Harmonic Generation Technique Research Articles

Probing motion of electric dipoles and carriers in organic monolayers by Maxwell Displacement Current and optical second harmonic generation

Probing and modeling of surface polarization phenomena originating from dipoles and electrons in organic materials are studied. We use an electrical measurement that can directly probe orientational dipolar motion in monolayers by Maxwell Displacement Current, and show that a two-dimensional domain shape is dependent on electrostatic energy stored in the domain owing to the polar orientational alignment of permanent dipoles. Also, we demonstrate a novel optical second harmonic generation technique that allows carrier motion in active layers of organic field-effect transistors to be directly probed as the migration of electric field. We conclude that probing of electric field generated from moving charged particles, e.g., dipoles and electrons, at the interface is a common concept for studying dielectric polarization and its related two-dimensional interfacial phenomena in organic materials.

Second harmonic generation in α-Si:H thin films: Thickness dependence

The second harmonic generation (SHG) technique has been applied to investigate the nonlinear optical properties of thin films of hydrogenated amorphous silicon on a glass substrate. SHG measurements have been performed in two transmission experimental schemes. Modeling of a film as a three-layer structure and considering the interference and absorption at the SH wavelength have been carried out. The dependence of the nonlinear susceptibility on the film thickness was used to separate the bulk electric dipole contribution.

Effect of pendant group on the second-order optical nonlinearity of sol–gel films

A series of organically modified sol–gel films with various acceptor groups were prepared and characterized. All the chromophores exhibit much larger microscopic optical nonlinearity compared with the classical chromophore DR1 in solvatochromic measurement. Using in situ second harmonic generation (SHG) technique, the optimal poling temperatures (Topt) for sol–gel films were obtained. The second harmonic coefficients (d33) of hybrid films at the wavelength of 1,064 nm were in the range of 50.1–70.3 pm/V after corona poling under their Topt. The NLO stabilities of these poled films were also investigated by tracing the d33 value as a function of temperature and time. One of the hybrid films, which was prepared from chromophore 2,4-dinitro-4′-(N,N-dihydroxyethyl) aminoazobenzene exhibited a combination of large optical nonlinearity and high NLO stability. Furthermore, the effects of molecular structure on the NLO property and thermal stability of the hybrid films were also discussed.

Optical concept of a compressor for XUV pulses in the attosecond domain

We discuss the phase properties of a double-grating compressor with grazing-incidence gratings in the off-plane mount, designed for the temporal compression of XUV attosecond pulses produced with the technique of high-order harmonic generation. Its purpose is to introduce a negative chirp that compensates for the intrinsic chirp of the pulse. The study is based on the path lengths of the rays at different wavelengths, and their control in order to achieve either positive or negative group-delay dispersion. We demonstrate that the sign and the amount of the dispersion introduced is controlled by a linear translation of a grating. Beside the instrument is expected to present a high throughput, constant along the spectrum of interest. The compressor can be designed for any spectral region in the XUV and soft X-ray domain. As a test case, the applications to the compression of attosecond pulses centered at 70 eV and at 160 eV are discussed.

Nonlinear optical properties of Zn 1− xMg xSe and Cd 1− xMg xSe crystals

Nonlinear optical properties of Zn 1− x Mg x Se and Cd 1− x Mg x Se crystals have been studied using degenerate four wave mixing (DFWM), nonlinear transmission (NLT), second harmonic generation (SHG) and third harmonic generation (THG) methods by Nd:YAG laser operating at 532 nm and 1064 nm, respectively. The studied Zn 1− x Mg x Se and Cd 1− x Mg x Se crystals were grown from the melt by the modified high-pressure Bridgman method. The quadratic nonlinear optical properties of ternary crystals were investigated by second harmonic generation (SHG) technique. We found that the highest value of the second order nonlinear optical susceptibility was obtained for Cd 0.7Mg 0.3Se. In the case of cubic nonlinear optical properties we used third harmonic generation (THG) and degenerate four wave mixing (DFWM) methods. We found that the values of third order nonlinear optical susceptibility for Cd 1− x Mg x Se crystals are higher than for Zn 1− x Mg x Se ones.

