Efficient Wave Generation Research Articles

Spatial and temporal thermal management of a spintronic terahertz emitter

This work presents methods for addressing undesirable thermal effects induced by the pump beam of an oscillator laser to improve the efficiency of a terahertz (THz) spintronic emitter. We explore two approaches: spatial distribution of pump energy using a 2D lens array and temporal modulation of the pump duty cycle. Optimizing the spatial distribution approximately doubles the THz signal by increasing local heat dissipation, delaying the saturation limit. Similarly, temporal spreading of pump pulses by adjusting the duty cycle allows greater thermal relaxation within the heterostructure, enhancing the overall efficiency of THz wave generation.

Study on energy conversion efficiency of wave generation in shake plate mode

In recent years, increasing attention has been paid to factors affecting the efficiency of converting wave energy from kinetic energy using shake plates. This research was based on the volume of fluid method to evaluate the energy conversion efficiency of waves generated in the shake plate mode. The effects of different shake plate parameters and water depths on the kinetic energy conversion efficiency were studied, and the results agreed well with experimental data. Results indicated that the angular velocity of the shake plate had the highest influence on kinetic energy conversion efficiency. In the range of ωx to 1.8ωx, the maximum kinetic energy conversion efficiency was 52.1 %. The influence of the oscillation amplitude of the shake plate on the kinetic energy conversion rate was relatively low. As a result, the kinetic energy conversion efficiency remained at approximately 18.10 % even with changes in oscillation amplitude. The kinetic energy conversion efficiency of the shake plate increased with an increase in water depth, with maximum increase 87.39 % seen between 0.15 and 0.2 m. Our research is expected to serve as a reference for the implementation of efficient wave energy generation systems worldwide.

Theoretical study of influence of laser pulse chirp on terahertz emission characteristics of gas induced by two-color laser field

With the development of terahertz (THz) wave research, the demand for controllable THz sources is increasing. How to obtain the regulated THz waves has been one of the research hotspots and key problem in the field of THz science. There have been researches in which the resulting THz wave is modulated by changing the wavelength, relative phase, energy, or chirp of the laser produced by a two-color laser. In this work, we establish a three-dimensional theoretical model of THz wave generation and subsequent propagation induced by two-color laser. And we investigate the influence of chirp modulation of different laser on THz wave by chirp modulation of the fundamental wave (FW) and the second harmonic wave (SHW) of two-color laser, including THz wave amplitude, THz wave center frequency and spectrum width, and analyze the physical mechanism of related phenomena. At the same time, the effects of different orders of magnitudes of laser chirp parameters (femtosecond and picosecond) and initial phase of laser pulse on THz wave parameters are also studied. The results are shown below. 1) In the two-color laser, the chirp of FW mainly affects the shape of THz wave when the initial phase is unchanged. The chirp modulation of SHW can cause the amplitude of THz wave to change significantly, and affect the center frequency and spectrum width of THz waves. 2) In the case of laser pulse width of femtosecond order, 40 fs is taken as an example. When the chirp parameter is of femtosecond magnitude, the chirp parameter has a great influence on the THz wave generation efficiency of two-color laser filament. At the picosecond magnitude, the chirp parameter has a weak effect on the THz wave energy and mainly affects the phase of the THz wave. 3) The initial phase of the two-color laser can aid in chirp modulation of THz wave to optimize the energy generated. 4) The initial phase of two-color laser can assist in the process of chirped laser modulation of terahertz waves to optimize the energy generated. Our research shows that the chirp modulation, as a controllable parameter, has multiple regulation effect on the properties of radiated THz waves. The related research results provide a new idea and basis for studying the generation and regulation of THz waves.

