Transducer profile effect on the second harmonic level

The measurement of nonlinear parameter of the propagating medium using finite amplitude techniques is based on the detection of the second harmonic generated nonlinearly in the investigated medium. This method requires an analytical expression for the second harmonic. Analytical expressions have been derived for the Gaussian source. For other shapes than Gaussian, a set of Gaussian beams can be used to approximate the pressure distribution at the source. Gaussian coefficients, in the literature, are provided for a uniform source. However, the sources used in many applications radiate non-uniformly because of the manner the piezoelectric element is fixed and because of Lamb waves generated in transducer’s active element. This is of a great importance to derive an analytical expression for the second harmonic for different profile “excitation” of the transducer. Our model is based on the quasilinear theory and a set of Gaussian beams. We used the K-Prony method in order to compute the Gaussian coefficients for each of the uniform, exponential, elliptic and Bessel sources. Using the obtained Gaussian coefficients we showed that the second harmonic magnitude is varying respectively to the used source’s profile. For the measurement of the nonlinear parameter one needs to compute the appropriate values of the Gaussian parameters according to the profile of the used source. One can also use the Gaussian parameters for the uniform source with a correction.     

微波与非均匀等离子体相互作用过程中坑子的形成与谐波发射

在稳态的等离子体装置中，利用微波和等离子体的相互作用，模拟了激光等离子体过程，研究了这个非线性相互作用过程中产生的坑子(caviton)，观察了坑子幅度增长，衰减及传播的演化过程。同时利用数值计算求解了在实验条件下的Zakharov方程，得到与实验较为符合的结果。实验测量了在相互作用过程中的二次谐波发射谱，并用参量衰变过程分析了二次谐波的频率红移。     

在GaAs-Al界面上红外表面二次谐波的产生及其特性研究

利用可调谐TEACO₂激光,在GaAs-Al界面上采用光栅耦合首次从实验上获得了红外表面二次谐波的共振激励输出,并研究了该界面上红外表面二次谐波的某些特性。理论值与实验值符合较好。 关键词：     

TDLAS波长调制法中调制深度与高次谐波中心幅值关系的研究

可调谐半导体激光光谱技术（TDLAS）是近年来发展十分迅速的光谱检测技术,相较于其他光谱检测技术,它具有高灵敏度、高分辨率、实时监测、便携性好、小型化等优点,在工业环保、医疗检测、气象监测等领域得到了广泛的应用。TDLAS波长调制法中谐波信号易受气压影响,经研究发现气压的影响是调制深度对谐波信号的影响,基于TDLAS技术谐波法的原理,研究了各次谐波与调制深度的关系,通过计算四次谐波与二次谐波中心幅值比,利用调制深度函数推算当前气压环境的调制深度,调整调制频率幅度,使得调制深度接近各次谐波最佳调制深度值,使谐波信号信噪比最佳,提高检测精度。实验通过国瑞智GRZ5031湿度发生器产生固定为1 000 ppm的水汽,调节气阀控制密封箱内不同的气压环境,采用TDLAS水汽检测系统获得了10.2~177.9 kPa气压条件下的二次谐波和四次谐波信号,并进行了仿真与实验分析。仿真结果显示：四次谐波与二次谐波中心幅值比的理论值和仿真值最大相对误差为-1.44%,调制深度的理论值与仿真值最大相对误差为1.78%,说明了仿真下的调制深度函数曲线与理论一致。实验结果显示：根据调制深度函数推算调制深度值,当m=2.226 7时,实测的二次谐波中心频率幅值达到最大值,当m=4.061 0时,实测的四次谐波中心频率幅值达到最大值,与理论结果一致;在30.2 kPa<p<177.9 kPa时,调制深度与气压乘积mp值相对误差较小,最大相对误差不超过±3.2%,说明了此气压条件下的mp值波动不大,通过调制深度函数推算的调制深度值与实际值近似,验证了调制深度函数理论的准确性。     

Synthesis,crystal growth and characterization of novel semiorganic nonlinear optical crystal: Dichloro(beta-alanine)cadmium(II)

Semiorganic nonlinear optical material of dichloro(beta-alanine)cadmium(II) (DCBAC) have been synthesized and single crystals were grown by solvent evaporation method at room temperature. The lattice parameters of the grown crystals are determined by single crystal XRD. The modes of vibration of different molecular groups present in the sample were identified by the FTIR spectral analysis. Thermal stability of the crystal was investigated using thermo gravimetric analysis (TGA) and differential thermal analysis (DTA). The dielectric constants of the crystal were studied as a function of frequency and the results are discussed. The grown crystals are subjected to microhardness studies and the variation of the microhardness with the applied load is studied. The optical transmission spectra and second harmonic generation (SHG) were investigated to study its linear and nonlinear optical properties. The nonlinear optical (NLO) property of the crystal was confirmed by powder second harmonic generation (SHG) test. SHG efficiency is comparable to that of KDP.     

