高频超声脉冲的激光激发

早在1880年Bell就发现了光激发声的光声效应,但直至1963年White和Asker’yan才分别提出利用光辐射在固体和液体中激发声波的理论。由于光激超声脉冲技术的非接触特点,以及能在各种形态的媒质中有效地激发高频声脉冲,光激超声脉冲技术已日益受到人们重视。 迄今为止,绝大多数光激超声脉冲的理论都是假设试样表面受到一阶跃的应力或几对偶极子力的作用而得到样品的位移场。因此,很难看出位移场与光一热一声之间的转换关系。本文将简要地介绍由双曲型热传导方程求得煤质内在δ(t)δ(r)光脉冲作用下的温度场,并对热厚不透光试样,由Navier-Stok…     

高强度超短脉冲单块BBO晶体的三倍频研究

对高强度超短脉冲单块BBO晶体产生三倍频的过程进行了理论及实验研究.定量分析入射基频光强度、晶体厚度、晶体失谐角和方位角等因素对三倍频光转换效率和时间波形的影响,并对实验参数进行了优化.针对脉冲宽度100 fs、带宽25nm、中心波长810 nm、能量为6 mJ左右的超短脉冲基频光,采用单块BBO晶体开展了三倍频实验研究,获得了0.8%的三倍频转换效率,并对实验结果进行了分析.在此基础上,进一步提出了提高单块BBO晶体超短脉冲三倍频转换效率的改进措施.     

固体光声检测的两维理论

本文给出了四层媒质理论模型中有两层具有光吸收特性时，各层媒质中两维交变温度场的严格解，并由此导出目前光声检测中常用的传声器检测，光热光偏转检测，热透镜、光热光位移检测、光声压电和热释电检测中先声信号的理论表达式，使固体光学检测理论更系统化和实用化。     

脉冲光激励对少数载流子寿命的理论分析

主要从理论上研究脉冲光信号激励对少数载流子寿命测试结果的影响.通过研究两种理想的脉冲光源--高斯脉冲、方脉冲与理想δ函数光源产生的光电导衰减曲线的对比,发现对于高斯脉冲光源,只有在脉宽的条件下才可等效于δ函数光源;而脉宽的方脉冲才能等效为δ函数光源.否则脉冲光过长的脉宽和下降沿将对光电导衰减曲线中进一步分离出的表面复合速度和少子体寿命结果造成误差.若样品受表面状况影响较小时(如体寿命较低或表面复合速度低时),则可对两种理想脉冲光源的要求适当放宽.     

新型低温光声传感器

新型低温光声传感器Ｍ．Ｍ．ＡＩ－ｋｈａｆａｊｉ等１．前言Ｒｏｓｅｎｃｗａｉｇ和Ｇｅｒｓｈｏ对固体的光声效应作了理论分析以后，又对固体的光声特性作了许多研究。最近几年已开发出许多新的固态激光材料，因而需对它们的性质，尤其是它们的量予效率进行表征。许多激...     

Analysis and research on acoustic efficiency of underwater plasma sound source

为研究水下等离子体声源放电系统的声效率问题,系统地分析了水下等离子体电晕放电和电弧放电两种放电方式的微观机理以及等离子体声源放电系统各部分之间的能量转换流程,建立了一套完整的系统声效率计算模型,同时设计了水下等离子体脉冲放电试验系统.通过比较和分析水下脉冲电晕放电和水下脉冲电弧放电的声效率发现,系统中电弧放电的声效率明显高于电晕放电.通过水下高压脉冲放电试验,详细分析了水下等离子体放电系统中关键部件对系统声效率的影响.分析结果表明:关键部件的设计和参数的优化配置对整个系统的声效率有重要影响;通过声效率的分析,能够准确评估水下等离子体声源的设计合理性.     

表面微裂纹材料分频效应的实验研究

0引言广义声表面波是指所有在固体近表面传播的声波,因其近表面传播特性,声表面波在材料的表面和近表面性质表征方面具有独特的优势[1]。一方面,由于传播媒质的弹性参数决定了材料表面传播的声表面波特性,利用声传播媒质与周围介质的相互作用关系,发展了各类声表面波传感器[2];另一方面,声表面波脉冲对材料表面和亚表面     

基于脉冲液流换能器的振动能量回收原理与实验研究

针对自供电磁流变阻尼调节系统的能量供给问题,提出基于脉冲液流换能器的振动能量回收方法,通过振动能-液流能-机械能-电能的变换,实现振动能量回收。建立了基于齿轮马达、飞轮和永磁发动机的脉冲液流换能器分析模型,利用Matlab软件对正弦振动下换能器的电压输出特性、功率转换特性和转换效率进行仿真。为验证理论分析的正确性,设计制作了基于脉冲液流换能器的振动能量回收装置,利用J95-I减振器测试台上对振动能量回收装置实验测试,比较了正弦激励条件下换能器的理论电压特性与实测电压特性、理论转换效率与实验转换效率。实验研究表明,脉冲液流换能器是能够实现振动能量的转换,在带30 负载情况下,效率可以达到 46.85%,平均输出功率可以达到37.4W。     

