Surface Second Harmonic Generation Intensity Research Articles

Restoring the silenced surface second-harmonic generation in split-ring resonators by magnetic and electric mode matching.

Surface second-harmonic generation (SHG) in plasmonic metal nanostructures provides a promising approach to design compact and ultrafast nonlinear nanophotonics devices. However, typical plasmonic nanostructures, such as those with tiny gaps that provide strong near-field-amplified nonlinear sources, often suffer from the cancellation of nonlinear fields in the gaps, which results in the so-called silenced SHG and consequently attenuates the overall nonlinear conversion efficiency. In this study, we propose and demonstrate that the silenced SHG in a gold split-ring resonator can be effectively restored by carefully tailoring its gap geometry to avoid the cancellation of nonlinear fields in the gap and simultaneously achieve both spatial and frequency mode matching between the magnetic and the electric dipolar resonances. As a result, the effective nonlinear sources in the gap can be dramatically amplified and the surface second-harmonic emissions can be efficiently coupled out, leading to an SHG intensity enhancement of 7 times compared to a conventional split-ring resonator. The overall SHG conversion efficiency can thus be enlarged to about 1.49 × 10-8 in the near-infrared excitation region. Importantly, the restored surface second-harmonic emission exhibits the scattering characteristics of an ideal electric dipole, which can be very useful for nonlinear far-field manipulation such as beam steering and holograms.

Second-harmonic generation from supported carbon nanotube films grown by chemical vapor deposition on fused silica

Surface second-harmonic generation (SHG) is shown to be observable for carbon nanotube (CNT) films grown by CVD on fused silica. The SHG intensity ratio for incident radiation polarized perpendicular and parallel to the plane of incidence at the fundamental radiation (1064 nm) is observed to have typical values for the two kinds of grown film: a film with aligned CNTs and a film with randomly oriented CNTs lying on a substrate sheet. SHG is discussed to originate not from the CNT bulk but from the film surface. The polarization ratio obtained for the two kinds of film was consistent with that calculated for a composite film of spheroidal particles with varying diameters employed to approximate the actual CNT films. The present findings may be applied to in situ evaluation of the initial stage in the CVD growth process of CNTs.

Relative SHG measurements of metal thin films: Gold, silver, aluminum, cobalt, chromium, germanium, nickel, antimony, titanium, titanium nitride, tungsten, zinc, silicon and indium tin oxide

We have experimentally measured the surface second-harmonic generation (SHG) of sputtered gold, silver, aluminum, zinc, tungsten, copper, titanium, cobalt, nickel, chromium, germanium, antimony, titanium nitride, silicon and indium tin oxide thin films. The second-harmonic response was measured in reflection using a 150fs p-polarized laser pulse at 1561nm. We present a clear comparison of the SHG intensity of these films relative to each other. Our measured relative intensities compare favorably with the relative intensities of metals with published data. We also report for the first time to our knowledge the surface SHG intensity of tungsten and antimony relative to that of well known metallic thin films such as gold and silver.

Study of the interfacial array behaviour of surfactant at very low concentration by second harmonic generation and molecular dynamic simulation

The array behaviour of anionic surfactant sodium dodecylbenzenesulfonate (SDBS) on the air/water interface was investigated using non-resonant surface second harmonic generation (SHG) and molecular simulation. It was found that the SHG intensities changed with surfactant concentrations non-monotonously under Critical Micelle Concentration (CMC). The mean orientational angle of SDBS molecule at the air/liquid interface was calculated, and the molecular array behaviour was analyzed by molecular dynamics (MD) simulation. The simulated results agreed with the experimental results very well, which provided the detailed information about the interfacial molecular configuration of surfactant, not only indicating the SHG intensity related to the Ns and r(θ), but also demonstrates that the microscopic information presented by the MD simulation method is reliable. According to the results, the surfactant molecules get clustered even at very low concentration far lower than the CMC.

