Mechanism Of Second Harmonic Generation Research Articles

Mechanism of second harmonic generation of photovoltaic grid-connected system faults and its impact on transformer protection

The short-circuit transient characteristics of photovoltaic(PV) power system are different from traditional power sources. When the grid voltage drops, the output current of PV will contain a high proportion of second harmonics which cannot be ignored. This paper takes a single-phase turn-to-turn short-circuit fault inside the PV grid-connected transformer as an example. First, it analyzes the mechanism of single-frequency input and double-frequency output of a three-phase grid-connected inverter based on a phase-locked loop (PLL), and secondly, considering the comprehensive grid-connected control strategy, the principle of current second harmonic in the minor faults is analyzed and the analytical expression of second harmonics is derived. It is pointed out that the transient dc component will produce second harmonic component in the grid-connected current through control system of the inverter. This current induces a second harmonic voltage in the impedance, and as a disturbance term it generates a new harmonic current through the control link. Superimposed second harmonic currents in two scenarios may threaten the reliability of transformer differential protection. Finally, the correctness of the theoretical analysis is verified by simulation based on MATLAB/Simulink.

Aerodynamic Sound Production from Flying Beetles

The flapping wings of insects generate sounds during their flight. Recent interdisciplinary studies have extensively investigated the lift and thrust mechanisms produced from flapping flight; however, the associated sounds produced are less understood. Most studies have examined the amplitude and frequency with respect to wing beat reporting on the associated harmonic structure. The directionality and phase relationships have been examined in more detail in a few recent studies for flies and mosquitoes. In this study, we examine the sound generation mechanism from two invasive beetle species to Hawaii which are the Oriental Flower Beetle and the Coconut Rhinoceros Beetle. The wing beat, amplitude, and phase relationships are measured and determined with an eight element spherical microphone array. The mechanism of second harmonic generation for the different species is investigated with adaptive signal processing (Empirical Mode Decomposition) and complementary high speed optical video. The rotational motion of wing has a significant role in the second harmonic for the Coconut Rhinoceros Beetle. The different location and phase oscillation of the elytron for the two species results in different vortex generation during the down stroke.

Second harmonic generation at fatigue cracks by low-frequency Lamb waves: Experimental and numerical studies

Abstract This paper presents experimental and theoretical analyses of the second harmonic generation due to non-linear interaction of Lamb waves with a fatigue crack. Three-dimensional (3D) finite element (FE) simulations and experimental studies are carried out to provide physical insight into the mechanism of second harmonic generation. The results demonstrate that the 3D FE simulations can provide a reasonable prediction on the second harmonic generated due to the contact nonlinearity at the fatigue crack. The effect of the wave modes on the second harmonic generation is also investigated in detail. It is found that the magnitude of the second harmonic induced by the interaction of the fundamental symmetric mode (S 0 ) of Lamb wave with the fatigue crack is much higher than that by the fundamental anti-symmetric mode (A 0 ) of Lamb wave. In addition, a series of parametric studies using 3D FE simulations are conducted to investigate the effect of the fatigue crack length to incident wave wavelength ratio, and the influence of the excitation frequency on the second harmonic generation. The outcomes show that the magnitude and directivity pattern of the generated second harmonic depend on the fatigue crack length to incident wave wavelength ratio as well as the ratio of S 0 to A 0 incident Lamb wave amplitude. In summary, the findings of this study can further advance the use of second harmonic generation in damage detection.

Intrinsic exciton-state mixing and nonlinear optical properties in transition metal dichalcogenide monolayers

Optical properties of transition metal dichalcogenides monolayers are controlled by the Wannier-Mott excitons forming a series of $1s$, $2s$, $2p$,... hydrogen-like states. We develop the theory of the excited excitonic states energy spectrum fine structure. We predict that $p$- and $s$-shell excitons are mixed due to the specific $D_{3h}$ point symmetry of the transition metal dichalcogenide monolayers. Hence, both $s$- and $p$-shell excitons are active in both single- and two-photon processes providing an efficient mechanism of second harmonic generation. The corresponding contribution to the nonlinear susceptibility is calculated.

Dynamical centrosymmetry breaking — A novel mechanism for second harmonic generation in graphene

We discover an unusual phenomenon that occurs when a graphene monolayer is illuminated by a short and intense pulse at normal incidence. Due to the pulse-induced oscillations of the Dirac cones, a dynamical breaking of the layer’s centrosymmetry takes place, leading to the generation of second harmonic waves. We prove that this result can only be found by using the full Dirac equation and show that the widely used semiconductor Bloch equations fail to reproduce this and some other important physics of graphene. Our results open new windows in the understanding of nonlinear light-matter interactions in a wide variety of new 2D materials with a gapped or ungapped Dirac-like dispersion.