温度对稀土夹心双萘酞菁LB膜光学性质的影响

 采用线性吸收谱和二次谐波产生技术研究了温度对具有中心对称结构的稀土夹心双萘酞菁化合物LB膜光学特性及其LB膜结构的影响。研究发现,加热可以使其LB膜的结构及分子间的相互作用发生变化,并形成J聚集体,从而使得吸收峰发生红移。稀土夹心双萘酞菁化合物约在40 ℃时其二次谐波信号有一个极小值,约在65 ℃时该化合物发生相变,使得二次谐波信号产生极大值,二次谐波信号最小时温度可高达180 ℃,说明该化合物的化学结构较为稳定。     

LB膜混合聚集体中极性基团相互作用对光学非线性的影响

通过紫外－可见吸收光谱、压力－面积等温线、二次谐波的测量，研究了不同活性分子混合聚集体的形成对ＬＢ膜光学性质的影响。分子极性基团间的相互作用，使激发态能量及平均偶极矩发生变化，导致ＬＢ膜的光学非线性明显地增强或减弱。     

Quantitative studies of copper plasma using laser induced breakdown spectroscopy

In the present work, we present the spatial evolution of the copper plasma produced by the fundamental harmonic (1064 nm) and second harmonic (532 nm) of a Q-switched Nd:YAG laser. The experimentally observed line profiles of neutral copper have been used to extract the electron temperature using the Boltzmann plot method, whereas, the electron number density has been determined from the Stark broadening. Besides we have studied the variation of electron temperature and electron number density as a function of laser energy at atmospheric pressure. The Cu I lines at 333.78, 406.26, 465.11 and 515.32 nm are used for the determination of electron temperature. The relative uncertainty in the determination of electron temperature is ≈10%. The electron temperature calculated for the fundamental harmonic (1064 nm) of Nd:YAG laser is 10500–15600 K, and that for the second harmonic (532 nm) of Nd:YAG laser is 11500–14700 K at a Q Switch delay of 40 μs. The electron temperature has also been calculated as a function of laser energy from the target surface for both modes of the laser. We have also studied the spatial behavior of the electron number density in the plume. The electron number densities close to the target surface (0.05 mm), in the case of fundamental harmonic (1064 nm) of Nd:YAG laser having pulse energy 135 mJ and second harmonic (532 nm) of Nd:YAG laser with pulse energy 80 mJ are 2.50×10¹⁶ and 2.60×10¹⁶ cm⁻³, respectively.     

Design considerations for tunnel diode non-linear transmission lines

A tunnel diode non-linear transmission line has been investigated for second and third harmonic generation purposes. Design criteria are given and performance predictions have been obtained simulating the single diode behavior by means of a polynomial expression. An optimization of the non-linear transmission line as a harmonic generator has been obtained as a function of the number of diodes, their separation distances and the bias voltage.     

宽带准连续光纤激光在周期极化铌酸锂中倍频特性的研究

双包层光纤激光器和非线性光学材料(如周期性极化的铌酸锂晶体,PPLN)相结合,开辟了实用性非线性光学器件的一个新领域。研究了准相位匹配周期性极化反转铌酸锂晶体对宽带准连续光纤激光倍频的温度特性和频谱特性。在理论上,从准相位匹配相位失配关系出发,推导了晶体温度与抽运源中心波长的关系以及温度响应带宽,并和已报道实验结果进行了比较,二者符合得很好。此外,还推导了倍频周期极化铌酸锂晶体对抽运基频光源的响应谱线带宽。在实验上,采用长度20mm,极化周期6．5μm,厚度0．5mm的周期极化铌酸锂晶体光纤激光器准连续宽带输出进行了倍频,获得了在不同控制温度下的倍频光光谱,并对此进行了详细分析。     

