in the third layer, on top of two-dimensional potassium islands in the second layer, the activation energy represent the barrier for descending from the 2D islands.

Characterization of unidirectional photopolymerization in poly(vinyl cinnamate) by surface optical second-harmonic generation.

Surface optical second-harmonic generation measurements were used to determine the change in the surface density and the surface orientational order parameter of the trans-cinnamoyl side groups of unidirectionally photopolymerized poly(vinyl cinnamate) (PVCN) films. The observed surface features were compared to the bulk properties of the films, which were probed by their UV absorption spectra and birefringence measurements. We found that the surface and the bulk photoreaction kinetics of the trans cinnamoyl side groups of the PVCN are very similar. The two photoreaction processes (trans-cis photoisomerization and photo-induced cross linking) appear to affect the linear and nonlinear optical properties of the polymer differently. This may explain the discrepancy between the birefringence and the azimuthal anchoring force of the unidirectionally photopolymerized PVCN.

In-Situ Second-Harmonic Generation and Luminescence Measurements for Structural Characterization of Ruthenium−Polypyridine Complex Monolayers with Two and Four Aliphatic Tails at the Air/Water Interface

Langmuir monolayer assemblies of ruthenium−polypyridine complexes having two and four aliphatic tails were studied by in-situ optical second-harmonic generation (SHG) and luminescence measurements, together with surface pressure−molecular area (π−Α) isotherms. As the monolayer was compressed, the four-tailed complex showed a distinct phase transition at a 30−40 mN/m region from phase I (1.3 nm2 mean molecular area) to phase II (1.1 nm2). Excitation with 1064-nm light gave an SHG signal at 532 nm that appeared almost synchronously with the increase of surface pressure, but the p-polarized component of the SHG signal, induced by either p- (parallel to the plane of incidence) or s- (perpendicular to the plane of incidence) polarized fundamental light, showed a different profile upon compression of the monolayer. In the phase II region, the p-polarized component increased steeply, whereas the s-polarized component decreased to almost zero. A significant blue-shift and a longer lifetime of the luminescence sig...

Photostimulated optical SHG in BEDO‐TTF polyiodide/polycarbonate composites

Measurements of photoinduced optical second-harmonic generation (PISHG) for the bis(ethylenedioxy)tetrathiofulvalene (BEDO-TTF)/Iodine complex in a polycarbonate matrix were performed. Two kinds of samples were studied: as-obtained, grey-violet material, showing a metal-like temperature dependence of conductivity down to liquid helium temperature (system 1), and electrochemically decoloured, transparent (slightly grey) material, showing metallic conductivity down to about 30 K and semiconductive properties below this temperature (system 2). An increase in the PISHG signal with decreasing temperature was observed below 10 K for both specimens. In the temperature range 40-88 K, an increase then a decrease in PISHG with increasing temperature was observed for specimen 2, while practically no PISHG signal was recorded for sample 1. The dependences obtained indicate unambiguously the essential influence of the BEDO-TTF(l) dopant on the PISHG of polycarbonate composites. Simultaneously, competition was demonstrated between the optical second- and third-harmonic signals at photoinducing fluxes near 1015 photons cm−2. A strong increase in the PISHG signal at low temperatures was observed for both systems (below 11 K for sample 2 and below 7 K for sample 1). In the temperature range 40-88 K a maximum of the PISHG was observed only for specimen 2. At higher temperatures the intensity of the PISHG was very low in both kinds of samples.

Optical second-harmonic generation measurements of molecular adsorption and orientation at the liquid/liquid electrochemical interface

The surface-sensitive spectroscopic technique of optical second-harmonic generation (SHG) is applied to the in situ study of molecular adsorption at the interface between two immiscible electrolyte solutions (ITIES). The resonant SHG form molecules which exhibit a large non-linear optical response at a specific wavelength can be used to measure the relative surface coverage of surfactants at the ITIES as a function of the external electro-chemical parameters. In addition, the polarization dependence of the resonant surface SHG can be used to estimate the average molecular orientation of adsorbates at the liquid/liquid electrochemical interface. As an example, the adsorption of the surfactant 4-(4′-dodecyloxyazobenzene)benzoic acid at the water/1,2-dichloroethane interface is characterized as a function of applied potential, surfactant concentration and aqueous pH with in situ resonant molecular SHG measurements. An analysis of the SHG data results in the determination of the local potential and surface pH experienced by the surfactant.

Nonlinear optical properties of sublimed C 60 thin films

Results of optical second-harmonic generation and wave-dispersed third-harmonic generation measurements in C 60 thin films are reported and discussed. In third-harmonic generation measurements two strong resonances in cubic susceptibility χ (3)(−3ω; ω, ω, ω) are observed. The first one occurring at 1.3 μm fundamental wavelength is interpreted in terms of a three-photon resonance with vibronic components of gerade states made partially allowed by vibronic mixing with the higher T 1u state. The second resonance at 1.064 μm is interpreted as a three-photon resonance with the T 1u state. The measured resonant values of χ (3)(−3ω; ω, ω, ω) are 6.1 × 10 −11 and 8.2 × 10 −11 e.s.u. at 1.3 and 1.064 μm, respectively. The second-harmonic generation measurements performed at 1.064 μm on native thin films lead to a significant second-order susceptibility d pp(−2 ω; ω, ω) = 3.8 × 10 −9 e.s.u. (1.6 pm/V).

Accurate cw Measurements of Optical Second-Harmonic Generation in Ammonium Dihydrogen Phosphate and Calcite

Optimally focused cw gas-laser beams were used to make an accurate absolute measurement of optical second-harmonic generation (SHG) in ammonium dihydrogen phosphate (ADP) and accurate relative measurements of the higher order quadrupole-type SHG and electric-field-induced SHG in calcite. The result of the first set of experiments was ${d}_{36}(\mathrm{ADP})=1.38\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9} \mathrm{esu}\ifmmode\pm\else\textpm\fi{}16%$, in excellent agreement with the value determined by Francois using unfocused beams. This agreement demonstrates that focused laser beams can be used to make accurate measurements of crystal nonlinearities, and it provides further evidence that the value of ${d}_{36}$ in potassium dihydrogen phosphate (KDP), normally taken as the standard nonlinearity, is considerably smaller than the value usually quoted. The nonlinearity describing quadrupole-type SHG in calcite was found to be $2.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{d}_{36}(\mathrm{ADP})\ifmmode\pm\else\textpm\fi{}14%$, and that for electric-field-induced SHG was $2.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}{d}_{36}(\mathrm{ADP})\ifmmode\pm\else\textpm\fi{}24%$. The second-harmonic power from calcite could not have been measured without the enhancements available from focusing. A displacement of the parabolic curve of second-harmonic power as a function of the applied electric field was noted, and it was shown that this shift was not caused by inhomogeneous applied electric fields. The very small induced birefringence in calcite which depends quadratically upon the applied electric field was also measured.

Measurement of optical second-harmonic generation in centrosymmetric and noncentrosymmetric crystals using focused Gaussian laser beams

Measurement of optical second-harmonic generation in centrosymmetric and noncentrosymmetric crystals using focused Gaussian laser beams