Second-order nonlinear optical properties of Ge-Ga-Ag-S glass irradiated by electron beam

Ge-Ga-Ag-S chalcogenide glasses with the composition Ge 30Ga 3Ag 4S 63 were obtained by the conventional melt-quenching method. According to the visible-infrared and infrared spectra, Ge 30Ga 3Ag 4S 63 chalcogenide glass possesses wide transmittance window from 510 nm in the visible region up to 11.5 μ in the infrared region. And the present glass has better glass-forming ability (the difference between glass transition temperature and the peak temperature of crystallization is larger than 100 °C). Utilizing maker-fringe technique, a prominent second-harmonic generation was observed in Ge 30Ga 3Ag 4S 63 chalcogenide glass after irradiated by an electron beam (Accelerating voltage: 25 kV; Irradiating current: 15 nA; Irradiating time: 10 min). And the mechanism of second-harmonic generation in the Ge-Ga-Ag-S system glasses was discussed.

Growth mechanism of second harmonic generation of optically poled disperse red 1∕polymer films

The writing process in the formation of χ(2) gratings for disperse red 1 (DR1)∕polymethylmethacrylate film at room temperature is investigated. The writing laser power and film thickness dependence for the growth of the second harmonic generation (SHG) intensity in optically poled films is delineated. The effective growth rate of the SHG intensity is shown to be proportional to the fourth power of the fundamental laser intensity. This result is not in agreement with the existing theory. A model to account for the experimental result associated with the writing process is proposed. The dependence of the induced SHG intensity on the film thickness dependence is also studied. An equation that can adequately describe the dependence on the fundamental laser intensity and the film thickness for the effective growth rate of the SHG intensity is provided.

Laser-Pulse-Induced Second-Harmonic and Hard X-Ray Emission: Role of Plasma-Wave Breaking

We report time resolved measurements of second-harmonic and hard x rays emitted during the interaction of an intense laser pulse (10(16) W cm(-2), 100 fs) with a preplasma generated on a solid target. We observe that for a particular length scale the second harmonic goes through a minimum, while hard x-ray emission on the contrary maximizes. Theoretical or numerical modeling of this anticorrelation in terms of wave breaking of strongly driven electron plasma waves clearly brings out hitherto unexplored links between the physical mechanisms of second-harmonic generation and hard x-ray emission.

Band-resolved analysis of nonlinear optical properties of crystalline and molecular materials

Through three case studies, plane wave pseudopotential density functional theory calculations are performed to investigate the mechanism of second harmonic generation (SHG) of the nonlinear optical materials. We want to know how SHG coefficients are affected by: (1) anion substitution in $\mathrm{AgGa}{({\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x})}_{2}$; (2) the isomeric effect in push-pull benzenes; and (3) the length of $\ensuremath{\pi}$-conjugate chain in push-pull polyenes. A sum-over-states type formalism is used for the evaluation of static SHG coefficients. Orbital contribution to the $\ensuremath{\beta}$ or ${\ensuremath{\chi}}^{(2)}$ can be decomposed on a (energy) level-by-level basis by partially summing only two out of all three band indices. Through such process that we proposed, the dominant orbitals that give major contribution to a SHG process can then be identified and analyzed. These so-called ``band-resolved'' plots of SHG coefficient are sensitive to the variation of structure and configuration of the materials, and can thus be used to study the detailed mechanism of SHG processes in crystals and molecules.

Second harmonic generation in plasma in the presence of SBS

The mechanism of second harmonic generation in the presence of intense-radiation SBS in plasma is considered under the condition that the incident beam has no time for filamentation. The mechanism is based on the formation of low-frequency plasma-density perturbations that are optimal for this effect in the field of scattered waves. The theoretical calculations are compared with the experimental data.

Mechanisms of second-harmonic generation in one-dimensional periodic media

A theory of second-harmonic generation in spatially periodic media is developed. This theory takes account of distortions of the atomic wave functions in the field of an external electromagnetic wave and also of the spatial dispersion. The results of experiments on second-harmonic generation in one-dimensional periodic media are discussed. The angular and polarisation dependences of the reflected second harmonic are calculated. A comparison of the experimental results with the theoretical calculations shows that they are in good agreement.

