Efficient multiple-beam Bragg acoustooptic diffraction with phase optimization of a multifrequency acoustic wave

The subject of investigation is the formation of a highly efficient multiple-beam diffraction field resulting from acoustooptic diffraction by a periodically modulated acoustic wave and acoustic signal composed of a set of independently generated equidistant frequency components. Conditions for minimization of optical losses associated with higher diffraction orders are analyzed. A highly efficient multiple-beam diffraction field is formed by optimizing the phases and amplitudes of signal independent components. A technique of acoustooptic measurements at a high laser radiation intensity is developed, and the basics of the theory are verified experimentally. An attempt to split the power of a laser beam propagating through an optical fiber into seven channels of equal intensities is realized with a net efficiency of 80%.     

不同抽运光强激发窄带隙半导体产生太赫兹辐射的研究

研究了窄带隙材料InAs和三种不同掺杂浓度的InN在不同抽运光强激发下产生太赫兹(THz)波的辐射特性.实验结果表明:在相同的抽运光强下,InN和InAs辐射的THz信号强度在同一量级,InAs较InN辐射效率要高一些.随着抽运光强的增大,这几种材料的发射光谱变得更宽,当抽运光增大到一定强度时,它们的发射光谱半极大值全宽(HMFW)趋于恒定.InN比InAs更容易在较低功率的抽运光作用下获得宽带太赫兹光谱.研究也表明,不同掺杂浓度对辐射THz波的强度及辐射效率有很大影响.这项研究对于探索半导体表面辐射太赫 关键词： InN InAs 太赫兹 抽运光强     

Pulse Compression by Filamentation in Argon with an Acoustic Optical Programmable Dispersive Filter for Predispersion Compensation

We have experimentally demonstrated pulses 0.4 mJ in duration smaller than 12 fs with an excellent spatial beam profile by self-guided propagation in argon. The original 52 fs pulses from the chirped pulsed amplification laser system are first precompressed to 32 fs by inserting an acoustic optical programmable dispersive filter instrument into the laser system for spectrum reshaping and dispersion compensation, and the pulse spectrum is subsequently broadened by filamentation in an argon cell. By using chirped mirrors for post-dispersion compensation, the pulses are successfully compressed to smaller than 12 fs.     

Highly effective acoustooptic diffraction of light by multifrequency sound using a nonaxial deflector

The subject of investigation is multiple-beam Bragg diffraction of light, i.e., the formation of a highly effective multiple-beam field representing a superposition of equidistant monochromatic acoustic waves with determinate phases and amplitudes using a nonaxial TeO₂ deflector. It is shown theoretically and experimentally that the anisotropic character of interaction considerably changes the basic parameters of multiple-beam diffraction (compared with the isotropic case): the net diffraction efficiency grows to 100%, and the frequency band expands noticeably. A modulator splitting the initial laser radiation into five beams of equal intensity with a net efficiency approaching 100% is designed.     

Acoustic characteristics of a barrier-discharge XeCl excilamp

The generation of an acoustic signal by means of voltage pulses (f = 15 kHz) applied to the electrodes of a barrier-discharge excilamp based on a Xe/Cl₂ = (50?C500)/1 mixture kept at a pressure of 5?C500 Torr is studied. It is shown that, from the time variation of the acoustic signal intensity, one can judge the time instant the excilamp starts operating in a steady mode. Optimal (in power and efficiency) operating conditions of the excilamp are found (Xe/Cl² = 240/1, p = 98 Torr, ?? ?? 9.5%). It is experimentally demonstrated that the discharge energy at a low pressure is spent largely on heating the gas. This is indicative of the volume heat release and volume glow discharge (as the pressure grows, the efficiency of this source of energy consumption drops and more and more energy is spent on acoustic vibration excitation). Under higher pressures, the Fourier spectrum of the acoustic signal becomes richer, the intensity of the spectrum rises, and the dispersion of the signal grows. At very high pressures, the intensity of the acoustic signal drops to a level corresponding to the natural vibrations of the excilamp envelope without the discharge (when the discharge is quenched, the Fourier spectrum of the signal becomes depleted and contains only harmonics corresponding to the carrier frequency of voltage pulses from the power source).     

