Harmonic Generation Technique Research Articles

Concentration Variation of Quadratic NLO Susceptibility in PMMA-DR1 Side Chain Polymer

Chemical synthesis and concentration dependence of two second order NLO tensor components: and was studied for a side chain polymer PMMA-DR1 in thin films by optical second harmonic generation techniques. The noncentrosymmetry was created by the corona poling technique. The results show that for small chromophore concentrations the susceptibility follows linearly the density number of active chromophores, reaches a maximum, than decreases and stabilizes.

A diffraction correction for the nonlinearity parameter measured by the harmonic generation technique.

Practical applications of harmonic generation to determine the nonlinearity parameter Beta generally tend toward the lower drive (fundamental) frequencies. As a result, diffraction effects on Beta become more significant. Derivations of a number of different diffraction correction formulas, which are applied for a piston source and a receiver located some distance away, are in the literature. In the paper by Blackburn and Breazeale [J. Acoust. Soc. Am. 76, 1755–1760 (1984)] the correction formulas were applied to the amplitude of the fundamental frequency and the results on Beta were experimentally tested. By building on this earlier work, a diffraction correction formula that includes a factor from a numerical integration algorithm applied to the harmonically generated wave is provided. The formulation of this algorithm is discussed. Experimental results are given for measurements of Beta in AA 2024 to illustrate the agreement.

Nonlinear acoustics: A transition from laboratory to a practical fatigue assessment tool.

The ultrasonic second harmonic generation technique measuring the nonlinearity parameter has been developed and used in laboratories for several decades throughout the world. Many students and visiting scholars from numerous countries devoted their passions at the physical acoustics laboratories led by Dr. Mack Breazeale for 50 years. During that period of time, nonlinear elastic properties of single crystals, polycrystalline alloys, superconductors, ceramics, and composites were measured. New measurement systems and techniques were also developed to understand temperature dependent linear and nonlinear elastic behaviors of these materials over a temperature range from the liquid helium to Curie temperatures of piezoelectric ceramics. It took years of research and development efforts with a persistent funding before a practical fatigue damage measurement technique was finally developed. The transition from a rack full of equipment to a portable fatigue damage measurement system, a continuous iteration pro...

The Role of Ion Pairs in the Second‐Order NLO Response of 4‐X‐1‐Methylpiridinium Salts

A series of 4-X-1-methylpyridinium cationic nonlinear optical (NLO) chromophores (X = (E)-CH=CHC(6)H(5); (E)-CH=CHC(6)H(4)-4'-C(CH(3))(3); (E)-CH=CHC(6)H(4)-4'-N(CH(3))(2); (E)-CH=CHC(6)H(4)-4'-N(C(4)H(9))(2); (E,E)-(CH=CH)(2)C(6)H(4)-4'-N(CH(3))(2)) with various organic (CF(3)SO(3)(-), p-CH(3)C(6)H(4)SO(3)(-)), inorganic (I(-), ClO(4)(-), SCN(-), [Hg(2)I(6)](2-)) and organometallic (cis-[Ir(CO)(2)I(2)](-)) counter anions are studied with the aim of investigating the role of ion pairing and of ionic dissociation or aggregation of ion pairs in controlling their second-order NLO response in anhydrous chloroform solution. The combined use of electronic absorption spectra, conductimetric measurements and pulsed field gradient spin echo (PGSE) NMR experiments show that the second-order NLO response, investigated by the electric-field-induced second harmonic generation (EFISH) technique, of the salts of the cationic NLO chromophores strongly depends upon the nature of the counter anion and concentration. The ion pairs are the major species at concentration around 10(-3) M, and their dipole moments were determined. Generally, below 5x10(-4) M, ion pairs start to dissociate into ions with parallel increase of the second-order NLO response, due to the increased concentration of purely cationic NLO chromophores with improved NLO response. At concentration higher than 10(-3) M, some multipolar aggregates, probably of H type, are formed, with parallel slight decrease of the second-order NLO response. Ion pairing is dependent upon the nature of the counter anion and on the electronic structure of the cationic NLO chromophore. It is very strong for the thiocyanate anion in particular and, albeit to a lesser extent, for the sulfonated anions. The latter show increased tendency to self-aggregate.