Efficiency of Magnetohydrodynamic Wave Generation in Weakly Ionized Atmospheres

The generation of Alfvén and slow magnetoacoustic waves in weakly ionized atmospheres by excitation of the charges-only component of the two-fluid (charge and neutral) plasma is shown to be more or less efficient depending on the energy fraction initially allocated to the three stationary flow differential modes, which characterize the inter-species drift. This is explained via detailed analysis of the full 10-dimensional spectral description of two-fluid linear magnetohydrodynamics. Excitation via the velocity of the charges only is found to be very inefficient, in accord with previous results, while excitation via the magnetic field perturbation alone is highly efficient. All 10 eigenvalues and eigenvectors are presented analytically in the high collision frequency regime.

Enhancing THz wave generation in silica nanofibers with Zinc Telluride nonlinear coating

This study investigates the use of Zinc Telluride (ZnTe) as a second-order nonlinear coating to enhance THz wave generation in silica nanofibers. Numerical simulations show that ZnTe coatings can significantly improve THz wave generation efficiency due to their large second-order nonlinear susceptibility and high transparency in the THz frequency range. Specifically, we observe a 2000-fold increase in THz wave generation efficiency with a 100nm thickness ZnTe coating compared to an uncoated silica nanofiber.

Резонаторная ячейка с открытым входом на основе диэлектрической пластины на металлической подложке

Possibility of converting the energy of an incident electromagnetic wave to the energy of a surface wave in a dielectric plate on a metal substrate has been confirmed through numerical simulation. To transfer energy between waves of different structures, a diffraction grating placed on the surface of the plate is used. Energy is retained in the plate through the formation of a surface wave resonator by introducing aflush metal walls along the plate’s ends. Thus, the resonant cell with open entrance has been suggested – the energy enters the cell through the whole lit surface of the plate. It is assumed that the dielectric has small losses, and the diffraction grating on its surface consists of the identical bars, which are made of the same dielectric and have small cross section. The resonance in the form of sharp increase of the standing wave amplitude inside the plate, compared to the incident wave amplitude, and correspondingly increase of the incident wave’s energy absorption by the plate takes place at some grating periods. The external manifestation of resonance is a decrease of specular scattering from the plate: the observed decrease of plate’s RCS in the ray reflection direction was from several times to more than three orders of magnitude under different conditions. Stability of numerical solution of the problem of diffraction on the suggested cell follows from the observed smooth variation of the condition number in the vicinity of even the deepest plate’s resonant RCS reduction. Efficiency of surface wave generation by diffraction gratings of different types or by arranging other periodical inhomogeneities on the surface or within the volume of dielectric can be compared according to the depth of resonance.

Advances in theoretical, experimental and applied studies on second-harmonic generation of ultrasonic Lamb waves

