Phase-matched second-harmonic generation in poled polymer waveguide based on a main chain polymer

The electric field-induced dynamic phase-matching of second harmonic generation (SHG) waveguide was demonstrated by using a main chain polyarylamine. The linear and nonlinear optical properties of this polymer are presented. The optimum phase-matching thickness was controlled by applying an electric field to the polymer waveguide. The effective phase-matching thickness variation induced by poling is about 0.025 μm that is six times larger than full width at the half-maximum (FWHM) of phase-matching thickness in conventional slab waveguide. The efficient phase-matched SHG was observed from a taperless slab wave-guide. The optical loss of poled polymer on glass substrate at 632.8 nm was 2.7 dB/cm. © 1995 John Wiley & Sons, Inc.     

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.     

适合倍频光波导的多层LB膜结构研究

制备非中心对称的NMOB/CdA交替Y型累积LB膜,测量了它的二阶非线性光学特性.为了改善多层膜的有序性,研究在拉制NMOB/CdA多层膜时提高膜均匀性的两种方法,增强了它的二阶非线性光学特性.最后,制备光栅耦合的NMOB/CdA多层膜光波导,测量了它的Cerenkov相位匹配光学二次谐波产生.     

Acoustic second harmonic generation from rough surfaces under shear excitation in liquids

Emission of compressional acoustic waves at the second harmonic frequency (second harmonic generation, SHG) is possible from rough surfaces undergoing oscillatory shear in liquids. This nonlinear response is a consequence of the inertial term in the Navier-Stokes equation. On a corrugated surface, the streamlines of the sheared liquid are not strictly parallel to the surface, leading to variation of pressure along the streamlines and a concomitant Bernoulli pressure. Being quadratic in speed, the Bernoulli pressure contains a static term and a term at the second harmonic frequency, 2omega. Pressure fluctuations at 2omega generate compressional waves.     

半花菁LB多层膜的光学非线性各向异性研究

用偏振紫外-可见吸收及旋转样品二次谐波（SHG）测量法研究了半花菁／花生酸Y型交替多层膜中分子在基极平面内分布的各向异性．拉膜过程引发的半花菁分子在平面内沿提拉方向的规则取向具有镜面对称性，并随着拉膜速度或层数的增大而增强．     

The First-principles Calculations of the Electronic Structures and Optical Properties of Ⅱ-Ⅲ_2-Ⅵ_4(Ⅱ = Zn,Cd; Ⅲ = In; Ⅵ = Se,Te)

The electronic structures and optical properties of II?III2?VI4(II = Zn, Cd; III = In; VI = Se, Te) compounds are studied by the density functional theory(DFT) using the Vienna ab initio simulation package(VASP). Geometrical optimization of the unit cell is in good agreement with the experimental data. Our calculations show that the valence band maximum(VBM) and conduction band minimum(CBM) are located at G resulting in a direct energy gap. The optical properties are analyzed, and the independent second harmonic generation(SHG) coefficients are determined. By an analysis of the band structure, we can get that SHG response of the system can be attributed to the transitions from the bands near the top of valence band that are derived from the Se/Te p states to the unoccupied bands contributed by the p states of In atoms.     

An in situ second harmonic generation study of the electrochemical oxidation of silicon in fluoride media

The electrochemical oxidation of p-type Si in fluoride solutions has been studied by in situ second harmonic generation (SHG) with the SHG signal being recorded simultaneously with the cyclic voltammogram. The SHG signal is shown to change in tandem with the electrochemical response enabling the identification of transition points between different surface conditions such as hydrogen-terminated, hydrated oxide and oxide. Interpretation of the changes in SHG suggests that the initial response is due to the electric field-induced second harmonic (EFISH) due to the electric field gradient at the interface. It then becomes dominated by the variation in the resonant surface non-linear susceptibility as the changes in local bonding affect the response. SHG signals display a much greater sensitivity to surface conditions than the voltammetric response and allow the real-time identification of the potentials at which significant changes take place in the state of the surface.     

Orientation Angle of Molecules at Hexadecane-Water Interface Studied with Total Internal Reflection Second Harmonic Generation

The orientation angle is an important parameter that reflects the structure of molecules at interfaces. In order to obtain this parameter, second order nonlinear spectroscopic techniques including second harmonic generation (SHG) and sum frequency generation-vibrational spectroscopy (SFG-VS) have been successfully applied through analysis of the nonlinear signal from various polarizations. In some SHG and SFG-VS experiments, total internal reflection (TIR) configuration has been adopted to get enhanced signals. However, the reports on the detailed procedure of the polarization analysis and the calculation of the orientation angle of interfacial molecules under TIR configuration are still very few. In this paper, we measured the orientation angles of two molecules at the hexadecane-water interface under TIR and Non-TIR experimental configurations. The results measured from polarization analysis in TIR configuration consist with those obtained from Non-TIR configuration. This work demonstrates the feasibility and accuracy of polarization analysis in the determination of the orientation angle of molecules at the interfaces under TIR-SHG configuration.     