Investigation of ferroelectric properties of strontium barium niobate crystals by second harmonic generation technique

The ferroelectric properties of SrxBa1 − xNb2O6 (SBN-x) crystals have been investigated using noncollinear (diffuse) second harmonic generation of IR radiation converted by a disordered domain structure. The switching characteristics have been determined from the dependences of the second harmonic intensity I2ω(t) and I2ω(E) for the SBN-0.75 crystals. These characteristics are in good agreement with the results of measurements by traditional methods and exhibit a number of specific features (long polarization relaxation times, irreproducibility of the shape of hysteresis loops, low-frequency dependence of the quantity Ec) associated with the relaxor nature of SBN solid solutions. The kinetics of the diffuse second harmonic generation can be described in the framework of the phenomenological approach to polarization relaxation processes in relaxor ferroelectrics. The intensity of the diffuse second harmonic in switching is determined by the parameters (duration, amplitude) of the switching field pulse.

Magnetization reversal process of ferromagnetic granular thin films probed by magnetization-induced second harmonic generation

We propose to use magnetization-induced second harmonic generation (MSHG) technique to explore the magnetization reversal process of cobalt granular thin films. The three-dimensional magnetization reversal process can be visualized by analyzing the polarization dependent MSHG hysteresis curves. It is found that the magnetization reversal process involves not only the in-plane but also the out-plane magnetization rotation because of the uniaxial anisotropy of cobalt granular thin films. This study demonstrates that MSHG can be a versatile optical technique to investigate the magnetic property of not only ferromagnetic epitaxial thin films but also ferromagnetic granular thin films.

Evolution of a femtosecond laser-induced plasma and energy transfer processes in a SiO2 microvolume detected by the third harmonic generation technique

A nonlinear optical method has been proposed for probing the evolution of a plasma and energy transfer processes in the microvolume of a transparent dielectric using the third-harmonic probe signal. The electron self-trapping time has been estimated as t = 150 ± 80 fs. A decrease in the third-harmonic probe signal has been detected at the picosecond time scale; this decrease can be attributed to a change in the third-order susceptibility χ(3) of a fused silica sample modified by laser radiation.

Influence of deviatoric stress on the pressure-induced structural phase transition of ZnO studied by optical second harmonic generation measurements

The pressure-induced B4⇆B1 structural phase transition of ZnO has been studied with the second harmonic generation (SHG) technique. Measurements in nonhydrostatic and hydrostatic pressure transmitting media show slightly different transition pressures (9–11 GPa) and a different pressure dependence of the SHG intensities. These observations are consistent with the presence of a tetragonal and hexagonal intermediate phase as a result of hydrostatic and axial compression, respectively. In contrast to earlier work, it is shown that it is not necessary to use nanocrystalline starting material to be able to recover the B1 phase at ambient conditions.

Phase transitions in Langmuir monolayer of long chain attached fluorescein studied by second harmonic generation technique

Optical second harmonic generation (SHG) measurements coupled with Π-A isotherms have been shown to be helpful, for the following and comprehension of orientational orders and phase transitions in Langmuir monolayers. Using the SHG-(Π-A) measurements, monolayers of 5-hexadecanoylaminofluorescein on the water surface were examined by monolayer compression. The phase transitions were noticeably revealed. Dependence of the square root of the intensities polarizations quotient in the molecules surface density, allowed establishing tilting orientation alignment phases. In addition, change in the monolayer symmetry C ∞ v → C 2 v as it goes through the LE–LC phase transition, was clearly recognized. It was concluded that a possible change in β is taking place due to aggregate formation.

Investigation of ferroelectric properties of bismuth ferrite films by the second optical harmonic generation technique

The specific features of the ferroelectric polarization switching in bismuth ferrite thin films doped with neodymium ions are investigated by the optical second harmonic generation technique. The structure and nonlinear optical properties of the samples prepared are studied in the course of ferroelectric polarization switching in planar geometry over wide ranges of film thicknesses and electric field frequencies.

