Optical Second Harmonic Generation Technique Research Articles

Magnetophotonic crystals based on yttrium-iron-garnet infiltrated opals: Magnetization-induced second-harmonic generation

Three-dimensional magnetophotonic crystals (MPCs) based on artificial opals infiltrated by yttrium iron garnet (YIG) are fabricated and their structural, optical, and nonlinear optical properties are studied. The formation of the crystalline YIG inside the opal matrix is checked by x-ray analysis. Two templates are used for the infiltration by YIG: bare opals and those covered by a thin platinum film. Optical second-harmonic generation (SHG) technique is used to study the magnetization-induced nonlinear-optical properties of the composed MPCs. A high nonlinear magneto-optical Kerr effect in the SHG intensity is observed at the edge of the photonic band gap of the MPCs.

Molecular beam investigation of hydrogen dissociation on Si(001) and Si(111) surfaces.

The influence of molecular vibrations on the reaction dynamics of H2 on Si(001) as well as isotopic effects have been investigated by means of optical second-harmonic generation and molecular beam techniques. Enhanced dissociation of vibrationally excited H2 on Si(001)2 x 1 has been found corresponding to a reduction of the mean adsorption barrier to 390 meV and 180 meV for nu=1 and nu=2, respectively. The adsorption dynamics of the isotopes H2 and D2 show only small differences in the accessible range of beam energies between 50 meV and 350 meV. They are traced back to different degrees of vibrational excitation and do not point to an important influence of quantum tunneling in crossing the adsorption barrier. The sticking probability of H2 on the 7 x 7-reconstructed Si(111) surface was found to be activated both by H2 kinetic energy and surface temperature in a qualitatively similar fashion as H2/Si(001)2 x 1. Quantitatively, the overall sticking probabilities of H2 on the Si(111) surface are about one order of magnitude lower than on Si(001), the influence of surface temperature is generally stronger.

Characterization of multiple Si∕SiO2 interfaces in silicon-on-insulator materials via second-harmonic generation

Charge generation, transport, and recombination processes in UNIBOND® silicon-on-insulator wafers are studied via an optical second-harmonic generation (SHG) technique. The electric fields at the interfaces vary with time due to charge trapping. The presence of a thin native oxide layer on the top Si film contributes significantly to the SH intensity due to the strong time-dependent electric field generated by electrons transported to the surface. For the thick buried oxide, the electric field is primarily due to carrier trapping at the interface, and it varies with time weakly. The SHG signals depend strongly on the externally applied electric field, which can differentiate the contribution of each interface to the total SH signal.

Optical Second-Harmonic Generation Technique for Impulse Electric Field Measurements

A novel approach of an optical second-harmonic generation (SHG) technique combined with an electro-optic effect is proposed for measuring an impulse electric field. The experimental measurements combined with theoretical estimates show that even though the temperature control is a critical requirement, the SHG technique with nonlinear, electro-optic crystals, such as MgO:LiNbO3 and KTiOPO4, can be effectively used for electric field measurements by utilizing the phase matching condition.

Concentration measurements downstream of an insoluble monolayer front

The surfactant concentration distribution on a planar uniform flow with a surface-piercing barrier was measured via the nonlinear optical technique of second-harmonic generation. The measurements were performed for an insoluble surfactant monolayer on the air/water interface. A theoretical model balancing surface elasticity and bulk shear at the interface was developed to predict the concentration profile for any insoluble monolayer. Measured equations of state, relating the surface tension to the surfactant concentration, were used in the model along with velocity data obtained using boundary-fitted digital particle image velocimetry. Theoretical concentration profiles were in agreement with experimental results. Additionally, global predictions from the model for four different insoluble surfactant systems also showed agreement with experimental measurements.

In-situ profiling of dipole polarization distributions in poled nonlinear optical polymers with electrothermal and optical techniques

Poling is an essential process step in the preparation of dipolar chromophore-based second-order nonlinear optical polymers. In single- and multilayer film structures, poling often yields a spatially nonuniform orientation of the chromophores across the film thickness. An electrothermal technique, based on the measurement of electrical transients after pulsed thermal excitation, as well as an optical second-harmonic generation technique have been developed for the depth profiling of the dipole polarization with a high spatial resolution around 100 nm.

