Surface Second Harmonic Generation Research Articles

Control of Carboxylic Acid and Ester Groups on Chromium (VI) Binding to Functionalized Silica/Water Interfaces Studied by Second Harmonic Generation

Resonantly enhanced surface second harmonic generation (SHG) measurements carried out at pH 7 and room temperature were performed to study how surface-bound carboxylic acid and methyl ester functional groups control the interaction of chromate ions with fused silica/water interfaces. These functional groups were chosen because of their high abundance in humic and fulvic acids and related biopolymers commonly found in soils. They were anchored to the silica surface using organosilane chemistry to avoid competing complexation processes in the aqueous solution as well as competitive adsorption of the organic compounds and chromate. The SHG experiments were carried out at room temperature and pH 7 while using environmentally representative chromate concentrations ranging from 1 x10(-6) to 2 x 10(-4) M. Chromate is found to bind to the acid- and ester-functionalized silica/water interfaces in a reversible fashion. In contrast to the plain silica/water interface, chromate binding studies performed on the functionalized silica/water interfaces show S-shaped adsorption isotherms that can be modeled using the Frumkin-Fowler-Guggenheim (FFG) model. This model predicts a coverage-dependent binding constant of K(ads) x exp(gtheta). Values for g are found to be 3.2(2), 2.1(2), and 1.3(2) for the carboxylic acid-, the ester-, and the nonfunctionalized silica/water interfaces, respectively, and are consistent with stabilizing lateral adsorbate-adsorbate interactions among the Cr(VI) species adsorbed to the functionalized surfaces. The FFG model allows for the parametrization of the solid-liquid partition coefficient and chromate retardation factors in silica-rich soil particles whose surfaces contain organic adlayers rich in carboxylic acid and methyl ester groups. The straightforward model presented here predicts that chromate retardation increases by up to 200% when carboxylic acid functional groups are present at the silica/water interface. Increases up to 50% are predicted for methyl ester-containing organic adlayers, and the retardation factor remains effectively near unity for the plain silica/water interface (no siloxanes present).

Orientation of liquid crystal molecules on command surface LB monolayer studied by the surface potential and SHG technique

Orientation of the liquid crystalline (LC) and command surface molecules was investigated by the surface potential and optical second-harmonic generation (SHG) measurement. LC ultra thin film was prepared on azobenzene command surface (6AZ10-PVA) LB monolayer by the evaporation. SHG measurements show that LC molecules are deposited on 6AZ10-PVA monolayer so as to compensate the hyper-polarizability of azo side chain. Surface potential measurements indicate that dipole moment of the azo side chain and LC molecule faces toward each other. Orientational change of the LC molecules due to the photoisomerization of 6AZ10-PVA surface was also observed by the SHG measurement. Oriented gas model demonstrates that tilt angle of the LC molecules increases with the photoirradiation.

Effect of the orientational disorder on the hyperpolarizability measurement of amphiphilic push-pull chromophores in Langmuir–Blodgett monolayers

First-order hyperpolarizabilties of two new amphiphilic push-pull chromophore derivatives of hemicyanine dyes prepared in Langmuir–Blodgett films and designed for membrane potential imaging of living cells are measured by surface second-harmonic generation (SHG). Assuming uniaxial symmetry, an effective hyperpolarizability coefficient βe is defined, and a procedure to measure this coefficient is described. Relationships between βe and the true hyperpolarizability coefficient β are derived on the basis of a stochastic model taking account of the tilt angle disorder of the chromophores. Maximum discrepancies between the true and the effective hyperpolarizabilities are calculated as a function of the orientation parameter D, and it is shown that the relative difference may not exceed 40% for usual distribution functions of the tilt angle. As a result, the dominant hyperpolarizability coefficient βz,z,z(2) of the chromophores in LB films is found to be in the range 0.3–0.7×10−27 esu at 1064 nm fundamental wavelength.