Microstructural Degradation Assessment in Pressure Vessel Steel by Harmonic Generation Technique

The harmonic generation technique has been used to characterize the isothermal degradation of pressure vessel steel. The isothermal degradation was conducted at 630°C with forged 2.25Cr-1Mo steel. The variation in the normalized ultrasonic nonlinearity parameter (β=β0) was interpreted as having resulted from microstructural evolution, which was supported by the results of electron microscopy and X-ray diffraction, in addition to Vickers hardness and ductile-brittle transition temperature. The normalized ultrasonic nonlinearity parameter increased progressively with the isothermal degradation time due to the increase in the volume fraction of equilibrium M6C carbide and the variation in the lattice parameter of M23C6 carbide. It was found that the nonlinearity parameter was very sensitive to the microstructure during the isothermal degradation of 2.25Cr-1Mo steel. The harmonic generation technique has the potential to assess microstructural changes due to isothermal degradation.

Third-order Nonlinear Optical Properties of Regioregular Poly(3-Hexylthiophene) Thin Films on Quartz Glasses Modified by Hexamethyldisilazane

Regioregular poly(3-hexylthiophene) [RR-P3HT] thin films were prepared on fused quartz glasses by spin-coating and drop-casting from the chloroform solutions. The effect of surface modification of quartz glass by hexamethyldisilazane (HMDS) on the RR-P3HT film structures was investigated. Film structures and morphologies were characterized by UV-visible absorption spectra, out-of-plane X-ray diffraction (XRD) and atomic force microscopy (AFM). The third-order nonlinear optical (NLO) susceptibilities Χ(3)(-3ω; ω, ω, ω) of the RR-P3HT films were determined by the third harmonic generation (THG) technique with an incident wavelength of 1064 nm. The drop-cast film shows enhanced Χ(3) of about three times of that of the spin-coated one, and HMDS treatment can increase the Χ(3) further to about 30 percent higher than the untreated one, keeping all the film thicknesses around 140 nm These results revealed the deposition method and surface modification effects on the RR-P3HT film structures, and the importance of higher ordering (enhanced inter-molecular interactions through π-π stacking) and increased crystallinity to the enhancement of the Χ(3) of the polymeric films for their applications in NLO devices.

Evaluation of plasticity driven material damage using Lamb waves

This letter reports on the experimental observation of a direct correlation between the acoustic nonlinearity measured with Lamb waves and the level of plasticity in a metal specimen. This correlation implies that even though Lamb waves are multimodal and dispersive, they will interact with a material’s plasticity in a manner similar to longitudinal and Rayleigh waves; there is a fundamental relationship between material plasticity and acoustic nonlinearity that is independent of wave type. As a result, Lamb waves can be used to quantitatively assess plasticity driven material damage using established higher harmonic generation techniques.

NLO properties of functionalized DNA thin films

In this paper we investigate the third-order nonlinear optical properties of spin deposited thin films of DNA-based complexes using the optical third harmonic generation (THG) technique at a fundamental wavelength of 1064 nm. We found that the third-order susceptibility, χ (3)( − 3 ω; ω, ω, ω), of DNA-based films was about one order of magnitude larger than that of our reference, a pure silica slab. In thin films doped with 5% of the chromophore disperse red 1 (DR1), a two order of magnitude larger value of χ (3)( − 3 ω; ω, ω, ω) was observed.

Inclusion of liquid crystalline azo-dyes in nanometric porous anodic aluminas: A comparative morphological and optical study

Prepared amorphous porous anodic alumina with average nanometric porous dimensions of 67 nm in diameter and 50 μm in depth was successfully used as host network for different liquid crystalline azobenzene compounds bearing oligo(ethylene glycol) segments. Since the implemented optical chromophores were specifically designed for quadratic non-linear optical applications, the obtained ordered hybrid structures were fully characterized according to the second harmonic generation technique following the Maker-Fringes method. Comparative linear optical absorption, Raman and X-ray photoelectron spectroscopic investigations were also carried out in order to evaluate the optical performance of these molecular systems within the 1D-nanotube-like confinement. Finally, atomic force microscopic studies of both the dissolved barrier layer and the porous cap were performed on the samples in order to explore surface morphology and assure best possible insertion of these azo-dyes within the pores of anodic alumina.

Harnessing Attosecond Science in the Quest for Coherent X-rays

Modern laser technology has revolutionized the sensitivity and precision of spectroscopy by providing coherent light in a spectrum spanning the infrared, visible, and ultraviolet wavelength regimes. However, the generation of shorter-wavelength coherent pulses in the x-ray region has proven much more challenging. The recent emergence of high harmonic generation techniques opens the door to this possibility. Here we review the new science that is enabled by an ability to manipulate and control electrons on attosecond time scales, ranging from new tabletop sources of coherent x-rays to an ability to follow complex electron dynamics in molecules and materials. We also explore the implications of these advances for the future of molecular structural characterization schemes that currently rely so heavily on scattering from incoherent x-ray sources.

