Second harmonic generation applying injection-locked radiation from a high-power,broad-stripe laser diode

406 nm laser light is produced by second harmonic generation in a LiIO₃ crystal using the fundamental radiation of an injection-locked, broad-stripe, high-power laser diode. The efficiencies of injection locking as well as of second harmonic generation are discussed.     

Theory for nonlinear Mie-scattering from spherical metal clusters

Using classical electrodynamics we determine the angular dependence of the light intensities radiated in second and third harmonic generation by spherical metal clusters. Forward and backward scattering is analyzed in detail. Also resonance effects in the integrated intensities are studied. By treating intermediate cluster sizes, we complete the scattering theory for spherical clusters between Rayleigh-type analysis for small spheres and geometrical optics for spheres much larger than the wavelength for nonlinear optics. The particle size sensitivity of Mie-scattering is increased by nonlinearity. The results suggest that small particle sizes can be extracted more easily from nonlinear optics.     

Characterization of the Chromophore Orientation of Rhodamine B Amphiphiles in Langmuir-Blodgett Monolayers

SHG (second harmonic generation) interferometry and linear dichroism measurements were applied to the characterization of Langmuir-Blodgett monolayers composed of rhodamine B derivatives on hydrophilic fused silica substrates. When excited with a Nd-YAG laser, the contribution of dimeric species composed of rhodamine B chromophores to the nonlinear optical behavior was large, even though the relative concentration of the dimers was small. This implies the formation of noncentrosymmetrically oriented dimers with a relatively higher nonlinear molecular susceptibility, beta. Therefore, a unique phenomenon was observed; that is, the relative phase of the SH light shifted depending on the incident light polarization (p and s). In addition, it was found that the relatively standing up orientation was favored for the dimer, while a strongly tilted orientation was favored for the monomer. Copyright 2001 Academic Press.     

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。     

Communication: Reactions and adsorption at the surface of silver nanoparticles probed by second harmonic generation

Even though nanoparticles have dimensions much smaller than the optical wavelength and shapes commonly with inversion symmetry, we show, for the first time, direct experimental evidence that second harmonic generation (SHG) can be detected from the surface layer of metallic nanoparticles, in this case 40 nm radius Ag particles. The SH intensity detected is shown to substantially decrease upon chemical bonding of thiol molecules to the Ag particle surface. The surface generated SH intensity can be used for probing properties and processes at the nanoparticle surface.     

Vibrational effects on the dynamic electric properties of hydrogen peroxide

In this work we present a method based on the perturbation theoretic approach of Bishop and co-workers [J. Chem. Phys. 95, 2646 (1991); 97, 5255 (1992); 108, 10013 (1998)] to calculate the effect of torsional motion on the polarizability and hyperpolarizabilities of hydrogen peroxide. The frequency dependence has been evaluated using the time-dependent Hartree-Fock method. The results obtained show that the zero-point vibrational averaging contributions are small compared to the corresponding electronic contributions. In the static limit the pure vibrational contributions are very large, specially for beta and gamma. These contributions are significant for the hyperpolarizabilities even in the visible region, except for the second harmonic generation and third harmonic generation processes.     

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.     

Dynamic imaging of molecules using high order harmonic generation

We review recent progress towards imaging the electronic wavefunctions and nuclear dynamics of small molecules using the high order harmonics emitted when a molecule experiences an intense laser field. We illustrate that the essence of high harmonic emission is contained in the recombination amplitude between the continuum portion of the electronic wavefunction, that is formed through field ionization and which is accelerated and driven back to recollide in the laser field, and the bound electronic state. We review for the non-specialist some recent experimental and theoretical work dealing with high harmonic generation (HHG) in molecules. Particular attention is paid to two types of experiment recently performed in our group. The first of these types of experiment is the measurement of signatures of molecular electronic structure using HHG from molecules with a fixed orientation in space. The second is the use of HHG to track extremely fast proton rearrangement following ionization in light molecules, using the intrinsic temporal variation of the recolliding electron energy to extract these dynamics from measurements of the high harmonics.     

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.     

Tuning the Dipolar Second‐Order Nonlinear Optical Properties of Cyclometalated Platinum(II) Complexes with Tridentate N^C^N Binding Ligands

Appropriate functionalization of the cyclometalated ligand, L , and the choice of the ancillary ligand, X, allows the dipolar second‐order nonlinear optical response of luminescent [Pt L X] complexes—in which L is an N^C^N‐coordinated 1,3‐di(2‐pyridyl)benzene ligand and X is a monodentate halide or acetylide ligand—to be controlled. The complementary use of electric‐field‐induced second‐harmonic (EFISH) generation and harmonic light scattering (HLS) measurements demonstrates how the quadratic hyperpolarizability of this appealing family of multifunctional chromophores, characterized by a good transparency throughout much of the visible region, is dominated by an octupolar contribution.     

