Surface Optical Second-harmonic Generation Research Articles

Optical second harmonic generation from plasmonic nanoshells using the nonlocal hydrodynamic model

In this paper, we investigate the linear and nonlinear optical response of spherical plasmonic nanoshells within the nonlocal hydrodynamical model for conduction electrons. The optical absorption and surface second-harmonic generation (SHG) associated with metallic core-shell nanoparticles is studied on the basis of the hydrodynamic Drude model. The influence of nonlocality on the optical SHG is investigated. We show that nonlocal effects can be applied to boost SHG from plasmonic nanoshells. It is found that the surface SHG radiation from the surface of nanoshells can be achieved by symmetry-breaking and is sensitive to nonlocality of the electron response. It is also shown that nonlocal effects lead to enhancement and shift of the resonant wavelengths of the optical absorption and SHG in metal nanoshells. Keywords: Optical absorption, Second harmonic generation, plasmonic nanoshell, nonlocal effects.

Optical surface second harmonic generation from plasmonic graphene-coated nanoshells: influence of shape, size, dielectric core and embedding medium

We study optical surface second-harmonic generation (SHG) from the plasmonic nanoshells because they can improve and enhance nonlinear optical effects. We also investigate the SHG from the surface of spherical and cylindrical core-shell nanoparticles coated with a strong nonlinear material such as graphene due to its attractive plasmonic behaviour and unique properties of the surface plasmons in graphene. We demonstrate theoretically a giant and tunable second-order harmonic radiation which enhanced through the excitation of the surface plasmon resonance, can be observed at the surface of both spherical and cylindrical nanoshells because of symmetry-breaking at interface, which makes SHG as a valuable and powerful technique for applications in sensing and surface spectroscopy. We present the surface SHG strongly depends on the particle shape and size, the dielectric of embedding medium and core, the type of metal and graphene as a coating nonlinear material.

Surface Detection of Strain-Relaxed Si1–xGex Alloys With High Ge-Content by Optical Second-Harmonic Generation

We investigated the strain and surface structural properties of a strain-relaxed Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloy layer with high Ge-content using optical surface second-harmonic generation. Here, the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloys are heteroepitaxial, and they are deposited onto Si substrates via ultra-high-vacuum-chemical vapor deposition. The in-plane strain and the composition x of the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloys were determined using Raman spectroscopy. The SH signals generated from three Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloy surfaces versus the rotational angle of the substrate were measured. The SH intensities for the combination of s-input/p-output polarization show fourfold symmetry; however, for s-input/s-output, the SH signals show eightfold symmetry with the rotational angle. Residual strain would induce an enhancement of the isotropic p-output SH component in the Fourier transform coefficient. Finally, the degree of symmetry of the SH signals from these three Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloy surfaces versus the rotational angle was related with the surface pit defects (densities and sizes) and surface roughness.

Optical second-harmonic generation induced by electric current in graphene on Si and SiC substrates

We find that the flow of direct electric current (dc) through graphene on substrate enhances surface optical second-harmonic generation (SHG) from the graphene/substrate system. The current can enhance surface SHG by about 300% for a chemical-vapor-deposition (CVD) graphene monolayer on a SiO${}_{2}/\mathrm{Si}(001)$ substrate, and by about 25% for an epitaxial four-layer-graphene film on a 3.5\ifmmode^\circ\else\textdegree\fi{}-miscut vicinal SiC(0001) substrate. The enhancement in both the CVD and epitaxial graphene samples is due to electric field-induced SHG, which is produced by the current-associated vertical electric field at the SiO${}_{2}/$Si interface or at the graphene/SiC interface. Measurements of rotational-anisotropy SHG (RA-SH) from both samples revealed that the current-induced SHG varies strongly with the measurement location along the current flow direction. By measuring RA-SH from the vicinal SiC(0001) substrate, we determined all three second-order susceptibility tensor elements (${d}_{33}$ = \ensuremath{-}52.0 pm/V, ${d}_{15}$ = 20.0 pm/V, and ${d}_{31}$ = 18.7 pm/V) that characterize the SHG response of hexagonal SiC at the fundamental wavelength of 740 nm. We further determined the three effective susceptibility tensor elements (${d}_{33}$ = \ensuremath{-}135.8 pm/V, ${d}_{15}$ = 18.5 pm/V, and ${d}_{31}$ = 14.6 pm/V) that characterize the surface SHG from the graphene/vicinal-SiC(0001) sample and finally showed that the current-dependent tensor element ${d}_{33}$ can be enhanced to a large value of ${d}_{33}$ = \ensuremath{-}199.0 pm/V by electric current in epitaxial graphene.