Theoretical Study of Millimeter Wave Harmonic Oscillator Stabilized with a Transmission Cavity

An equivalent circuit model of millimeter wave second harmonic oscillator stabilized with a transmission cavity has been proposed for constructing analytical formulations between performance parameters of the oscillator and parameters of the circuit. The model consists of an equivalent circuit of fundamental wave and that of second harmonic wave. Each of the circuits comprises circuit models of main cavity, transmission waveguide, and transmission cavity. Absorbing material placed between the transmission waveguide and the transmission cavity can suppress additional resonances originated from transmission cavity. The behavior of the second harmonic oscillator can be effectively described by the circuit model. Furthermore, based on this model, mechanical tuning characteristics have been studied at first, and then analytical formulas for quality factor and efficiency depending on circuit parameters have been derived. The circuit parameters can be conveniently extracted by electromagnetic field simulation. Hence the formulas exhibit both compact form and enough accuracy. Thereafter, general rules of performance parameters varying with circuit parameters have been deduced for the harmonic oscillators. Then some design considerations have been derived according to the corresponding analysis. The equivalent circuit model is useful for designing and adjusting millimeter wave second harmonic stabilizing oscillator with a transmission cavity.     

Spatio-temporal characterization of self-formed hollow light pulses in the pump depletion regime of second harmonic generation

Depletion of the pump pulse in second harmonic generation (SHG) and its back-regeneration from the second harmonic pulse is a well known process in nonlinear optics. Nevertheless how the pump pulse reshapes as a three-dimensional object in space and time has never been investigated. In this work we apply a three-dimensional mapping technique to record this transformation and we experimentally show the formation of a hollow as well as more complex layered spatio-temporal structures of the fundamental pump pulse in different regimes. The results are compared with numerical simulations.     

Third order harmonic imaging for biological tissues using three phase-coded pulses

Compared to the fundamental and the second harmonic imaging, the third harmonic imaging shows significant improvements in image quality due to the better resolution, but it is degraded by the lower sound pressure and signal-to-noise ratio (SNR). In this study, a phase-coded pulse technique is proposed to selectively enhance the sound pressure of the third harmonic by 9.5 dB whereas the fundamental and the second harmonic components are efficiently suppressed and SNR is also increased by 4.7 dB. Based on the solution of the KZK nonlinear equation, the axial and lateral beam profiles of harmonics radiated from a planar piston transducer were theoretically simulated and experimentally examined. Finally, the third harmonic images using this technique were performed for several biological tissues and compared with the images obtained by the fundamental and the second harmonic imaging. Results demonstrate that the phase-coded pulse technique yields a dramatically cleaner and sharper contrast image.     

Effect of coupled oscillations on microbubble behavior

Ultrasound contrast agents are encapsulated microbubbles whose nonlinear acoustic scattering signatures have been the foundation of their use in diagnostic imaging. The coupled oscillations of microbubbles along their lines of center are investigated theoretically using radial equations in the monopole approximation and an energy balance, which is obtained for the system. Coupled microbubble pairs of different initial radii are investigated numerically relative to the normal modes for the linearized system. For microbubble pairs of different size bubbles driven below the mode of the smaller bubble and above the mode of the larger bubble, it is shown that oscillations of the smaller agent are affected substantially more by the coupling than those of the larger one. For separation distances of 10 and 500 microns, a difference of approximately 10 dB occurs in the second harmonic output of a 1.0-micron radius agent coupled with a 2.2-micron radius agent forced at 2.0 MHz and 0.3 MPa. The subharmonic spectral peak is shown to decrease approximately 19 dB for the coupling of 1.5- and 2.2-micron radius agents at 10- and 500-micron distances under the same acoustic forcing conditions. These coupling effects on the radiated pressure and its spectral power are highlighted for contrast agent imaging applications.     

Theoretical study of harmonic injection locking of millimeter wave harmonic gunn oscillators

Harmonic injection locking behavior of millimeter wave second harmonic Gunn oscillators is studied based on an equivalent circuit model. A large signal model of Gunn device in harmonic mode operation is employed. Injection locking curves of voltage amplitude and phase difference between injected current and harmonic voltage are calculated by means of describing function technique. Stability of the locked harmonic oscillators is also investigated. It is revealed that the harmonic locking bandwidth is much smaller than that of fundamental wave oscillators and is closely related to the susceptance slope parameter of fundamental wave circuit. It is also found that the stable region is smaller than that of fundamental wave oscillators. Some conclusions have been made for the application of harmonic injection locking technique.     