Linear and non-linear optical properties of 2,5-disubstituted pyrroles supported by a catalyst-free SiO 2 sonogel network

The preparation and optical characterization of pyrrole based sol–gel hybrid materials generated by ultrasonic irradiation (Sonogel composites) are presented in this work. Pyrrole compounds were recently synthesized in our group by a modification of the Schulte–Reisch reaction; these molecular systems were dissolved at different concentrations in tetrahydrofuran (THF) and optimally embedded into a catalyst-free SiO 2 sonogel network. For this purpose, we exploited the novel catalyst-free (CF) sonolysis route to produce highly pure sol–gel glasses, generated via sonochemical reactions. This approach has been recently developed in our research group and has been successfully implemented to develop several hybrid composites for optical applications. By this method, homogeneous and stable solid-state hybrid samples suitable for optical characterization can be produced. The high porosity exhibited by the sonogel matrix allowed us to prepare several pyrrole doped composites with variable dopant concentration. The linear and nonlinear optical (NLO) properties of these amorphous hybrid structures were determined by absorption- and photoluminescent (PL)-spectroscopies, and by the optical third harmonic generation (THG) techniques, respectively. The implemented catalyst-free sonolysis route produced SiO 2-host networks of high chemical and optical purity, suitable for optical and photonic applications.

Oxide interface studies using second harmonic generation

We report experiments using a non-invasive second harmonic generation (SHG) technique to characterize buried Si/SiO 2 interfaces and also SIMOX thin film silicon-on-insulator (SOI) wafers. The measurements demonstrate that the SHG response can provide an indication of the quality of the buried oxide interfaces, by providing information on surface roughness, strain, defects, and metallic contamination. The potential application of SHG for comprehensive buried interface characterization and as a non-contact metrology tool for process control is described.

Second Harmonic Generation as a Probe of Multisite Adsorption at Solid−Liquid Interfaces of Aqueous Colloid Suspensions

Chemistry at the surface of solid particles in colloidal dispersions is important in the description of such diverse topics as nutrient and contaminant flow in the environment and the creation of better paints and cosmetics. Particles in these systems are often characterized by molecular-level, surface heterogeneity. This molecular heterogeneity influences many aspects of the chemical processes of interest, among the simplest of which is physisorption. In this study we construct adsorption isotherms for the triphenylmethane dye Malachite Green on various latex particles using the flow through Second Harmonic Generation (ftSHG) technique. This technique makes it possible to construct significantly higher resolution isotherms than conventional separation methods. Here we chose both particles that we expect to be chemically homogeneous (plain polystyrene) and particles whose surface we expect to have molecular level heterogeneity (carboxylated and hydroxylated polystyrene). We find that, using the ftSHG isotherms, we are able to quantify the adsorption energies and site densities of multiple adsorption sites on the carboxylated and hydroxylated polystyrene particles but find only a single type of adsorption site on the plain polystyrene. We expect the ability to measure adsorption site energy and density on chemically heterogeneous latex colloids will be useful in the creation of better biosensors and that the application of ftSHG to other chemically heterogeneous colloids should provide insight into a variety of natural and engineered processes.

Third harmonic generation in undoped and X doped ZnO films (X: Ce, F, Er, Al, Sn) deposited by spray pyrolysis

There is a current interest in research of wide band gap semiconductor materials for the purposes of third order nonlinear optical properties in view of optoelectronics applications. Materials for nonlinear optics should present important changes of nonlinear intensity, dependence on changes of nonlinear refractive index, short response time, and weak absorption losses. We report the results of the third order nonlinear optical susceptibilities of undoped and doped (cerium, fluorine, erbium, aluminum, and tin) zinc oxide films using the third harmonic generation technique at 1064 nm wavelength region in picoseconds regime. Thin films were grown on glass substrate by the spray pyrolysis technique at different temperatures of substrates and characterized by using the x-ray diffraction, scanning electron microscope, transmission, and photoluminescence. A strong third harmonic signal was obtained from the studied films with a good crystallinity and roughness. We have found that at high conductivity, there is a big conversion of the third harmonic signal at different dopants and at an appropriate concentration. We might say that the morphology and the crystalline quality of the films are the main factors for this high conversion.