<p indent=0mm>Nondestructive evaluation and testing of material degradation or micro-damages in an early stage are essential for the maintenance of the safety-critical structures. Therefore, it is important to detect defects in a very early stage or even better to detect changes in the microstructure before the defect initiation even occurs. Linear ultrasonic characteristics are not sufficiently sensitive to the kind of microscopic degradation of the material. However, on the other hand, even very small imperfections can produce very significant excess nonlinearity, which can be orders of magnitude higher than the intrinsic nonlinearity of the intact material. The excess nonlinearity is produced mainly by the strong local nonlinearity of microcracks. In order to assess the damage in components at an early stage, the use of nonlinear acoustics has been shown to be the leading and one of the most promising techniques. Second harmonic generation is a typical acoustic nonlinear effect of ultrasonic waves, which can be used to assess the material degradation and detect the micro-defects in their early stages. Furthermore, ultrasonic Lamb wave has been widely used in large-scale industrial structures for health monitoring and material nondestructive testing. Possibility to inspect inaccessible or hidden areas and great cost-effectiveness are the key advantages of ultrasonic Lamb wave-based damage detection approaches. Second harmonic Lamb wave, which combines the advantages of Lamb wave and the high sensitivity of second harmonic waves to micro damage, is drawing increasing attention in recent years. The measurement of second harmonic Lamb waves is investigated as a promising feature extraction technique to characterize material nonlinearity in plate-like structures. However, the second harmonic fields of Lamb waves are much more complex due to the dispersion and multi-mode nature of propagating Lamb waves. In general, the effect of second harmonic generation is often very small and can easily be overlooked experimentally. Consequently, proper mode tuning with physically based feature is essential to enhance the efficiency of second harmonic Lamb wave generation and reception. In experimental work, it is also difficult to observe and measure the second harmonic Lamb waves. Although significant development has been made in theoretical analysis and experimental investigation of second Lamb waves, the use of second harmonic Lamb wave for structural damage assessments is still limited by a series of scientific and technical difficulties. In this paper, the theoretical and experimental advances concerning second harmonic Lamb wave are systematically reviewed. First, theoretical analysis of second harmonic Lamb waves based on nonlinear reflection of acoustic waves is reviewed. Then, a general approach of perturbation method and normal modal expansion for theoretical study of the second harmonic Lamb waves propagation in a plate is introduced. The physical insight of nonlinear feature among different Lamb modes is clarified. As revealed in these pieces of theoretical work, the second harmonic modes of Lamb waves display an accumulative effect when conditions of phase matching and non-zero power transfer from the fundamental mode to the second harmonic mode are satisfied. The conditions for strong nonlinear effect of Lamb wave are discussed. Experimental observation and measurement of second harmonic Lamb waves are reviewed. Cumulative effect and efficiency of second harmonic generation of Lamb waves are discussed. The guideline for choosing the Lamb modes with higher sensitivity to material nonlinearity is provided. Based on the theoretical and experimental studies, the applications of second harmonic Lamb waves for material damage evaluation are summarized. In addition, some obscure understanding and disputed issues during the study in this research area are clarified. Finally, prospective priorities and research trends of second harmonic Lamb waves are discussed.

Study on the Dynamic Soil-Pile-Structure Interactive Behavior in Liquefiable Sand by 3D Numerical Simulation

The dynamic behavior of structures in liquefiable sand exhibits more complicated characteristics, due to the development of excess pore pressure caused by cyclic loading, than that in dry sand. Therefore, it is crucial to accurately predict the soil–pile structure behavior during liquefaction to prevent damage to the structures. In this study, three-dimensional numerical modeling was performed to predict the dynamic soil–pile behavior during liquefaction. To directly simulate pore pressure generation due to soil shear deformation, the Finn liquefaction model was applied and coupled with the Mohr-Coulomb elasto-plastic model. Soil nonlinearity was considered by applying hysteretic damping, and the interface model was applied to simulate various dynamic phenomena between the soil and pile. Simplified continuum modeling was introduced to prevent reflection wave generation and increase analysis efficiency. The applicability of the proposed numerical model was validated using the experimental results. Thereafter, a parametric study was conducted to provide a better understanding of the dynamic behavior of pile foundation during liquefaction. From a series of parametric studies, several important factors that can affect the dynamic pile responses in liquefiable sand were identified. Also, the characteristics of the dynamic soil–pile structure interactive behavior, which are significantly different from each other in liquefied and dry sand, were analyzed qualitatively and quantitatively.

X‐Shaped Alignment of Chromophores: Potential Alternative for Efficient Organic Terahertz Generators

AbstractEfficient broadband organic terahertz (THz) generators using X‐shaped alignment of the nonlinear optical chromophores, as an alternative to the parallel alignment of chromophores in benchmark organic crystals, are reported. All the developed six organic benzothiazolium crystals exhibit an isomorphic X‐shaped alignment of chromophores, resulting in an unprecedentedly large off‐diagonal optical nonlinearity (&gt;100 × 10−30 esu), which presents one of the largest off‐diagonal optical nonlinearity of organic crystals. The benzothiazolium crystals exhibit efficient broadband THz wave generation employing the off‐diagonal optical nonlinearity, in contrast to the present state‐of‐the‐art organic THz generators that mostly utilize diagonal optical nonlinearity. For using off‐diagonal and diagonal optical nonlinearities, the polarization of the optical pump is perpendicular and parallel, respectively, to the polar axis of crystals. In addition to a large THz wave generation efficiency with one order of magnitude higher peak‐to‐peak THz electric field than that generated in a 1.0‐mm‐thick inorganic benchmark ZnTe crystal, the benzothiazolium crystals generate broadband THz spectra with an upper cut‐off frequency of near 8 THz, and the absence of strong absorption dimples in the range of 0.5−4 THz. Therefore, the X‐shaped alignment of chromophores presents an interesting potential alternative for efficient broadband organic THz generators.