Step and Domain Boundary Effect of Surface Reconstruction to Si(111)-√ 3×√3-Ag

The influence of step and domain boundary on growth of Si(111)-√ 3×√3-Ag has been studied in situ using optical surface second-harmonic generation and low energy electron diffraction. The second harmonic intensity shows a difference of about 50% for Si(111) surfaces with different miscut angles and domain boundary densities, although no significant difference has been observed in low energy electron diffraction patterns, indicating a significant impediment to the growth of Si(111)-√ 3×√3-Ag by step and domain boundaries. Simulation results reveal a 90% coverage of Si(111)-√ 3×√3-Ag on the vicinal substrate with an miscut angle of 0.41o, consistent with the dynamics of Ag atoms on Si(111)-7×7 surface. The influence of two dimentional adatom gas on surface structure has also been discussed.

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.

Optical second-harmonic generation and scanning tunneling microscopy study of the self-assembly process of cyanine dyes on Br-Ag(111) substrates

Surface optical second-harmonic generation (SSHG) was used to monitor in situ the adsorption kinetics of cyanine dyes on bromide modified Ag(111) substrates and to probe the structural ordering and orientation of the self-assembled aggregates. The observed SSHG changes are compared to the nanoscopic configuration of the aggregates determined by scanning tunneling microscopy and to the macroscopic degree of the surface coverage that was deduced from optical absorption measurements. Due to substrate-adsorbate interaction, the nonlinear optical response of the surface covered with ordered aggregates is significantly different from the case of disordered adsorption. Our observations demonstrate that SSHG can be used to identify the character of molecular self-aggregation on model silver-halide surfaces.

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.

Axial and Polar Orientational Changes by Rubbing/Photoalignment Processes in a Liquid Crystal Alignment Layer Studied by Optical Second-Harmonic Generation

Molecular reorientation by rubbing and photoalignment processes in polymer alignment layer for liquid crystals has been studied by linear optical absorption and surface optical second-harmonic generation. It was found that rubbing produces polar and axial orders in the photoisomerizable polyamic acid (PAA) containing azobenzene groups in the main chain, while photoalignment after rubbing destroys the polar order but is capable of reorienting the PAA main chain without creating further polar order.

Alignment of liquid crystals on a photosensitive substrate studied by surface optical second-harmonic generation

The adsorption kinetics and orientational ordering of 4(')-n-octyl-4-cyanobiphenyl (8CB) liquid crystal films evaporated onto photosensitive poly(vinyl-cinnamate) (PVCN) substrates were investigated by surface optical second-harmonic generation. The adsorption rate of the first monolayer decreases with increasing degree of photochemical modification of the polymer. The in-plane orientational anisotropy of the 8CB films grown on unidirectionally photopolymerized PVCN substrates is considerably lower than the intrinsic surface orientational anisotropy of these substrates, which can explain the generally found weak surface anchoring of liquid crystals on PVCN alignment layers.

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.

Orientational Changes of Side Chain Nonlinear Chromophore in Polyimide Langmuir-Blodgett Films

The orientational changes in optical nonlinear active side chains of polyamic acid alkyl amine salt monolayers before and after the imidization process were studied by optical surface second-harmonic generation (SHG). The polymeric Langmuir-Blodgett (LB) monolayer composed of the triphenylamino-(p-nitrophenyl)-ethylene group as a side chain of polyamic acid was prepared by the precursor method. The analysis of the experimental results showed that the symmetry of the side chain unit in the monolayer remains C∞v after imidization process. Moreover, the side chain molecular orientational distribution functions (ODFs) of the polymeric monolayer before and after imidization were obtained using the modified maximum entropy method. From the obtained ODFs, it was found that the polar angle of the side chain increases by imidization and this causes the decrement of the nonlinear optical components. However the half-width (Δθ) of the major molecular tilt angle decreases by imidization, suggesting that the distribution of the chromophore is more uniformly packed through the imidization process.

Determination of surface complex nonlinear optical susceptibilities and molecular orientational distribution functions using resonant surface second-harmonic generation

Using the resonant optical surface second-harmonic generation (SHG), we have determined the relative values of the complex nonlinear optical (NLO) components $({\ensuremath{\chi}}_{\mathrm{zzz}},$ ${\ensuremath{\chi}}_{\mathrm{zxx}},$ and ${\ensuremath{\chi}}_{\mathrm{xxz}}$) at isotropic interfaces ${(C}_{\ensuremath{\infty}v})$ of a polymer with SHG active side chains. The introduced configuration of the SHG experiment was a polarizer--rotating quarter wave plate--sample--analyzer. It was shown that this configuration gives information on complex NLO coefficients without using the Kleinmann symmetry. For the experiments, we measured resonant surface SHG from the air-polymer and the substrate-polymer interfaces of a thick polymer film. By theoretically fitting the SHG data, we unambiguously determined the nonlinear susceptibility components at the both interfaces of the polymer film. Moreover, unbiased molecular orientational distribution functions (ODFs) at both interfaces were also determined using the modified maximum entropy method. The obtained ODFs were found to be quite different from the previous ones obtained by assuming the Kleinmann symmetry, indicating the important role of the imaginary part of \ensuremath{\chi}'s played when determining ODFs.