熱ポーリングした石英ガラスの第2高調波発生の機構

The mechanism of second-harmonic generation (SHG) was investigated for the thermal poled fused quartz and synthesized silica glasses. The current was measured for silica glasses during the thermal poling process. As a result, fused quartz with a relatively higher current generated SHG. We discussed the relationship between crystallization from glass to crystal and second-order nonlinearity, based on a continuous-heating transformation (C-H-T) diagram and the sample temperature, which was calculated from the Joule’s heat of the poling process. It was observed that grain size of the α-quartz and α-cristobalite crystal was about 20 nm at the depth at 5 μm from the anodic surface in the poled fused quartz (553 K, 4 kV, 103.56 s), by a field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD) analysis. The distribution of intensity of the second-harmonic (SH) signal was measured in a layer of SHG near the anodic surface. As a result the mechanism of SHG was deemed due to crystallization from silica glass to the orientated α-quartz possessing second-order nonlinearity under a high electric field and Joule’s heat in the thermal poling condition. It was considered that the intensity of SH signal was higher with increasing quantity of orientated nano-crystals, but was lower with decreasing orientation of the nano-crystals, but was lower with decreasing orientation of the nano-crystals, along with decreasing electric field.

Microwave-range determination of the autocorrelation function of the intensity of ultrashort light pulses

A new method for determination of the autocorrelation function of the intensity of ultrashort laser pulses is proposed and implemented. The method is based on measurement of this function in the microwave range. The use of microwave radiation extends the spectral limits of the correlation method to the VUV and IR parts of the spectrum. The method can be used in those cases when the mechanism of second-harmonic generation is inoperative or inefficient.

The influence of 1064-nm light on x(2) gratings at 790/395 nm in Tm-doped fibers

We have investigated the influence of Nd: YAG laser light at 1064 nm on the growth and erasure of x (2) gratings using 790/ 395-nm light in Tm-doped fibers. The addition of 1064-nm light to the writing pair of 790/395 nm was observed to cause at most a slight increase in writing speed in a low-Tm (4 ppm) fiber; Nd: YAG irradiation was furthermore observed not to erase a previously written 790/395-nm grating. In a high-Tm (600 ppm) fiber, however, the growth rate of a 790/395-nm grating decreased substantially when 1064-nm light was added during writing. Also, exposure of a previously written grating at 790/395 nm to 1064-nm light caused a rapid drop in second-harmonic generation (SHG). These observations are determined to result from Nd: YAG-induced photodarkening, which can be partially reversed by prolonged exposure to 790-nm light. These results imply either that intense Nd: YAG irradiation does not produce mobile electrons or that the mechanism of second-harmonic generation in these fibers does not involve static macroscopic electric fields.

Relationship between the anisotropy of reflected second harmonic radiation and the orientation of the crystal surface

The anisotropic properties of the reflected second harmonic are analyzed for an arbitrary orientation of the crystal surface relative to the crystallographic coordinate system for crystals of the symmetry classes m3m and 432. It is shown that misorientation of the surface relative to the (001) face by an angle of up to a few minutes may be controlled by a nonlinear optical method. It is established that polarization ss forbiddenness may apply under conditions of the quadrupole mechanism of second harmonic generation in crystals of the symmetry classes m3m and 432.

Theory of second-harmonic generation in an inhomogeneous hot plasma

The authors have developed a consistent theory concerning the generation of a second harmonic when an electromagnetic wave is incident on a weakly inhomogeneous isotropic plasma, and have studied the dependence of the effect on plasma temperature. They point out the qualitative difference between the mechanisms of second harmonic generation in cases involving low and high temperatures and indicate the oscillatory nature of the intensity of second-harmonic emission. Attention is drawn to the strong enhancement of the effect when the density profile has inflection or extremum points. The influence of changes in density profile due to the high-frequency pressure of the incident wave on the generation of the second harmonic is discussed. The significance of second-harmonic generation in the laser-power heating problem is also discussed.

Mechanism of Second Harmonic Generation of Optical Maser Beams in Quartz

This paper is concerned with the experimental determination and the interpretation of the relative magnitudes of the two independent coefficients, ${d}_{11}$ and ${d}_{14}$, that appear in the tensor which describes the symmetry of second harmonic generation (SHG) of optical maser beams in quartz. Experimental data on these co-efficients aid in determining the physical process involved in optical SHG. Data have been obtained for both ruby (6934 \AA{}) and CaW${\mathrm{O}}_{4}$: ${\mathrm{Nd}}^{+3}$ (10 582 \AA{}) unfocused laser beams. These experiments, which failed to give any evidence of SHG due to ${d}_{14}$, show that $\frac{{d}_{14}}{{d}_{11}}<\frac{1}{30}$ for the ruby laser, and $<\frac{1}{40}$ for the Nd laser. The result ${d}_{14}\ensuremath{\ll}{d}_{11}$ shows that the mechanism involved in SHG in quartz is nearly lossless and dispersionless at the frequencies of the laser beams and their second harmonics. This further shows that in quartz the linear electro-optic effect and optical SHG cannot be due to the same mechanism. It is concluded that optical SHG in this material is due to a high-frequency electronic mechanism.