Improving the time-averaged thermal efficiency of a laser beam by acoustooptic correction of the directional pattern

Applicability of the acoustooptic method for raising the time-averaged thermal efficiency of laser radiation is substantiated theoretically and confirmed experimentally. The effect produced by laser radiation on materials being processed (laser cutting, welding, engraving, etc.) has a threshold in light intensity. Importantly, a beam with the most frequently used normal (Gaussian) angular distribution of intensity is not optimal from the technological viewpoint. A method proposed for its optimization is based on acoustooptic refraction, i.e., fast nonlinear scanning of the initial beam around its central position, which improves (at certain values of the parameters) the time-averaged angular distribution of the beam intensity. In the experiment, the thermal efficiency of laser radiation is raised by several times.     

Analysis of liquid surface films by pulsed laser photoacoustic spectroscopy

The photoacoustic generation of plane acoustic waves in strongly absorbing or opaque liquids by pulsed laser radiation is discussed both experimentally and theoretically. The regimes of a confined and a free surface of the liquid are considered. The model which takes the temporal shape of the laser pulses applied in the experiments into account, implies that spectroscopic studies are feasible with direct photoacoustic generation and detection also for opaque liquids. The experiments are performed with a tunable hybrid CO₂ laser and piezoelectric detection. For the first time liquid/liquid interfaces are studied by this technique. We demonstrate that the presence of an absorbing liquid film with a thickness of >1 m on the surface of another liquid amplifies the acoustic signal which is detected in the bottom liquid. The enhancement depends on the thickness and the optical and thermal properties of the film medium. The surface layer can be analyzed on the basis of the photoacoustic spectrum. It is also shown that this non-contact method is surface-film selective and should thus prove useful for pollution analysis of liquid surfaces.     

纳秒激光诱导空气等离子体的红外辐射特性研究

纳秒激光诱导空气等离子体存在从紫外、可见、近红外乃至射频微波的宽谱段辐射,但目前的研究大多关注紫外到可见波段的光谱辐射。激光等离子体作为一种新型的红外辐射源具有很多优势,相比于红外干扰弹以及红外干扰手段而言,空气等离子体红外辐射源可以灵活布置,成本低廉,因此研究空气等离子体的红外辐射特性就很有必要。针对目前脉冲激光诱导空气等离子体的红外干扰研究需要,对激光波长为532 nm的纳秒脉冲激光诱导空气等离子体的红外辐射特性进行实验研究,探讨激光能量对空气等离子体红外辐射强度的影响规律,以及空气等离子体红外辐射的角度分布特性,分析了等离子体红外辐射的可能产生机制。实验结果表明,激光诱导空气等离子体在950～1 700 nm范围内的红外光谱为线状谱和连续谱的叠加。其中线状谱主要是氮和氧的中性原子谱线,并且氮原子红外辐射占主导。随着激光能量的增加,由于空气击穿产生的氧和氮原子数量增加,导致空气等离子体红外辐射的谱线强度逐渐增大。随着红外探测角度的变化,在探测角度为75°时,OⅠ 1 128.63 nm和NⅠ1 246.96和1 362.42 nm谱线强度达到最大,在探测角度为120°时,NⅠ 1 011.46和1 053.96 nm谱线强度达到最大,这是因为空气等离子体红外辐射强度随探测角度变化呈现空间非对称性,表明空气等离子体内不同粒子的空间分布呈现非对称性。     

利用棱镜减小自衍射信号光角色散的研究

自衍射效应在飞秒激光领域有非常重要的应用,如提高飞秒脉冲的时域对比度、作为自参考光谱干涉脉冲测量法的参考光、作为频率分辨光学开关法的信号光等.然而,具有较宽光谱带宽的飞秒激光在自衍射效应过程中产生的信号光存在明显的角色散,这给自衍射效应的应用带来不利影响.本文研究发现通过在自衍射效应的一个光路中加入等腰直角棱镜,当到棱镜的入射角为23fi时可以明显地减小飞秒激光脉冲自衍射信号的角色散.这就为以后将自衍射效应更好的应用到飞秒激光脉冲的研究提供了有用的参考.     