Nanocrystalline SrTiO3:Eu3+ and BaTiO3:Eu3+: fluorescence spectroscopy and optical studies of structural phase transitions

AbstractThe results of optical studies of Eu3+ ions doped nanocrystalline materials which undergo structural phase transitions: SrTiO3:Eu3+ and BaTiO3:Eu3+ are presented. Nanocrystalline SrTiO3 and BaTiO3 samples with crystallite sizes about 25–50 nm were produced by the sol‐gel process. The Eu3+ 5D0,1 → 7Fn fluorescence spectra were studied. Time‐resolved fluorescence spectra enabled to separate different kinds of Eu3+ centers in BaTiO3 samples. Fluorescence spectroscopy of probe Eu3+ ions was used to detect the structural Oh – D4h phase transition in nanocrystalline SrTiO3. The Eu3+ excited states lifetimes were studied in a wide temperature range and the effects of modified refractive index of the media on radiative transitions were observed. The study of ferroelectric C4v – Oh phase transition in nanocrystalline BaTiO3 with optical second harmonic generation technique revealed remarkable hysteresis in the temperature dependence of the second harmonic intensity. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characterization of 2-amino-5-chlorobenzophenone (2A-5CB) single crystals

Organic nonlinear optical single crystals of 2-amino-5-chlorobenzophenone (2A-5CB) were grown in ethanol by slow solvent evaporation technique. The grown crystals were characterized by single-crystal XRD, FTIR, FT-Raman and UV–vis–NIR techniques. The UV–vis–NIR spectrum ascertains the cut-off wavelength of the sample as 390 nm. The powder second harmonic generation (SHG) technique reveals that 2A-5CB crystal has its SHG efficiency nearly three times that of KDP. The dielectric response of the sample was studied in the frequency region of 50 Hz–1 MHz at varying temperatures. The photoconductivity studies indicate that the 2A-5CB crystal exhibits negative photoconductivity. TGA–DTA studies confirm the melting point of the sample as 101.5 °C.

Second harmonic generation analysis in hydrogenated amorphous silicon nitride thin films

Surface and interface electronic properties of plasma-deposited hydrogenated amorphous silicon nitride films have been investigated by means of optical second harmonic generation (SHG) technique. Polarization analysis shows that the nonlinear field origins from isotropic interfaces (film/substrate interface and film/air interface) whose spectral features are ascribed to surface/interface Si dangling bonds and strained Si–Si bonds. Differences and similarities with SHG spectra of pure amorphous silicon (a-Si:H) are discussed in terms of compositional inhomogeneities of the ternary alloy.

Optical second harmonic generation study of the ferroelectric phase transition in crystals and nanocrystals of lithium heptagermanate

The D2h-C2v ferroelectric phase transition in crystals of lithium heptagermanate Li2Ge7O15 was revealed by the optical second harmonic generation technique. The phase transition temperature Tc was observed to shift from 10 to 6°C when the samples were doped with chromium (0.05%). No second harmonic signal was detected in the ferroelectric phase of nanocrystalline Li2Ge7O15 samples.

Probing the Interfacial Structure of Aqueous Electrolytes with Femtosecond Second Harmonic Generation Spectroscopy

The existence of polarizable anions at the outermost layer of electrolyte solutions has received much recent attention from both theory and experiment, but remains controversial. Anions can be probed directly in the UV via their strong charge-transfer-to-solvent (CTTS) transitions. We have recently described experimental characterizations of enhanced concentrations of several anions at the air-water interface, using the surface-specific technique of second harmonic generation. Here we present a detailed description of the experimental design and methodology used in these experiments, as well as a proof of principle experiment with the known surfactant tetrabutylammonium iodide (TBAI), yielding surface enhancements in excellent agreement with surface tension measurements. Furthermore, we analyze the observed increase in the nonresonant contribution to the SHG response from the water background of alkali halide solutions. The observed change in the water structure of alkali halide (except iodide) solutions is linear in concentration and correlates with the fractional saturation concentration of the salt and with the ionic volume fraction. Finally, the surface adsorption of iodide at high bulk concentrations is analyzed, but it is not possible to differentiate between a Gibbs free energy of adsorption of zero (surface concentration proportional to the bulk) or -0.8 kcal/mol, as predicted by recent molecular dynamics simulations.