Direct Determination of Effective Interfacial Optical Constants by Nonlinear Optical Null Ellipsometry of Chiral Films

Nonlinear optical null ellipsometry (NONE) measurements of chiral interfaces allowed direct experimental measurement of the linear interfacial optical constants in surface second harmonic generation (SHG) measurements. Since phase information is retained in NONE measurements, the real and imaginary components of the interfacial refractive index (n and k, respectively) were uniquely obtained from the measured chiral chi((2)) tensor elements of a fluorescein-labeled bovine serum albumin film. The sensitivity of the calculated chi((2)) tensor elements on the assumed values of the interfacial optical constants allowed measurements of n and k to four significant figures with no additional adjustable parameters and independent of molecular symmetry. The optical constants measured by SHG agreed within a relative error of 0.8% with values predicted independently using a simple effective medium approximation, also with no adjustable parameters. Additionally, those same optical constants produced relationships between the achiral chi((2)) tensor elements in excellent agreement with predictions for systems exhibiting weak orientational order. This study suggests that the far-field intensity and polarization state of the nonlinear optical beam may be largely independent of the near-field optical constants within the interfacial layer in the limit of a film thickness much less than the wavelength of light.

Carrier-wave Rabi flopping: role of the carrier-envelope phase.

Recently, a dependence of Rabi flopping on the carrier-envelope phase of the exciting laser pulses was predicted theoretically [Phys. Rev. Lett. 89, 127401 (2002)] for excitation of a thin semiconductor film with intense few-cycle pulses. Here, we report corresponding experiments on 50-100-nm thin GaAs films excited with 5-fs pulses. We find a dependence on the carrier-envelope phase arising from the interference of sidebands from the fundamental or the third-harmonic Mollow triplet, respectively, with surface second-harmonic generation.

Effect of ultrathin Cr layers on surface second-harmonic generation from Cr-SiO2-Si(001) structures

An ultrathin semitransparent Cr coating layer on oxidized Si(001) significantly alters surface second-harmonic generation (SHG) behavior. The peak locations of the rotational-anisotropy SHG (RA-SHG) signals from Cr coated SiO2-Si(001) surfaces are shifted by 45° from that of uncoated surfaces consistently for several polarizations, but only for specific photon energies. Moreover, for appropriate polarizations, a peak-to-valley transition of the RA-SHG signal by varying photon energy is observed for Cr coated samples but not for uncoated samples. The time-dependent SHG signal decreases with continuous laser illumination for Cr coated surfaces, but it increases for uncoated surfaces. Through these comparisons, we find that the Cr coating introduces an additional SHG. Interference between the additional and original SHG contributions modifies both the phase and amplitude of the net SHG, and thus the measured spectroscopic and time dependence of SHG. To determine the source of this additional SHG, we perform further surface SHG measurements on a thick Cr film and the SiO2-Cr interface. We find that it mainly comes from the outermost surface of the Cr coating layer. In addition, we investigate the effect of the thickness of the SiO2 layer on the SHG response in reflection from Cr-SiO2-Si(001) structures.

Chirp effects in femtosecond laser-induced surface second-harmonic generation from metals

Chirp effects are studied in femtosecond laser-induced surface second-harmonic generation (SHG) from gold. We observe a higher SHG yield when the incident laser pulses are negatively chirped, compared to using the transform-limited or positively chirped pulses. This study indicates that SHG is critically dependent on the exciting wavelength sequence within the femtosecond pulse bandwidth when electrons contributing to SHG are heated to a sufficiently high temperature and are in thermal nonequilibrium distribution with the lattice.