BaAl4Se7: a new infrared nonlinear optical material with a large band gap

The new compound BaAl(4)Se(7) has been synthesized by solid-state reaction. It crystallizes in the non-centrosymmetric space group Pc and adopts a three-dimensional framework built from AlSe(4) tetrahedra and with Ba(2+) cations in the cavities. The material has a large band gap of 3.40(2) eV. It melts congruently at 901 °C and exhibits a second harmonic generation (SHG) response at 1 μm that is about half that of AgGaS(2). From a band structure calculation, BaAl(4)Se(7) is a direct-gap semiconductor with strong hybridization of the Al 3s, Al 3p, and Se 4p orbitals near the Fermi level. The calculated birefractive index is about 0.05 for wavelength longer than 1 μm and major SHG tensor elements are: d(15) = 5.2 pm V(-1) and d(13) = 4.2 pm V(-1).     

A second harmonic generation study of the oxidation of copper electrodes

The oxidation of copper in basic media has been studied by in situ second harmonic generation (SHG), where the SHG signal was recorded alongside the cyclic voltammogram. The SHG signal changes markedly as the copper surface is oxidised to first Cu₂O and then CuO in a duplex structure. The development of Cu₂O gives rise to a resonant SHG signal because of the band-gap of the material then the upper CuO layer produces an electric-field induced second harmonic (EFISH) response. A correlation of charge with the SHG signal is informative with regard to the mechanism of reduction of CuO and SHG is shown to be a useful method for the examination of oxidation of electrode surfaces.     

Second harmonic generation phase measurements of Cr(VI) at a buried interface

Surface second harmonic generation (SHG) phase measurements are carried out on methyl ester-functionalized fused quartz/water interfaces in the presence and absence of Cr(VI). The experiments are performed at pH 7, room temperature, and a chromate concentration of 10(-4) M, which corresponds to monolayer Cr(VI) coverage. The liquid/solid interface is probed from the fused quartz side by directing the probe light field at 580 nm onto the interface together with an SHG reference signal at 290 nm that is collinear with the fundamental. The phase difference of the SHG signals generated at the interface in the presence and absence of Cr(VI) is 85 degrees, which is consistent with SHG resonance enhancement observed for the surface-bound Cr(VI) near 290 nm. The optical arrangement discussed here does not require vacuum technology or optics that compensate for the dispersion of the fundamental and the second harmonic E-fields in the two condensed-phase media. This approach is general and can be applied for analyzing thermodynamic and kinetic data derived from SHG measurements of physical and chemical processes occurring at any buried interface.     

Mechanism of linear and nonlinear optical effects of chalcopyrite AgGaX2 (X=S, Se, and Te) crystals

The electronic band structures for AgGaX(2) (X=S, Se, Te) chalcopyrites have been calculated using a pseudopotential total energy method. First-principles calculations of the linear and nonlinear optical properties are presented for these crystals, with the electronic band structures obtained from pseudopotential method as input. The theoretical refractive indices and nonlinear optical coefficients are in good agreement with available experimental values. The origin of the nonlinear optical effects is explained through real-space atom-cutting analysis. The contribution of the GaX(2) group (X=S, Se, Te) for second harmonic generation (SHG) effect is dominant while that of the cation Ag is negligible. In addition, the percentage contribution to the SHG coefficients from the different bonds increase with increase of the bond order.     

用激光第二谐振光研究阴离子在多晶铜电极表面上的吸附

首次用激光产生的第二谐振光(SHG)检测到金属/水溶液界面上阴离子在多晶铜电极表面上的吸附,阴离子吸附特性对SHG强度影响明显,由多晶铜电极在(0.5-x)mol/L NaClO_4+xmol/L NaBr溶液中的SHG强度-电位曲线表明铜电极表面对ClO_4~-的吸附非常弱,对Br~-有特定的吸附,SHG强度随Br~-浓度增加而增强,结果表明SHG是定量研究电化学界面区吸附特性的灵敏有效的探针,可揭示金属与吸附质间相互作用的本质。     

酞菁铜/金属薄膜界面电位与光二次谐波特性分析

Using the Kelvin probe and the optical second harmonic generation (SHG) measurement, the Space charged phenomenon and the non-linear optical effects at the interfaces of the coppertetra-tert-butyl phthalocyanine (CuttbPc) Langmuir-Blodgett (LB) film deposited on a metal (AlorAu) coated glass slide substrate were in vestigated. The surface potentials decreaseas the film thickness increases and eventually approaches a saturated value. The SHG has been detected although there is a centro-symmetric systemin the Cuttb Pcmolecular, and avery strong SH signal can be investigated at 1260 nm band for Cuttb Pc/Al samples. According to a proposed physical model for Cuttb PcLB film/metal, the nonlinear mechanism were analyzed by using electromagnetic wave theory. It is considered that the enhanced SH peak of CuttbPc/Al is attributed to the strong surface potential aroused by SCIEF at the interface, and shows that the production of SH signal is correlated closely with the electrostatic phenomena at the interface.     