Determination of Surface Viscosities by Surfactant Concentration and Velocity Field Measurements for an Insoluble Monolayer

A new technique has been developed to determine the sum of the surface (excess) shear viscosity and surface dilatational viscosity for insoluble monolayers on a gas−liquid interface. The flow on the interface is produced by the rise and the subsequent spread of a laminar vortex pair which dilates the interface. The terms in the tangential component of the stress boundary condition equation for the interface are directly evaluated through measurements of the velocity in the liquid and on the interface, obtained through digital particle image velocimetry, and measurements of the unsteady surfactant concentration on the interface, made with the nonlinear optical technique of second-harmonic generation. For the experiments, the water surface was initially covered by a uniform monolayer of hemicyanine dye, 4-[4-(dimethylamino)styryl]-1-docosylpyridinium bromide, which was chosen for its strong nonlinear optical response. The sum of the surface viscosities for hemicyanine was found to be 1.2 (±0.5) sp at a conc...

Second-Harmonic Generation Measurements of Electrostatic Biopolymer−Surfactant Coadsorption at the Water/1,2-Dichloroethane Interface

The electrostatic coadsorption of the synthetic polypeptide poly-l-glutamic acid (pGlu) and the cationic hemicyanine surfactant trans-4-[4-(dibutylamino)styryl]-1-methylpyridinium iodide (D+−CH3) is observed at the water/1,2-dichloroethane (DCE) interface with the spectroscopic technique of optical second-harmonic generation (SHG). The adsorption isotherm of D+−CH3 to the water/DCE interface is obtained with a combination of resonant surface SHG and interfacial tension measurements. In the presence of polyanionic pGlu in the aqueous phase at a pH of 8.5, the limiting surface coverage of the cationic surfactant is enhanced by a factor of 3.7. In contrast, no such enhancement is observed in the presence of pGlu for the adsorption of the neutral zwitterionic hemicyanine surfactant trans-4-[4-(dibutylamino)styryl]-1-(3-sulfopropyl)pyridinium hydroxide inner salt hydrate (D+−(CH2)3−SO3-) to the water/DCE interface. Additionally, no enhancement in surfactant adsorption is observed for D+−CH3 when the pGlu in th...

Van der Waals-induced distortions in nematic liquid crystals close to a surface.

By a discrete-layer model, we investigate theoretically the alignment of a nematic liquid crystal close to a flat amorphous substrate. We show that the presence of van der Waals interactions induces a strong distortion localized in a few molecular lengths. We compare these predictions with that of a continuum model recently developed by some of us. In the continuum model, we derive approximate analytical expressions for the distortion profile, showing that, asymptotically, the distortion decays as the inverse of the distance from the substrate. The amplitude of the distortion according to the two models is different, but the main qualitative features remain unchanged. We show that the main distorting mechanism can be attributed to an effective surface field. The existence of a subsurface distortion is compatible with some recent measurements of the alignment of the first molecular layer, performed with optical second-harmonic generation techniques. \textcopyright{} 1996 The American Physical Society.

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.

Probing surface chemical processes during epitaxial semiconductor crystal growth at near-atmospheric pressures using photon-based techniques

It is demonstrated here that for the study of epitaxial semiconductor crystal growth from vapour-phase precursors, photon-based methods may be applied under true ‘in situ’ conditions and can, where appropriate, reveal a great deal regarding the nature of the growing surface. While several techniques exist which may probe the gaseous phase, the study of the surface phase under conditions of high pressures (up to 1000 mbar) and high temperatures (up to 1000 K) is far from trivial. This paper sets out to emphasise recent developments in technique which permit the study of the surface phase during epitaxial crystal growth. Data are presented for several semiconductor/substrate systems obtained using the linear technique of reflectance anisotropy and the non-linear technique of optical second-harmonic generation (SHG). In particular, the technique of reflectance anisotropy is shown to be a very powerful probe of surface stoichiometry, real-time monitoring of layer-by-layer growth processes, adsorption and desorption kinetics and also the detection of surface reconstructions that occur under high vacuum and near-atmospheric pressure conditions. For the SHG technique it is shown that the application of this method to centroasymmetric substrates such as GaAs is not trivial, yet under certain conditions surface SHG responses can be observed which are directly related to the structure of the surface layers. While the data presented are for special epitaxial semiconductor systems, throughout the paper emphasis is placed upon the applicability of the experimental approach towards the study of crystal growth systems in general.