Surface second-harmonic generation in Au: TiO $\mathsf{_{2}}$ cermet thin films

The Surface Second-Harmonic Generation (SSHG) in Au: TiO2 thin films of nanocermets is studied with samples of different metallic volume fractions using an IR Nd: YAG pulsed laser. We worked in a reflection geometry by varying the incidence angle and the directions of the incident beam polarization and of an analyzer of the Second Harmonic beam. We show that the SH signal is not due to electric dipolar processes only but can be treated within the general a, b, and d formalism where surfaces and bulk SH currents are well distinguished, and by describing the dielectric function of cermets within the effective-medium theory. The effective values obtained for a, b, and d are attributed to gold but the rather small discrepancies between them and those given by models are understandable by the fact that our samples are heterogeneous and not semi-infinite.

A study of kinetic molecular exchange processes in the medium frequency range by surface SHG on an oscillating bubble.

The dilatational properties of fluid surfaces and interfaces have been comprehensively investigated in recent years. For example, an improved oscillating bubble device provided experimental results that allow for critical testing of established surface models, such as the Lucassen/van den Tempel (LvdT) model. The comparison of the LvdT model with the oscillating bubble experiments demonstrates a mismatch between the model parameters. For example, near the CMC or the limit of solubility the calculated parameters of surfactant solutions become unrealistically large. The deviation can be explained by the introduction of more detailed surface models, in particular by the modification of the effective thickness of the surface layer, its internal structure and the molecular exchange processes between these structures. For the verification of such processes an experimental setup was realized which allows for an independent determination of the instantaneous adsorption state at the surface of an oscillating bubble inside a surfactant solution. The setup utilizes the Second Harmonic Generation (SHG)--effect at the air-solution interface generated by the light of a pulsed LASER. The set-up is described in detail, and the results of a first experimental series are presented and discussed in this paper. As system, aqueous solutions of the fluortenside F381 were used.

Statistical analysis of second harmonic generation experiments: a phenomenological model

We discuss issues arising in fitting theoretically derived nonlinear models with complex coefficients to data from surface Second Harmonic Generation (SHG) experiments conducted at the air/liquid interface. We explore different parametrisations for the complex parameters and show that the Euler (magnitude and phase angle) parametrisation is preferable to the real and imaginary parts parametrisation, both theoretically and empirically. We emphasise the importance and value of diagnostic plots for evaluating the quality of model fit. We derive approximate standard errors for the parameter estimates and discuss issues of making inference about ratios of parameters. We consider approximate confidence intervals (using the approximate standard errors), profile likelihood intervals, Fieller's method and bootstrap intervals. Fieller's method (and the bootstrap intervals) provide useful information on the value of the simpler approximate confidence intervals. We also propose and implement a likelihood ratio test to assess whether a common model can be fitted to several independent data sets. Finally, the methods are applied to data sets obtained from SHG experiments on l-phenylalanine at the air/water interface and toluene.

Quenching of optical second harmonic generation at the Si(001) surface by hydrogen adsorption

A detailed study of the surface second harmonic generation (SHG) efficiency at $\mathrm{Si}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ is presented. We have measured the SHG efficiency as a function of hydrogen coverage, relative to that of the clean surface, at a fundamental wavelength of 1064 nm, for substrate temperatures between 200 and 600 K, and for different polarization directions of both the fundamental and second harmonic light beams. The dependence of hydrogen-induced SHG quenching on H coverage is explained with a statistical mechanical model that has previously been invoked to describe the kinetics and dynamics of ${\mathrm{H}}_{2}$ adsorption/desorption. The model predicts an increase in the SHG intensity upon rearranging randomly adsorbed H atoms by thermal annealing, which was confirmed experimentally. An empirical parametrization of the SHG efficiency curves is presented which may be used as a practical calibration to determine hydrogen coverage on Si(001) as a function of SHG signal and temperature.

Nonlinear Optical Spectroscopic Studies of Polymer Surfaces for Liquid Crystal Alignment: Photo-Irradiated Polyimide and Rubbed Polystyrene Surfaces

Surface specific sum-frequency vibrational spectroscopy (SFVS) and second-harmonic generation (SHG) were used to study the structures of polymer surfaces modified by linearly polarized UV irradiation or mechanical rubbing. The spectroscopic results show that the surface anisotropy of a polyimide surface resulting from bond breaking by linearly polarized UV irradiation is extremely small compared to a rubbed surface. On a rubbed polystyrene surface, the phenyl sidegroups are well aligned by rubbing in the direction perpendicular to rubbing but tilt from the surface normal with a broad distribution.