脉冲激光线源在半无限各向同性媒质中的声激发

如果在ｚ≥０的半无限各向同性媒质中，在ｚ＝ｈ处有一平行于Ｙ轴的线状Ｐ－ＳＶ波力源ｆ，该力源可以用势函数ＴΦ和Ψ来表示：且而在ｚ≥０空间的位移场ｕ，也可用势函数和表示：势函数和满足方程：（３）式中：Ｃ＝（λ＋２μ）／ｐ，ｃ＝μ／ｐ分别为媒质中的纵波和横波波速，ｐ是媒质密度，而λ和μ是媒质的Ｌａｍｅ常数。对式（３）作双重变换，（ｘ，ｔ）→（ｋｘ，ｓ）：ｅ－ｓｔｄｔ对于ｚ＝０是自由界面时，可以求得：式中：Ｒｐｐ，Ｒａｓ和Ｒｓｐ，Ｒｓｐ分别是反射和模式转换系数，且：Ｒ（户）一４ｐ。＜７＋（ｃＦ。一Ｚｐ。…     

激光偏振方向对光声信号的影响

在高强度激光照射下材料表面会产生蒸发,形成等离子体。如果激光是斜入射材料表面的,则材料光声信号的大小与激光的偏振方向有关。本文通过实验研究了在桢的激光入射角的情况下,光声信号的大小随激光偏振方向的变化关系,得出了激光偏振方向与激光入射面的夹角为０度时,光声信号最强;而其有为９０度时,光声信号为极小值。     

Size-dependent effect of gold nanospheres on the acoustic pressure pulses from laser-irradiated suspensions

We carried out the experimental measurements of photoacoustic responses, where the suspensions of spherical gold nanoparticles (NPs) of different diameters (20, 40, and 50 nm) in water at different concentrations and different temperatures (4 and 20 °C) were irradiated by 0.8-ns laser pulses. In the case of 20 °C, the values of photoacoustic signals normalized by the light absorbance of the NP suspension decreased with increasing the NP size. The photoacoustic signals at 4 °C were significantly reduced compared with those at 20 °C. These experimental results are in fair agreement with the estimations from our phenomenological model, where the acoustic pressure pulse is represented by a sum of two contributions from the NPs and the surrounding liquid medium.     

Change of ultra-short laser pulse shape after scattering by a nanosphere in a dielectric matrix

The scattering of ultra-short laser pulses by a metallic nanosphere embedded in a dielectric matrix was studied theoretically in the frame of the dipole approximation. Calculations were performed for the corrected Gaussian shape of the incident laser pulse, which enabled us to consider both multi-cycle and sub-cycle pulses. Analysis of the influence of the scattering process on the shape of scattered pulse is made for various durations and carrier frequencies of the incident pulses.     

New insight into photoacoustic conversion efficiency by plasmon-mediated nanocavitation: Implications for precision theranostics

The probe-assisted integration of imaging and therapy into a single modality offers significant advantages in bio-applications.As a newly developed photoacoustic (PA) mechanism,plasmon-mediated nanocavitation,wherebyphotons are effectively converted into PA shockwaves,has excellent advantages for image-guided therapy.In this study,by simulating the laser absorption,temperature field,and nanobubble dynamics using both finite-element analysis and computational fluid dynamics,we quantified the cavitation-induced PA conversion efficiency of a water-immersed gold nanosphere,revealing new insights.Interestingly,sequential multi-bubble emission accompanied by high PA signal production occur under a single high-dose pulse of laser irradiation,enabling a cavitation-induced PA conversion efficiency up to 2％,which is ～50 times higher than that due to thermal expansion.The cavitation-induced PA signal has unique frequency characteristics,which may be useful for a new approach for in vivo nanoparticle tracking.Our work offers theoretical guidance for accurate diagnosis and controllable therapy based on plasmon-mediated nanocavitation.     

Thermoelastic Model for Impulsive Stimulated Scattering Monitoring the Evolution from Capillary to Rayleigh Type Wave Propagation on the Surface of Viscoelastic Materials Throughout the Glass Transition

In recent decades, the impulsive stimulated scattering (ISS) method, which is based on photothermal and photoacoustic phenomena, has been successfully used to simultaneously investigate the thermal and elastic properties in a four-wave mixing configuration, both in transmission in semitransparent materials and on reflecting surfaces of solids. In this report, an extension of the technique is proposed to study a laser-induced thermoelastic response at the free surface of glass-forming liquids. The employed all-optical configuration allows extraction of information about the acoustic shear modulus in the MHz frequency range, and hence is complementary to the classical ISS configuration in the transmission mode, which is suitable to study the relaxation of the longitudinal acoustic modulus, and to another earlier reported ISS configuration, which is exciting and probing laser-induced thermoelastic phenomena at a liquid?Csolid interface. A theoretical model is presented and numerically illustrated for the glass transition of glycerol, and experimentally validated for water at room temperature.     