Interactions of deuterium and hydrocarbon species with the diamond C(111) surface

AES, XPS, TDS and LEED have been used to study the adsorption, the desorption and the reaction behavior of D atoms and CH x (1 ⩽ x ⩽ 3) species on the diamond C(111) surface. It is found that a small amount of chemisorbed D atoms can induce the (2 × 1) to (1 × 1) phase transition and hold the sp 3 structure up to nearly 1400 K. The structural transformation can also be monitored by the optical second-harmonic generation technique. Thermal desorption of deuterium occurs mainly around 1300 K. Adsorption of CH x can also induce the phase change but not as readily as the D atoms. Annealing of the adlayer at a high temperature results in the partial conversion to graphitic carbon. D atoms can react with both the adsorbed CH x and the surface graphite. The surface bonding characteristics and the relevance of present results to the diamond CVD process are briefly discussed.

Interactions of deuterium and hydrocarbon species with the diamond C(111) surface

AES, XPS, TDS and LEED have been used to study the adsorption, the desorption and the reaction behavior of D atoms and CH x (1≤ x≤3) species on the diamond C(111) surface. It is found that a small amount of chemisorbed D atoms can induce the (2 × 1) to (1 × 1) phase transition and hold the sp 3 structure up to nearly 1400 K. The structural transformation can also be monitored by the optical second-harmonic generation technique. Thermal desorption of deuterium occurs mainly around 1300 K. Adsorption of CH x can also induce the phase change but not as readily as the D atoms. Annealing of the adlayer at a high temperature results in the partial conversion to graphitic carbon. D atoms can react with both the adsorbed CH x and the surface graphite. The surface bonding characteristics and the relevance of present results to the diamond CVD process are briefly discussed.

Investigation of surface-induced alignment of liquid-crystal molecules by optical second-harmonic generation.

We apply the technique of optical second-harmonic generation to study homogeneously aligned liquid-crystal cells. The surface dipole sensitivity of the technique made it possible to study the monolayer in the absence and presence of a bulk of liquid crystal. By comparing the monolayer orientational distribution functions of three surface treatments (rubbed polymer-coated substrates, rubbed surfactant-coated substrates, and substrates made from oblique evaporation of ${\mathrm{SiO}}_{\mathit{x}}$ film), we find that two different surface-originated mechanisms are effective in aligning liquid-crystal films. For rubbed polymer samples, it is shown that a short-range molecular interaction is responsible for alignment of the first monolayer, which then aligns the bulk via an epitaxylike interaction. Results on polymers with various structures and compositions and rubbed with a variety of rubbing strengths are presented. For the other surface treatments, the first monolayer is isotropically distributed, indicating that a bulk elastic interaction is responsible for the bulk alignment.

In situ determination of induced dipole moments of pure and membrane-bound retinal chromophores.

By using the technique of optical second-harmonic generation, we have succeeded in obtaining the orientation, spectrum, and the change in dipole moment upon optical excitation for monolayers of retinal chromophores, which are of relevance for visual excitation. The change in dipole moment of the chromophore embedded in the purple membrane of Halobacterium halobium is found to be identical to that of the free chromophore. The spectrum of all-trans retinal mono layers exhibits a subsidiary one-photon forbidden peak in addition to the main absorption peak.

Nonlinear Optical Study of Si Epitaxy

AbstractThe applicability of the nonlinear optical technique of surface second-harmonic generation to in-situ studies of epitaxial and non-epitaxial crystal growth of centrosymmetric materials is demonstrated. In measurements of the deposition of atomic Si on Si(111)-7×7 surfaces, the (anisotropic) second-harmonic response is seen to be sensitive to the ordering of fractional monolayers of adatoms. For deposition on a substrate held at room temperature, the second-harmonic data are consistent with the formation of a disordered adlayer on top of the original reconstructed surface. The results of real-time measurements of the thermal annealing of disordered Si adlayers of monolayer thickness are also presented.