Mode Selection for a Terahertz Gyrotron Based on a Pulse Magnet System

The TE_6,11 mode has been selected as a candidate for the second harmonic operation of a terahertz gyrotron at 1007.68 GHz. The predicted efficiency is 8.6 percent for the output power 0.38 kW. Time-dependent, multi-mode calculations have been carried out to investigate stability of a single-mode operation at second harmonic. It has been found that with the beam current 0.111 A and the magnetic field 19.282 T the second harmonic operation in the TE_6,11 mode is possible.     

Subharmonic imaging of contrast agents

Ultrasound contrast agents promise to improve the sensitivity and specificity of diagnostic ultrasound imaging. It is of great importance to adapt ultrasound equipment for optimal use with contrast agents e.g., by exploiting the nonlinear properties of the contrast microbubbles. Harmonic imaging is one technique that has been extensively studied and is commercially available. However, harmonic imaging is associated with problems, due to second harmonic generation and accumulation within the tissue itself. Given the lack of subharmonic generation in tissue, one alternative is the creation of subharmonic images by transmitting at the fundamental frequency (fo) and receiving at the subharmonic (fo/2). Subharmonic imaging should have a much better lateral resolution and may be suitable for scanning deep-lying structures owing to the higher transmit frequency and the much smaller attenuation of scattered subharmonic signals. In this paper, we will review different aspects of subharmonic imaging including implementation, in-vitro gray-scale imaging and subharmonic aided pressure estimation.     

Space harmonic analysis of sound radiation from a submerged periodic ring-stiffened cylindrical shell

An analytical method is developed to study radiated sound power characteristics from an infinite submerged periodically stiffened cylindrical shell excited by a radial cosine harmonic line force. The harmonic motion of the shell and the pressure field in the fluid are described by Flügge shell equations and Helmholtz equation, respectively. By using periodic theory of space harmonic analysis, the response of the periodic structure to harmonic excitations has been obtained by expanding it in terms of a series of space harmonics. Radiated sound power on the shell wall along the axial direction and the influence of different parameters on the results are studied, respectively. A conclusion is drawn that the stiffeners have a great influence at low and high frequencies while have a slight influence at intermediate frequencies for low circumferential mode orders. The work will give some guidelines for noise reduction of this kind of shell.     

Optical rectification coefficient in an oxide quantum dot with different confinement potentials

Exciton binding energies, oscillator strength, optical rectification coefficients and second harmonic generation are investigated using three different confinement potentials in a CdO/ZnO core/shell quantum dot. The bare potential, Smorodinsky–Winternitz potential and Woods–Saxon potential are employed in the Hamiltonian. The position dependent dielectric function is used. The electronic properties are found using variational formulism within a single band effective mass approximation whereas the optic properties are investigated using compact density matrix approach. The results show that different confinement potentials lead to significant changes in the coefficients of optical rectification and the second harmonic generations and the effects of confined potentials are more pronounced in the strong confinement region. The resonant peaks in the nonlinear optical rectification coefficients and second harmonic generation are blue shifted to larger photon energies with the various confined potentials and the results are enhanced using Smorodinsky–Winternitz potential. The obtained results can be applied for the potential applications for fabricating opto-electronic devices.     

马铁菊头蝠声道声学特性的数值分析

采用有限元数值计算得到了马铁菊头蝠声道内部的声场分布,给出了马铁菊头蝠声道内几种特殊的腔体结构在蝙蝠发声过程中的作用。通过微型CT扫描并经过三维重构得到了马铁菊头蝠声道的三维立体模型用于有限元数值计算,通过在声门处放置单位声源计算得到了整个声道内部以及鼻孔周围的声压分布。结果表明,马铁菊头蝠声道包含了鼻腔结构后声波在声门上方的声压幅度明显大于不含鼻腔结构的情况,从传输曲线来看,声门上方鼻腔的存在使得系统对声波传输在二次谐波频率处呈现低阻抗效果,同时鼻腔的改变还可影响二次谐波的位置。而声门下方的气管空腔主要影响声波的背向转播,声门下方的气管空腔的存在可明显降低蝙蝠发声时声场在声道声门下方的声压幅度,同时抑制声音背向传播时二次谐波成分的强度。