Visualization of the Molecular Shape and the Orientational Distribution in the Langmuir−Blodgett Film of a Bent-Shaped Achiral Molecule As Studied by Surface Second-Harmonic Generation

We have studied the anisotropic molecular orientational distributions of two-dimensional bent-shaped molecular monolayers by optical surface second-harmonic generation (SHG). The molecular monolayer of 1,3-phenylene bis[4-(((4-dodecylphenyl)imino)methyl)benzoate] (12-PIMB) was prepared by the Langmuir−Blodgett (LB) technique. Oblique incident surface SHG measurements from the monolayer showed that the bent-shaped molecules preferably aligned toward the dipping direction. The analysis of the experimental results provided the nonlinear-optical tensor components of the 12-PIMB LB monolayer. Using these components and a simple superposition model, the molecular orientational distribution function was determined. We successfully showed that the distribution map provides a view of the molecular shape.

Correlation between surface and bulk orientations of liquid crystals on rubbed polymer surfaces: Odd-even effects of polymer spacer units.

Using surface optical second-harmonic generation, we have determined the molecular tilt angle and the in-plane orientational order of the liquid-crystal molecules 4'-n-octyl-4-cyanobiphenyl (8CB) on a series of polyimide surfaces. The polyimides differ in the length of the flexible spacers in between neighboring aromatic cores. Using birefringence measurements, we obtained the bulk tilt angle of liquid-crystal cells having the same series of surfaces. The surface molecular tilt angle, the in-plane orientational ordering near the surface, and the bulk tilt angle all exhibit an odd-even effect related to the odd or even number of spacer units in the polyimides. The results are explained by the fact that polyimide surfaces with even numbers of spacer units are smoother and capable of inducing higher surface ordering in the 8CB surface layer. The orientational order at the surface is shown to be intimately correlated with the bulk tilt angle.

Rotationally anisotropic second-harmonic generation studies of the structure and thermal stability of Cu(110)

Rotationally anisotropic surface second‐harmonic generation (SHG) has been used to investigate the surface structure and thermal stability of a clean, well‐characterized Cu(110) single crystal surface in ultrahigh vacuum. The results show clear evidence of the sensitivity of optical SHG to changes in surface structure. At a fixed Cu(110) sample azimuthal angle, the temperature dependence of the second‐harmonic radiation field from the surface was measured at two different wavelengths and found to be in agreement with other experiments performed using very different surface probes that show a sharp change in slope above ∼600 K associated with a possible order–disorder transition. The rotationally anisotropic SH intensity from Cu(110) was found to dramatically decrease as the surface temperature was increased above ∼600 K indicating a loss of surface order and symmetry. A simple model is proposed where thermally induced one‐dimensional disordering accounts for all of the experimental observations between ∼600 and ∼800 K, and where the Cu(110) surface has thermally disordered in two‐dimensions above ∼800 K.

Coverage dependence of surface optical second-harmonic generation from CO/Ni(110): Investigation with a nonlinear-interference technique.

A nonlinear-optical-interference technique in surface second-harmonic generation is developed to measure directly the variation of surface nonlinear susceptibility of CO/Ni(110) as a function of the CO coverage. It is found that some of the susceptibility elements vary linearly with the coverage and others nonlinearly. A simple model suggests that the former are dominated by contributions from nearly free electrons, while the latter by contributions from the more localized electrons.

Adsorbate resonance enhancement in optical surface second-harmonic generation: S on Pd(100)

Optical second-harmonic generation from a fixed-frequency laser at λ = 1064 nm has been used to monitor the adsorption of sulfur atoms on a Pd(100) single-crystal surface. The signal observed was dominated by the contribution from the adsorbate atoms to the second-order susceptibility due to a resonance enhancement from a nearby electronic state of the S atom. This enhancement rendered an adsorbate sensitivity of 0.01 monolayer possible.

Local and nonlocal surface nonlinearities for surface optical second-harmonic generation.

We investigate the physical origins of the surface nonlinear susceptibility responsible for surface optical second-harmonic generation. Experiments performed on simple covalent systems were designed to distinguish between the nonlocal electric-quadrupole-type nonlinearity induced by field discontinuity and the local intrinsic, electric-dipole-type nonlinearity of a surface or interface. We find that both mechanisms could operate: The latter usually dominates when the surface layer has a strong structural asymmetry with the fundamental or the second-harmonic frequency at resonance with a dipole-allowed transition.