An oscillating bubble device for direct measurement of molecular exchange processes at the air–liquid interface in the medium frequency range

An experiment for the investigation of the exchange dynamics of adsorbed and dissolved surfactants at the air–water interface is described. The experiment combines two established techniques: the method of the oscillating bubble generates in a well-defined fashion a nonequilibrium state by a periodic compression and expansion of the surface layer. The state of the adsorption layer is then investigated by surface second harmonic generation (SHG). SHG is a second order nonlinear optical technique, which probes only the adsorbed molecules. Contribution of the bulk is widely suppressed and information on the orientational order of the surfactant and the corresponding number density of the adsorbed species are gained. These data are of utmost importance to assess the established models of Gibbs adsorption layer. The combination of the nonlinear optical technique with a rapidly oscillating bubble imposes severe experimental hurdles such as the synchronization of the laser pulses with the state of the bubble or problems arising from a bad signal to noise ratio. The experimental setup discussed in this article overcomes these problems and outlines all relevant design parameters.

Orientation of cavitands at air/water and air/solid interfaces studied by second harmonic generation

Surface second harmonic generation (SHG) was used to probe orientation of cavitand molecules deposited on water as Langmuir films and on fused silica substrates either bare or functionalized with a surfactant. Measurements performed during compression of the Langmuir films reveal that while the surface density may vary, molecules always appear more or less oriented with their long axis perpendicular to the water surface. Phase measurements indicate that the vase-like heads of the molecules cover the water for Langmuir films, but face the air for monolayers spread on surfactant-coated substrates.

Kinetics of Chromate Adsorption and Desorption at Fused Quartz/Water Interfaces Studied by Second Harmonic Generation

It is now recognized that an understanding of hexavalent chromium pollution in the environment is tied to an understanding of how hexavalent chromium binds to geosorbent surfaces. We have applied the nonlinear optical laser spectroscopy surface second harmonic generation (SHG) to study the adsorption and desorption kinetics of submonolayer amounts of chromate interacting with the fused quartz/water interface at pH 7 and at room temperature. The chromate concentrations are varied between 10-6 and 10-5 M. The adsorption and desorption behavior of chromate at the fused quartz/water interface can be described by a Langmuir adsorption and a first-order chromate desorption model with an adsorption rate constant of 3(1) × 103 s-1 M-1 and a desorption rate constant of 0.9(7) × 10-3 s-1. At 300 K and pH 7, the resulting equilibrium constant for chromate binding is in good agreement with equilibrium constants obtained from Langmuir isotherm measurements carried out between pH 4 and 9. Thus, thermodynamic and kineti...

On-line monitoring of laser modification of titanium dioxide using optical surface second harmonic

TiO2 photocatalysis is a promising technology for the destruction of organic pollutants in both waste and potable waters with the mineralisation of a wide range of compounds having been reported. TiO2 has many advantages over other semiconductors, it is highly photoreactive, cheap, non-toxic, chemically and biologically inert, and photostable. The photocatalytic activity of TiO2 has been shown to depend upon many criteria including the ratio of anatase/rutile crystal phase, particle size and oxidation state. This paper reports the use of optical surface second harmonic generation (SSHG) to monitor modifications in TiO2 powder induced following laser treatment. SSHG is a non-contact, non-destructive technique, which is highly sensitive to both surface chemical and physical changes. Results show that three different SSH intensities were observable as the TiO2 samples were irradiated with the laser light. These regions were related to changes in chemical characteristics and particle size of the TiO2 powder.