Generation of intense attosecond x-ray pulses using ultraviolet laser induced microbunching in electron beams

We propose a scheme that combines the echo-enabled harmonic generation technique with the bunch compression and allows one to generate harmonic numbers of a few hundred in a microbunched beam through up-conversion of the frequency of an ultraviolet seed laser. A few-cycle intense laser is used to generate the required energy chirp in the beam for bunch compression and for selection of an attosecond x-ray pulse. Sending this beam through a short undulator results in an intense isolated attosecond x-ray pulse. Using a representative realistic set of parameters, we show that 1 nm x-ray pulse with peak power of a few hundred MW and duration as short as 20 attoseconds (FWHM) can be generated from a 200 nm ultraviolet seed laser. The proposed scheme may enable the study of electronic dynamics with a resolution beyond the atomic unit of time ($\ensuremath{\sim}24$ attoseconds) and may open a new regime of ultrafast sciences.

Comparison of high-order harmonic generation from various cluster- and ion-containing laser plasmas

We apply the technique of high-order harmonic generation from laser-produced plasma to investigate the harmonics generated from a medium that contains an abundance of micron- and sub-micron-sized structures. We compare harmonic efficiencies from the plasma plumes containing silver nanoparticles (110 nm spherical), colloidal silver (blocks of 100–1000 nm), aluminium particles (2–14 µm) and copper oxide nanorods (50 nm thick, few µm long). We compare these results with those obtained using solid bulk targets. We show that plasma media that are abundant with small-sized structures have higher harmonic generation efficiencies. Results also reveal that the harmonic cutoff is not affected significantly by using the nanostructured targets. The maximum conversion efficiency we observe is 4 × 10−5 using silver nanoparticles.

Evaluation of fatigue damage using nonlinear guided waves

This research develops an experimental procedure for characterizing fatigue damage inmetallic plates using nonlinear guided waves. The work first considers the propagation ofnonlinear waves in a dispersive medium and determines the theoretical and practicalconsiderations for the generation of higher order harmonics in guided waves. By usingresults from the nonlinear optics literature, it is possible to demonstrate that both phaseand group velocity matching are essential for the practical generation of nonlinear guidedelastic waves. Next, the normalized acoustic nonlinearity of low cycle fatigue damagedaluminum specimens is measured with Lamb waves. A pair of wedge transducers is used togenerate and detect the fundamental and second harmonic Lamb waves. The results showthat the normalized acoustic nonlinearity measured with Lamb waves is directly related tofatigue damage in a fashion that is similar to the behavior of longitudinal and Rayleighwaves. This normalized acoustic nonlinearity is then compared with the measuredcumulative plastic strain to confirm that these two parameters are related, and toreinforce the notion that Lamb waves can be used to quantitatively assess plasticitydriven fatigue damage using established higher harmonic generation techniques.

Second harmonic generation in nonsymmetrical squaraines: tuning of the directional charge transfer character in highly delocalized dyes

The interest in squaraine compounds, a relatively old class of cyanine-like dyes, has been recently renewed due to their potential usefulness in a large number of technologically relevant fields such as two photon absorbing materials, field effect transistors, solar cells, NIR emitting fluorescent probes and sensitizers for photo dynamic therapy. In the case of symmetric compounds, the squaraine electronic structure is reminiscent of that of symmetric cyanines. In the case of nonsymmetric squaraines the HOMO–LUMO transition is characterized by a charge transfer component responsible for a sometimes sizeable band broadening, also reflected in a relevant second order nonlinear optical behaviour. The aim of the present paper is the design, synthesis and multidisciplinary characterization of a series of nonsymmetrical heterocycle-based squaraines, a study directed towards a general understanding of the deviation from the cyanine limit in this class of dyes. We exploit the electric field induced second harmonic generation technique as a tool for the charge transfer character evaluation and, together with other hints coming from UV-Vis, NMR and cyclic voltammetry, we show how the directionality of the HOMO–LUMO excitation can be characterized and accounted for in terms of the most relevant contribution of canonical structures for the squaraine ground state description. DFT/TDDFT calculations provide further insight to the electronic structure of representative compounds.