Tunable UV generation at 283 nm by frequency doubling and sum frequency mixing of two semiconductor lasers for the detection of Pb

High resolution atomic absorption measurements of lead at 283 nm in a vapor cell were performed by frequency doubling an 850 nm laser diode to obtain 425 nm light, followed by sum frequency generation of the harmonic radiation with a second 850 nm laser diode.     

中心对称双酞菁铥LB膜二次谐波产生特性

利用二次谐波产生(SHG)方法研究了中心对称分子稀土夹心双酞菁铥(TmPc₂) Langmuir-Blodgett (LB)膜二阶非线性光学特性, 测量了二次谐波强度随入射基频光入射角的关系, 并对其二阶非线性产生机制进行了讨论. 实验结果表明, TmPc₂分子LB膜具有较好的二次谐波信号, 二次谐波信号强度的最大值在基频光入射角为45°的地方, 其二阶非线性极化率χ⁽²⁾和分子超极化率β分别为1.152×10^-8和1.905×10^-30 esu. 通过测量样品二次谐波信号的偏振特性, 并与理论分析相比较, 得出其二阶非线性起源于电四极子作用机制.     

Multifunctional Molecular Material Combining Photocurrent Generation and Second-Order Optical Nonlinearity in LB Monolayer Films

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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.     

Probing Structure,Spectroscopy, Kinetics,and Dynamics on Metal Surfaces by Optical Second Harmonic Generation

With the intrinsic surface sensitivity, superior frequency and time resolution, and adaptability to all pressure conditions, optical second harmonic generation has proven a versatile and powerful diagnostic tool for surface science studies. Examples are provided in the use of surface second harmonic generation in elucidating the adsorption kinetics and structural transitions of molecular adlayers, detecting surface electronic transitions, and determining transient temperature changes on metal surfaces.     

Enhancement of second harmonic generation in helical Sc liquid crystals

In a ferroelectric liquid crystal, a special type of phase-matching for optical second harmonic generation (SHG) is possible, where two counter-propagating fundamental waves create second harmonic waves at the edge of the selective reflection band. We compute the SHG intensity in such a situation and show that, at slight detuning from exact phase-matching, useful resonance enhancement can be obtained. A considerable amount of SHG also appears when the second harmonic frequency is in the reflection band, where the SHG wave is non-propagating.     

Interaction of nitrate, barium, strontium and cadmium ions with fused quartz/water interfaces studied by second harmonic generation

Inorganic anions and cations are ubiquitous in environmental chemistry. Here, we use second harmonic generation to track the interaction of the environmentally important metal cations barium, strontium, and cadmium and the nitrate anion with fused quartz/water interfaces at pH 7. Using a dynamic flow system, we assess the extent of reversibility in the binding process and report the absolute number density of adsorbed cations, their charge densities, and their free energies of adsorption. We also present resonantly enhanced second harmonic generation experiments that show that nitrate is surface active and report the free energies and binding constants for the adsorption process. The second harmonic generation spectrum of surface-bound nitrate shows a new adsorption band that cuts further into the solar spectrum than nitrate in the aqueous or solid state. The results that we obtain for all four inorganic ions and the implications for tropospheric and aquatic chemistry as well as geochemistry are discussed in the context of fundamental science as well as pollutant transport models.     

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.     

Organized Aggregation of Porphyrins in Lipid Bilayers for Third Harmonic Generation Microscopy

Nonlinear optical microscopy has become a powerful tool for high‐resolution imaging of cellular and subcellular composition, morphology, and interactions because of its high spatial resolution, deep penetration, and low photo‐damage to tissue. Developing specific harmonic probes is essential for exploiting nonlinear microscopic imaging for biomedical applications. We report an organized aggregate of porphyrins (OAP) that formed within lipidic nanoparticles showing fingerprint spectroscopic properties, structure‐associated second harmonic generation, and superradiant third harmonic generation. The OAP facilitated harmonic microscopic imaging of living cells with significantly enhanced contrast. The structure‐dependent switch between harmonic (OAP‐intact) and fluorescence (OAP‐disrupted) generation enabled real‐time multi‐modality imaging of the cellular fate of nanoparticles. Robustly produced under various conditions and easily incorporated into pre‐formed lipid nanovesicles, OAP provides a biocompatible nanoplatform for harmonic imaging.