Quantitative analysis of orientational order in the molecular monolayer by surface second harmonic generation

We have developed a systematic approach to quantitatively determine the orientational order of molecular monolayer based on surface second harmonic generation (SHG) measurement. Traditionally, the experimentally obtainable orientational parameter D=〈cos θ〉/〈cos3 θ〉 has been used to determine the averaged orientational angle θ with SHG. Also, it has been widely accepted that without the help from other techniques, SHG was insufficient for determination of both the orientational angle θ0 and its distribution Δθ. Here, we derived an explicit and general functional form, R(θ)=|〈cos θ〉−c * 〈cos3 θ〉|2, which is orientationally dependent and is proportional to the SH intensity measured from the molecular monolayer. We further showed that R(θ) contains the full information of the orientational order of the molecular monolayer, and the orientational parameter D represents only a special case of R(θ). Through orientationally sensitive R(θ) and D, both the orientational angle and its distribution could be uniquely and accurately determined from the SHG-only measurements in chosen polarization configurations. Using this approach, we were able to accurately determine both the orientational angle and its distribution in the Langmuir monolayer of a liquid crystal molecule 4′-n-octyl-4-cyanobiphenyl (8CB) at the air/water interface. With such information, we were not only able to show that the phase transition of 8CB from the gas–liquid coexistence region to liquid region is accompanied with a continuous orientational angular distribution narrowing process, but also for the first time we were able to accurately determine a less than 3° tilt angle change of the 8CB chromophore in the whole liquid phase region of the 8CB monolayer. This quantitative approach to orientational analysis of molecular monolayer or multilayer is general for both SHG and sum frequency generation, and should lead the way to more quantitative understanding of the orientational order of molecular films per se.

Surface Studies of Chromate Binding to Fused Quartz/Water Interfaces

While it is well known that chromium contamination in groundwater represents a considerable threat to the environment, little is known about the heterogeneous processes that govern chromium interaction with solid materials in soil. Using the nonlinear optical laser spectroscopy surface second harmonic generation (SHG), we have studied chromate adsorption and desorption at the fused quartz/liquid water interface in real time, at room temperature and at chromate concentrations between 1 × 10-6 and 2 × 10-4 M. Adsorbed chromate is spectroscopically identified via a two-photon resonance of one of its ligand-to-metal charge-transfer bands with the fundamental probe light. Adsorption isotherm measurements at 300 K result in a free chromate adsorption energy ΔGads of 38 ± 1 kJ/mol at pH 7. Real-time kinetic measurements of chromate adsorption and desorption show reversible chromate binding to the fused quartz/water interface, consistent with the high mobility of Cr(VI) in soils and the ΔGads determined from our adsorption isotherm measurements. The pH dependence of chromate binding to the fused quartz/water interface is discussed.

Polarization-vortex-driven second-harmonic generation.

Polarization vortices exist in the focus of a class of vector beams, the lowest order of which possess full vector symmetry about the axis of propagation of the beam. At high numerical apertures these beams are known to exhibit large, local, longitudinal fields in the focal region. At an interface these fields can be many times stronger than the largest available transverse component and are therefore candidates for a variety of different experiments in surface physics. The observation of vortex-driven surface second-harmonic generation at smooth metal and semiconductor surfaces and thin films is reported. By comparing the response to that of a purely transverse field, we show that the smooth surface responds primarily to the longitudinal field component.

Local second-harmonic generation enhancement on gold nanostructures probed by two-photon microscopy.

The localization of surface second-harmonic generation (S-SHG) enhancements from granular gold structures that exhibit local plasmon resonance was investigated. A two-photon microscopy technique was used to perform high spatial resolution S-SHG imaging. The magnitude and the spatial density of S-SHG enhancement confined in submicroscopic regions are strongly dependent on the morphology of the gold's surface. Polarization measurements of local S-SHG responses reveal the local field anisotropy in enhancement regions and furthermore prove the incoherent and strongly depolarized nature of the emission, which is attributed to ultrafast fluctuations of the enhancement location in the focal volume.