Three Dimensional Landslide Generated Tsunamis: Numerical and Physical Model Comparisons

Tsunamis are one of the most catastrophic natural hazards that can devastate coastal regions. Most common mechanisms that generate tsunamis are undersea earthquakes and submarine landslides. However, in enclosed basins, such as fjords, reservoirs, and lakes, subaerial landslides can also generate devastating tsunamis with similar or worse consequences, because of a more efficient wave generation. Here, we present the validation of a three-dimensional (3D) numerical model, TSUNAMI3D, by comparing numerical results with a set of subaerial landslide laboratory experiments carried out in the large tsunami wave basin at the Oregon State University (OSU). Results obtained from the laboratory experiments show that subaerial landslides generate highly non-linear waves with complex wave patterns and runup, challenging existing numerical models. The 3D numerical model is first verified with a commercial code, FLOW3D, to obtain a compromise between the model’s spatial-time resolution accuracy and computational cost. Second, the 3D numerical model and the commercial code are validated with a set of OSU laboratory experiments comprising three different layouts or basin configurations, namely, fjord, curved headland, and basin-wide. Simplified material rheology and key parameters required for modeling subaerial landslides are defined. Numerical models’ relative errors are estimated and analyzed, and the models’ limitations are discussed. In general, numerical models’ relative errors are found to be acceptable in most of the validation tests except those tests located close to the wave generation region. Validation results confirm that the 3D numerical models with simplified landslide rheology can be used to understand and reproduce the complex non-linear wave propagation and runup generated by subaerial landslides. Thus, TSUNAMI3D can help assess tsunami hazards in communities located in proximity to potential subaerial landslides. Also, the set of physical experiments can be used for further numerical validation efforts, helping tsunami organizations, e.g., the National Tsunami Hazard Mitigation Program, to amend existing or develop better numerical models and thus, improve inundation/evacuation mapping products that would save lives and property.

Impact of laser-ionized liquid nonlinear characteristics on the efficiency of terahertz wave generation.

The generation of terahertz (THz) radiation during the propagation of subpicosecond pulses in liquid media is investigated using a theoretical model considering the relative contribution of Kerr and plasma nonlinearity. The dependences of the THz emission generation efficiency on the contribution of plasma nonlinearity with a fixed third-order nonlinearity value revealed the existence of weak and strong ionization modes. It is shown that the transition between these modes is determined by the ratio of plasma to Kerr nonlinearity coefficients and the pump energy. In the strong ionization mode and with the fixed contribution of plasma nonlinearity, the optical-to-THz conversion efficiency decreases with increasing Kerr nonlinearity due to the redistribution of the pump energy for the third-order effects. These results contribute to estimating the potential of liquid media as highly efficient THz sources.

New nonlinear ultrasonic method for material characterization: Codirectional shear horizontal guided wave mixing in plate