Surface second harmonic generation from silicon pillar arrays with strong geometrical dependence

We present experimental demonstration and analysis of enhanced surface second harmonic generation (SHG) from hexagonal arrays of silicon pillars. Three sets of Si pillar samples with truncated cone-shaped pillar arrays having periods of 500, 1000, and 2000 nm, and corresponding average diameters of 200, 585 and 1550 nm, respectively, are fabricated by colloidal lithography and plasma dry etching. We have observed strong dependence of SHG intensity on the pillar geometry. Pillar arrays with a 1000 nm period and a 585 nm average diameter give more than a one order of magnitude higher SHG signal compared to the other two samples. We theoretically verified the dependence of SHG intensity on pillar geometry by finite difference time domain simulations in terms of the surface normal E-field component. The enhanced surface SHG light can be useful for nonlinear silicon photonics, surface/interface characterization, and optical biosensing.

Characterization studies on the additives mixed l-arginine phosphate monohydrate (LAP) crystals

l-arginine phosphate monohydrate (LAP), potassium thiocyanate (KSCN) mixed LAP (LAP:KSCN) and sodium sulfite (Na 2SO 3) mixed LAP (LAP:Na 2SO 3) single crystals were grown by slow cooling technique. The effect of microbial contamination and coloration on the growth solutions was studied. The crystalline powders of the grown crystals were examined by X-ray diffraction and the lattice parameters of the crystals were estimated. From the FTIR spectroscopic analysis, various functional group frequencies associated with the crystals were assigned. Vickers microhardness studies were done on {1 0 0} faces for pure and additives mixed LAP crystals. From the preliminary surface second harmonic generation (SHG) results, it was found that the SHG intensity at (1 0 0) face of LAP:KSCN crystal was much stronger than that of pure LAP.

Nucleation studies and surface SHG analysis of l-arginine phosphate monohydrate (LAP) family crystals

The metastable zone width and the nucleation parameters, such as interfacial tension, radius of the critical nucleus and critical free energy change have been estimated for pure l-arginine phosphate (LAP), potassium thiocynate (KSCN) mixed LAP (LAP:KSCN) and sodium sulfite (Na 2SO 3) mixed LAP LAP:Na 2SO 3 single crystals. Pure and additive mixed LAP single crystals are grown by slow cooling technique. The surface second harmonic generation (SHG) analysis is done on (100) face of the grown crystals. The SHG intensity on (100) face of the crystals are measured.

Field localization and enhanced Second-Harmonic Generation in silicon-based microcavities

High-quality amorphous Silicon Nitride (a-Si(1-x)N(x):H) Fabry-Pérot microcavities can show resonant surface Second Harmonic Generation (SHG) effect. We consider two different layouts of planar microcavities with almost identical linear reflectance and show how the structure geometry can strongly affect SHG yield. In particular, a difference of more than one order of magnitude in the SHG intensity is observed when the fundamental beam is tuned at the cavity resonance frequency. We explain this finding on the basis of a theoretical model taking into account the spatial distribution of the electric fields of the pump and harmonic frequencies inside the structure. A satisfactory matching of experimental data with the theoretical model is obtained by considering the source of second-order nonlinearity as limited to surface contributions.

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.

Characterizing Metal Phosphonate Surface Coverage Using Surface Second Harmonic Generation. Evidence for the Coexistence of Ordered and Disordered Domains

We report on the use of surface second harmonic generation (SHG) intensity measurements to understand the molecular details of potentially heterogeneous surface coverage. Using a rigid chromophore with a large second-order nonlinear susceptibility (χ(2)) and an adlayer diluent molecule with negligible nonlinear response, the incidence angle dependence of the SHG intensity was determined as a function of surface chromophore coverage. The SHG data at fixed incidence angle suggest that the average orientation of the adsorbed chromophores does not change significantly with surface coverage, resulting in the expected square-law relationship between SHG intensity and loading density. The form of the SHG data points to the presence of more than one domain in the monolayer. The data can be fit successfully using a two-domain model in which the dominant domain is characterized by chromophores oriented along the surface normal and the minority domain is comprised of a nominally random orientational distribution, with the relative amount of the disordered domain decreasing with increasing fractional chromophore coverage.