Trapping centers engineering by including of nanoparticles into organic semiconductors

This paper reports the characteristics of an organic field-effect transistor (OFET) with silicon nanoparticles (NPs) on a semiconductor-gate insulator interface, which work as trapping centers of charge carriers. Charge transport and injection phenomenon were studied by electrical measurements and optical time-resolved microscopy second harmonic generation (TRM-SHG) technique sensitive to injected carrier distribution and internal electric fields. We found that OFETs with low concentration of intrinsic carriers and operating in terms of injection type OFET are extremely sensitive to the internal electric field. An enormous threshold voltage shift due to trapped charge was observed, and the possibility to adjust it by controlling the NP density was found. We demonstrate that the NP film can serve to design the accumulated charge in OFET and thus change in charge injection time and transport properties. The detailed analysis of pentacene OFET based on dielectric properties and the Maxwell–Wagner model reveals internal electric field created by NPs. Additionally, the effect of NPs is discussed with respect to mobility estimated by electrical and TRM-SHG experiment; its decrease is related to deceleration of carrier propagation by trapping effect.

Diagnostic study of the roughness surface effect of zirconium on the third-order nonlinear-optical properties of thin films based on zinc oxide nanomaterials

Zinc oxide (ZnO) and zirconium doped zinc oxide (ZnO:Zr) thin films were deposited by reactive chemical pulverization spray pyrolysis technique on heated glass substrates at 500 °C using zinc and zirconium chlorides as precursors. Effects of zirconium doping agent and surface roughness on the nonlinear optical properties were investigated in detail using atomic force microscopy (AFM) and third harmonic generation (THG) technique. The best value of nonlinear optical susceptibility χ (3) was obtained from the doped films with less roughness. A strong third order nonlinear optical susceptibility χ (3) = 20.12 × 10 −12 (esu) of the studied films was found for the 3% doped sample.

In-Situ Observation of the Inside-to-Outside Molecular Transport of a Liposome

The first in-situ and real-time observation of the molecular transport from inside to outside of a liposome was shown by using the second harmonic generation (SHG) technique. The transport of an organic cationic molecule across the liposome bilayer could be switched on and off using the structural change of the lipid bilayer caused by temperature change. This approach can be helpful for the understanding and control of the molecular transport in the liposome vehicle.

Incorporation in Langmuir and Langmuir–Blodgett films of symmetric fluorine substituted bent-core liquid crystals: Morphological and optical properties

Molecular mono- and multilayer films of a symmetric fluorine substituted bent-core liquid crystalline (LC) compound with hydrocarbon end-chains were prepared by the Langmuir and Langmuir–Blodgett (LB) technique. Langmuir films were characterized by surface pressure isotherms and Brewster angle microscopy. Likewise LB-films deposited onto amorphous glass substrates were characterized by ultraviolet–visible spectroscopy, the optical second harmonic generation technique and atomic force microscopy (AFM). Results show optimal non-centrosymmetric Z-type LB-film deposition for moderate number of layers ( n < 20); thereafter, an apparent collapse of the Z-type structure occurs, giving rise to a decrease of the nonlinear optical (NLO) properties. AFM, NLO and linear optical measurements suggest a uniform molecular coverage on the glass substrates and anisotropic orientational distribution; where, according to NLO-experimental data, the LC-bent-shaped molecules are preferably aligned toward the dipping direction with the net molecular polarization perpendicularly aligned to the substrate layer. This observation leads us to implement a simplified model based on the monomeric rod-like approximation, in order to estimate the significant NLO-tensor components.

A nonlinear-guided wave technique for evaluating plasticity-driven material damage in a metal plate

This research develops an experimental method for evaluating material damage due to plastic deformation in a metal plate using nonlinear-guided waves. An improved version of a previously proposed measurement technique is used. The material nonlinearities of aluminum specimens loaded to produce different levels of plastic strain are measured with Lamb waves. A pair of wedge transducers is used to generate and detect the fundamental (s1) and second harmonic (s2) Lamb waves. The weak amplitude of the second harmonic wave is extracted from the spectrogram of a received signal. The measured acoustic nonlinearity increases monotonically with the level of plasticity (plastic strain), which is similar to the behavior of longitudinal and Rayleigh waves. This result indicates that Lamb waves can be used to assess plasticity-driven material damage in the established framework of the second harmonic generation technique. The effects and the importance of signal processing in determining the nonlinearity parameter are also discussed.

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.