A nonlinear ultrasonic method is proposed based on a group of newly discovered wave triplets where two primary codirectional shear-horizontal SH0 waves mix in a weakly nonlinear plate and generate a cumulative S0 Lamb wave at the sum frequency. Theoretical analyses show that any combination of two primary SH0 waves whose frequencies sum to the frequency at which the SH0 mode intersects the S0 Lamb wave mode results in an internally resonant secondary S0 Lamb wave. Moreover, the relationship between the frequency combination and the nonlinear Lamb wave generation efficiency is revealed, which guides further engineering applications. Finite element validations are carried out with the aid of a subtraction method for the nonlinear feature extraction. The cumulative effect of the generated S0 Lamb wave at the sum frequency as well as the influence of the frequency combinations on the nonlinear Lamb wave generation efficiency is confirmed. Experiments are performed to validate the proposed method as well as demonstrate its use for material characterization. The experiments require a gel filter to mitigate the influence of the undesired nonlinear sources. With the gel filter, the cumulative effect of the secondary S0 Lamb wave is verified and the corresponding slope is extracted and further used to characterize the material status of the fatigue samples. Results demonstrate the proposed method provides high sensitivity to early fatigue damage, which makes it promising for the further early damage detection applications.

Multifunctional vibrational source of seismic waves

The paper deals with a powerful low-frequency surface source of seismic waves with an electrohydraulic drive and a force closure in the “inertial mass-soil system”. A mathematical model of a hydromechanical vibromodule is developed, and according to the computer simulation, an estimate of the seismic wave generation efficiency is given as well as the operating modes are determined for signals reproduction over a wide range of frequencies and shapes.

Yellow‐Colored Electro‐Optic Crystals as Intense Terahertz Wave Sources

AbstractThis study presents newly developed yellow‐colored organic electro‐optic crystals to provide high terahertz (THz) wave generation efficiency. Compared with currently existing red‐ or orange‐colored electro‐optic crystals, which are used for most benchmark organic THz sources, yellow‐colored crystals have additional superior advantages for THz wave generation, e.g., higher transparency in the visible wavelength range with accompanying different phase‐matching possibilities. The new yellow‐colored crystals consist of a highly nonlinear optical 4‐(4‐hydroxystyryl)‐1‐methylpyridinium (OHP) cation, with a relatively short wavelength of maximal absorption at 390 nm in solution, and various halogen‐substituted benzenesulfonate anions, with strong secondary‐bonding ability. OHP 4‐chlorobenzenesulfonate (OHP‐CBS) crystals exhibit large off‐resonant macroscopic optical nonlinearity and high transparency, with a cut‐off wavelength for solid‐state absorption near 490 nm. OHP‐CBS crystals provide excellent THz wave generation characteristics based on optical rectification. A 0.53 mm thick OHP‐CBS crystal delivers ≈27 times higher optical‐to‐THz conversion efficiency and a much broader spectrum bandwidth compared with the standard 1.0 mm thick ZnTe at 1300 nm pumping. Particularly, compared with a benchmark organic quinolinium crystal with a similar thickness of 0.55 mm, OHP‐CBS crystals exhibit 1.7 times higher optical‐to‐THz conversion efficiency, and show a significantly different THz spectral shape.

Anthropogenic impact signatures revealed in the travelling ionospheric disturbances by regional GPS interferometry

With the technique of the radio interferometry of the navigational satellite signals, we investigate the travelling ionospheric disturbances generated by large urban agglomeration. We resolve detailed structure of the ionospheric disturbances field and attribute disturbances to particular atmospheric wave sources in the agglomeration. Wave generation efficiency revealed from the observational data well agrees with the theoretical prediction derived from energetic considerations.

Benzothiazolium Single Crystals: A New Class of Nonlinear Optical Crystals with Efficient THz Wave Generation.