Pulsed indirect photoacoustic spectroscopy: application to remote detection of condensed phases

The technique of pulsed indirect photoacoustic spectroscopy is applied to the examination of free liquid surfaces, and the prospects are assessed for remote detection and identification of chemical species in a field environment. A CO(2) laser (tunable within the 9-11-mum region) provides pulsed excitation for a variety of sample types; the resulting photoacoustic pulses are detected at ranges of the order of a few centimeters. The phenomenon is investigated as a function of parameters such as temperature, sample depth, laser-pulse energy, pulse length, and beam diameter. The results are in good agreement with a theoretical model that assumes the mechanism to be expansion of air resulting from heat conduction from the laser-heated surface of the sample under investigation. Signal and noise processing issues are discussed briefly, and the possible extension of the technique to ranges of the order of 10 m is assessed.     

Photoacoustic holographic imaging of absorbers embedded in silicone

Light absorbing objects embedded in silicone have been imaged using photoacoustic digital holography. The photoacoustic waves were generated using a pulsed Nd:YAG laser, λ=1064 nm, and pulse length=12 ns. When the waves reached the silicone surface, they were measured optically along a line using a scanning laser vibrometer. The acoustic waves were then digitally reconstructed using a holographic algorithm. The laser vibrometer is proven to be sensitive enough to measure the surface velocity due to photoacoustic waves generated from laser pulses with a fluence allowed for human tissue. It is also shown that combining digital holographic reconstructions for different acoustic wavelengths provides images with suppressed noise and improved depth resolution. The objects are imaged at a depth of 16.5 mm with a depth resolution of 0.5 mm.     

Influence of spectral broadening on femtosecond wavelength conversion based on four-wave mixing in silicon waveguides

Femtosecond wavelength conversion in the telecommunication bands via four-wave mixing in a 1.5 mm long silicon rib waveguide is theoretically investigated. Compared with picosecond pulses, the spectra are greatly broadened for the femtosecond pulses due to self-phase modulation and cross-phase modulation in the four-wave mixing process, and it is difficult to achieve a wavelength converter when the pump and signal pulse widths are close to or less than 100 fs in the telecommunication bands because of the spectral overlap. The influence of the spectral broadening on the conversion efficiency is also investigated. The conversion bandwidth of 220 nm and peak conversion efficiency of -8 dB are demonstrated by using 500 fs pulses with higher efficiency than the picosecond pulse-pumped efficiency when the repetition rate is 100 GHz.     

Picosecond-pulse wavelength conversion based on cascaded second-harmonic generation-difference frequency generation in a periodically poled lithium niobate waveguide

The wavelength conversion of picosecond optical pulses based on the cascaded second-harmonic generation-difference-frequency generation process in a MgO-doped periodically poled lithium niobate waveguide is studied both experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40 GHz mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to 1570 nm, and the pulse width can be tuned from 2 to 7 ps. New-frequency pulses, i.e., converted pulses, are generated when the picosecond pulse train and a cw wave interact in the waveguide. The conversion characteristics are systematically investigated when the pulsed and cw waves are alternatively taken as the pump at the quasi-phase-matching wavelength of the device. In particular, the conversion dependences on input pulse width, average power, and pump wavelength are examined quantitatively. Based on the temporal and spectral characteristics of wavelength conversion, a comprehensive analysis on conversion efficiency is presented. The simulation results are in good agreement with the measured data.     

脉冲激光成像探测系统回波信号仿真

研究激光脉冲回波信号特性并建立其数学模型,是应用回波信号处理技术处理回波,生成目标三维激光仿真图像的基础.首先建立了激光器发射脉冲信号能量在时间和空间上的分布模型,然后依据成像目标的激光图像仿真模板,采用累加激光脚印各采样区发射脉冲信号与对应目标散射面单位冲激响应卷积值的方法,生成了探测器接收回波仿真信号,最后分析了影响回波信号仿真精度的因素.通过对激光脚印采样区个数的合理设置实现了激光脉冲回波波形的精确仿真.     

An Efficiency Study of Transient Wave Generation in a Thermoelastic Layer with a Pulsed Laser

The efficiency of transient wave generation in a thermoelastic silicon layer excited by a pulsed laser is considered. First a principle-based transfer matrix formulation with relaxation effect, also referred to as the generalized dynamic theory of linear thermoelasticity, is used in obtaining transfer functions between the input heat field and the elements of the thermoelastic state vector. The second sound effect, through this relaxation time term, is included to eliminate the thermal wave travelling with infinite velocity as predicted by the diffusion heat transfer model. By employing the fast Fourier transform (FFT) algorithm, the transient response of a silicon thermoelastic layer under a thermal excitation (by a pulsed laser) is investigated to quantify the conversion efficiency from thermal to mechanical energy. The transient acceleration, stress, heat, temperature, and mechanical power flux responses are presented. The pulse duration of the laser excitation is submicrosecond level and, consequently, a large number of modes of motion are excited. Rigid body singularities are eliminated by considering the higher order time derivatives of the state variables. A layer made of bulk silicon under this laser excitation is considered and it is found that the amplitude ratio of the applied heat field to the propagating heat flux at the data points is in the order of 10°. The ratio of the applied power (heat flux) to the generated mechanical power flux is in the order of 10°. The resulting rigid body motion of the layer due to the laser excitation is excluded in calculating the mechanical power.