Surface enhanced second harmonic generation from macrocycle, catenane, and rotaxane thin films: experiments and theory

Surface enhanced second harmonic generation (SE SHG) experiments on molecular structures, macrocycles, catenanes, and rotaxanes, deposited as monolayers and multilayers by vacuum sublimation on silver, are reported. The measurements show that the molecules form ordered thin films, where the highest degree of order is observed in the case of macrocycle monolayers and the lowest in the case of rotaxane multilayers. The second harmonic generation activity is interpreted in terms of electric field induced second harmonic (EFISH) generation where the electric field is created by the substrate silver atoms. The measured second order nonlinear optical susceptibility for a rotaxane thin film is compared with that obtained by considering only EFISH contribution to SHG intensity. The electric field on the surface of a silver layer is calculated by using the Delphi4 program for structures obtained with TINKER molecular mechanics/dynamics simulations. An excellent agreement is observed between the calculated and the measured SHG susceptibilities.     

有机晶体材料DAST和DSTMS的电子结构与光学性质

采用基于赝势平面波基组的密度泛函理论(DFT)方法,研究了有机晶体材料4-(4-二甲氨基苯乙烯基)甲基吡啶对甲苯磺酸盐(DAST)和4-(4-二甲氨基苯乙烯基)甲基吡啶2,4,6-三甲基苯磺酸盐(DSTMS)的电子结构和光学性质.结果表明,两种化合物具有相似的能带结构,其中价带顶和导带底分别含有较多二甲氨基和甲基吡啶中N原子的2p轨道成分.在线性光学性质方面,两种化合物具有较高的双折射率(Δn>0.5),在中远红外区均具有较好的透过性能.在二阶非线性光学性质方面,该类晶体具有较强的二阶倍频(SHG)效应,静态倍频系数d11约为150 pm V-1.由能带结构分析结果可知,体系的SHG效应与推拉电子基团之间的电荷转移密切相关,同时乙烯桥键在该电子转移过程中也起着重要作用.     

Electrical field induced phase-matched second harmonic generation in low Tg polymer waveguides

Second-harmonic generation in the guided waveguide configuration is very attractive because a high fundamental power density can be coupled over long propagation length therefore remarkably high conversion efficiencies can be expected compared to bulk materials.¹ Organic SHG devices in optical waveguides have not been developed extensively because of the difficulty encountered in phase-matching. To avoid this problem, the use of an artificial periodic structure, Cerenkov radiation, and non-colinear light path geometry have already been demonstrated. Recently, we reported an electric field-induced dynamic phase-matching in a guided wave configuration using a main-chain polymer in which the effective phase-matching thickness can be controlled by an applied electric field.² This technique is able to increase the waveguide dimension tolerances of phase-matching condition. In the case of a main-chain polymer, the thermal optic effects due to the heating prevent to satisfy the optimum phase-matching conditions, which causes a reduction in the conversion efficiency of devices. In order to overcome this problem, we have synthesized novel low glass transition temperature (Tg) nonlinear optical (NLO) polymers. In this presentation, we will discuss an electric field-induced dynamic phase-matching of a multilayer waveguide at room temperature using a low Tg NLO polymer which can increase both the waveguide dimension tolerances and overlap integral. Using this technique, efficient phase-matched SHG was generated from p-nitroaniline grafted NLO materials. The dimension tolerance of waveguides under an electric field will be described.     

Optical second harmonic generation in ferroelectric liquid crystals under oblique incidence

The second harmonic generation (SHG) in ferroelectric liquid crystals under oblique incidence was studied by using a numerical band matrix method that uses boundary conditions directly. No approximations were used except for the nondepleted fundamental wave. By matching the edge of the full-pitch band with that of the half-pitch band, the SH intensity increased significantly. The thickness (L) dependence of the SH intensity showed L ^6.93–L ^7.09 dependence without the limitation of the thickness.     

Significant enhancement of the optical second harmonic generation in a poled azopolymer thin grating

Nonlinear optical properties of an electrically poled surface relief grating inscribed on a thin film of azopolymer have been investigated. The linear and nonlinear optical far-field diffraction patterns of the grating are compared, and they show a clear angular separation of the fundamental (lambda = 1.064 mum) and second harmonic generated beams that are diffracted at different angles. The intensity of the zero order transmitted second harmonic generation (SHG) beam from inscribed surface relief grating (SRG) areas has been recorded using the Maker fringe technique and compared to the response from flat areas that were only poled. Poled gratings exhibit a sharp second harmonic generation enhancement for coupling angles of theta(i) = +/-52 degrees due to a quasi-phase matching process. A simple phenomenological model allows one to explain both the second harmonic intensity generated from the thin polar film and the dispersion curve observed under "pp" polarization due to the mismatch of the angular coupling condition: this gives us a good indication of the intensity enhancement of the SHG beam in the forward direction.