Highly efficient nonlinear optical organic crystals are very attractive for various photonic applications including terahertz (THz) wave generation. Up to now, only two classes of ionic crystals based on either pyridinium or quinolinium with extremely large macroscopic optical nonlinearity have been developed. This study reports on a new class of organic nonlinear optical crystals introducing electron-accepting benzothiazolium, which exhibit higher electron-withdrawing strength than pyridinium and quinolinium in benchmark crystals. The benzothiazolium crystals consisting of new acentric core HMB (2-(4-hydroxy-3-methoxystyryl)-3-methylbenzo[d]thiazol-3-ium) exhibit extremely large macroscopic optical nonlinearity with optimal molecular ordering for maximizing the diagonal second-order nonlinearity. HMB-based single crystals prepared by simple cleaving method satisfy all required crystal characteristics for intense THz wave generation such as large crystal size with parallel surfaces, moderate thickness and high optical quality with large optical transparency range (580-1620 nm). Optical rectification of 35 fs pulses at the technologically very important wavelength of 800 nm in 0.26 mm thick HMB crystal leads to one order of magnitude higher THz wave generation efficiency with remarkably broader bandwidth compared to standard inorganic 0.5 mm thick ZnTe crystal. Therefore, newly developed HMB crystals introducing benzothiazolium with extremely large macroscopic optical nonlinearity are very promising materials for intense broadband THz wave generation and other nonlinear optical applications.

Electrical property and structural analysis of amphoteric impurity Ge doped GaSe crystal grown by liquid phase growth

In order to improve conversion efficiency of THz wave generation, Germanium (Ge)-doped gallium selenide (GaSe) single crystals have been grown by Temperature Difference Method under Controlled Vapor Pressure (TDM-CVP). In this article, electrical property and lattice constant of Ge doped GaSe (GaSe:Ge) crystal are compared with that of not-intentionally doped GaSe crystal. Compared with non-doped GaSe crystal, carrier concentration p was decreased by Ge-doping (not-intentionally doped GaSe p=3.2×1015cm−3 at 255K, GaSe:Ge p=4.9×1014cm−3 at 255K). In addition, it has been confirmed that lattice constant of GaSe crystal increased with doping Ge concentration increased.

Threshold and Efficiency of an Optial Parametric Generator as Functions of Cavity and Pump Parameters

A simple method for optimizing the parameters of ring optical parametric generator cavities is proposed. Expressions describing the threshold and efficiency of continuous wave generation as functions of the cavity parameters at the frequencies of the interacting waves are obtained. It is shown that the threshold is proportional to the product of the losses of interacting waves and that the generation efficiency as a function of the output mirror reflection coefficient for the signal wave has a maximum. The expressions determining the height and position of this maximum are given.

Efficiency of dispersive wave generation in dual concentric core microstructured fiber

We describe the generation of powerful dispersive waves that are observed when pumping a dual concentric core microstructured fiber by means of a sub-nanosecond laser emitting at a wavelength of 1064 nm. The presence of three zeros in the dispersion curve, their spectral separation from the pump wavelength, and the complex dynamics of solitons originated by the pump pulse breakup all contribute to boost the amplitude of the dispersive wave on the long-wavelength side of the pump. The measured conversion efficiency toward the dispersive wave at 1548 nm is as high as 50%. Our experimental analysis of the output spectra is completed by the acquisition of time delays of the different spectral components. Numerical simulations and an analytical perturbative analysis identify the central wavelength of the redshifted pump solitons and the dispersion profile of the fiber as the key parameters for determining the efficiency of the dispersive wave generation process.

Nonlinear dynamics of double-pass cross-polarized wave generation in the saturation regime.

The conversion efficiency of cross-polarized wave (XPW) generation can be improved using two separate thinner nonlinear crystals versus a single thick one, due to the evolution of the beam sizes and individual phases after the first crystal. In this paper, we present an alternative scheme in which a curved mirror is used to reimage a plane just after the BaF2 crystal for a second pass. We also develop a simple analytic model for XPW conversion that describes the origin of a nonlinear phase mismatch and nonlinear lensing for both the fundamental wave and XPW. Coupled with the numerical solution for the process and the Fresnel propagation after the first pass, we also explore the factors that affect the efficiency of saturated, seeded XPW conversion. These include the development of the on-axis relative phase difference in the first crystal and after it (during free-space propagation), mode matching, wavefront curvature difference, and crystal tuning angle. We also experimentally demonstrate that the beam quality of the XPW signal after the second pass can be improved by the reimaging.