Studies on the nucleation, dynamics and structure of the Si(1 1 1)– [formula omitted]-Ag surface using surface second-harmonic generation

We have used surface second-harmonic generation (SHG) to study the dynamics of surface processes during the adsorption of silver at room temperature as well as the formation of the Si(1 1 1)– 3 × 3 -Ag surface at elevated temperatures on Si(1 1 1)-7×7 surface. Further, the temperature and the flux dependence of the nucleation of the Si(1 1 1)– 3 × 3 -Ag phase is also studied. The SHG intensity is found to depend on the density of the two-dimensional adatom gas on the 3 × 3 -Ag surface as a function of temperature and silver flux. The results from the experiments slightly differ depending on the probe wavelength and this has been attributed to the different origins of the SH at the two wavelengths studied here. The polarization sensitivity of SHG to the surface symmetry is used to infer structural changes on the surface. The temperature dependence of the ratio of the SH intensity in the vertical and horizontal polarization directions (extinction ratio) is related to the surface symmetry and its change is interpreted mostly by the variation of the time dependence of the fluctuation in the 3 × 3 -Ag structure. The possibility of finding the structural phase transformation is also discussed.

Surface plasmon effects on surface second harmonic generation during Au nanoparticle deposition onto H–Si(1 1 1)

Deposition of Au nanoparticles from aqueous HF onto H–Si(1 1 1) was studied in situ by surface second harmonic generation (SHG) and ex situ by extinction spectroscopy and non-contact atomic force microscopy (AFM). AFM measurements indicate that the maximum SHG intensities occur at lateral particle diameters of approximately 90–100 nm independent of solution phase composition, but with an intensity that depends on solution phase composition. Employing the evolution of SHG intensity to monitor lateral cluster growth, simultaneous Au deposition and Si oxidation exhibit apparent kinetic reaction orders of 1/2 and zero with respect to HF and Au(CN) 2 −, respectively. These results are similar to those obtained purely from ex situ AFM analysis. The variations in SHG intensity with Au(CN) 2 − concentration can be related to particle nucleation densities. These results demonstrate the utility of SHG as an in situ probe of particle growth.

Surface second harmonic generation studies of the dodecane/water interface: the equilibrium and kinetic behaviour of p-nitrophenol and tri-n-butyl phosphate

The behaviour of p-nitrophenol (PNP) absorbed at the dodecane/water interface and the interaction with tri-n-butyl phosphate (TBP) were studied by surface second harmonic generation (SHG). The polarisation dependence of the PNP SHG signal, which is the same for all concentrations of PNP and TBP, is best described in terms of a limiting weak orientational order. In the presence of TBP, the SHG signal initially increases with PNP concentration, passes through a maximum and then it decreases. At low PNP concentrations, increasing the TBP concentration leads to an increase in the SHG intensity, but for higher PNP concentrations the intensity decreases. The free energies of adsorption for PNP and TBP to the dodecane/water interface were determined using Frumkin and Langmuir isotherms. The kinetics of the interaction between TBP and PNP was observed by monitoring the SHG signal along a liquid–liquid flow cell and found to be first-order at high interfacial coverage with rate constant, k, of 0.1±0.01 s−1.

Photochemistry of O2 on Ag(110) using picosecond ultraviolet laser pulses

Surface second-harmonic generation (SHG) was used to study the photochemical reaction of O2 with Ag(110) using ∼6 ps laser pulses. A clean well-ordered Ag(110) surface produces a strong SHG signal at 580 nm. The SHG signal measured in real time during O2 exposure is sensitive to submonolayer coverages of O2 and to the subsequent exposure of this system to picosecond ultraviolet laser pulses. Upon O2 adsorption at 130 and 200 K, the signal decreases by more than a factor of 2 at saturation coverage. Langmuir adsorption kinetics are used to fit the O2 dosage curves relating the decrease in SHG intensity to the oxygen coverage. Subsequent exposure of the surface dosed at 130 K to 290 nm light of the same pulse width also produces a change in the second-harmonic response. Combined with surface SHG temperature programmed desorption measurements, these results are correlated to the photochemical reaction taking place on the surface in which chemisorbed O2 photodissociates and photodesorbs. The results of the surface photochemical reaction initiated by picosecond UV pulses are compared to those previously studied using continuous wave radiation and are found to have similar overall behavior while possibly showing some new features.