Measurement of the moments of the relaxation time spectrum of a liquid by pulsed acoustic spectroscopy

On the basis of theoretical results describing the propagation of short acoustic pulses in relaxation media, the temporal characteristics of a signal that carry the information on the first five moments of the relaxation time spectrum (RTS) are determined. The measurement of these characteristics forms the basis of the proposed variant of pulsed acoustic spectroscopy of relaxation media. An experimental setup was developed in which short acoustic pulses were excited by a neodymium glass laser. Test measurements of RTS moments for an acetic acid, the liquid with a single relaxation time, are carried out.     

Thermooptical Mechanism of Sound Generation by High-Power Laser Radiation Propagating in the Atmosphere

In the present paper, the results of experimental and theoretical investigations into the thermooptical mechanism of sound generation by high-power pulsed laser radiation (HPLR) propagating in the atmosphere are given. Experiments were performed with collimated and weakly-focused beams of a pulsed CO₂ laser with a wavelength of 10.6 m. A sound pulse was recorded with omnidirectional microphones placed at distances of 0.2–0.5 m from the laser beam axis and spaced along the laser beam propagation path whose length was a few hundred meters. A sound pressure level was measured with a precision impulse sound level meter, and time histories of acoustic pulses were also displayed on the screen of an oscilloscope. Based on these investigations, the methods of thermoacoustic sounding of the effective HPLR beam radius and divergence angle, the laser energy absorption coefficient, and the total laser energy have been suggested and realized together with the method of the diagnostics of HPLR propagation regime from the waveform or spectrum of the received acoustic signal.     

Angular chirp and tilted light pulses in CPA lasers

We investigate analytically and experimentally various aspects of the angular chirp of ultrashort laser pulses. This type of chirp is easily produced by slight misalignment of standard pulse stretcher and/or compressor setups. Angular chirp leads to tilted pulse fronts in the near field and to a strong reduction of intensity in the focus. The effect is rather difficult to observe with standard diagnostic techniques. We present a method that is based on interferometric field autocorrelation and allows us to measure the angular chirp reliably. Suggestions on how to avoid this effect are outlined as well.     

On the use of laser-ultrasonics technique to excite selectively cylinder acoustic resonances

Acoustic propagation in the transverse plane of a cylinder is considered in this paper. The acoustic source is a line along the axis z of the cylinder coordinates, obtained by focusing the beam of a pulsed laser on the surface of the cylinder. Point detection is performed with a laser interferometer. By adequately combining elementary signals measured for a large number of relative source-receiver positions, a signal is synthesized containing the basic acoustic resonances of the cylinder. Various elementary signal combinations allow us to select the observed acoustic modes related to Whispering Gallery modes and to Rayleigh surface waves. The changes in the signal spectrum are discussed. These acoustic modes are experimentally selected and observed at ultrasonic frequencies for the first time.     

Photoacoustic and laser-induced evaporation effects in liquids

It is shown experimentally that the photoacoustic pressure response in absorbing condensed matter to laser pulses with high frequency modulation of intensity can be used to monitor displacement of the irradiated surface. Some aspects of the processes under investigation are also discussed, including acoustic diffraction effects, features of acoustic reflection from liquid–air and liquid–solid boundaries, explosive boiling, and interference between thermoacoustic and vaporization pressure signals.     

Near field properties of nanoparticle arrays fabricated by laser annealing of thin Au and Ag films

In this work we show the properties of the electromagnetic field in the vicinity of a monolayer nanoparticle array on SiO₂ substrate. The nanoparticle array is produced by a simple experimental procedure, where thin gold and silver films are deposited on a substrate by pulsed laser deposition technique and they are annealed by nanosecond laser pulses. At certain conditions the laser annealing leads to a homogeneous decomposition of the film into nanoparticles with diameters in the range of few tens of nanometers. Using FDTD simulations the near field distribution in array structures taken from SEM images are obtained. The distribution shows presence on “hot spots” where the near field intensity is enhanced more than two orders of magnitude compared to the incident one. The existence of enhanced field intensity is assumed to be the main reason on enhancement of the Raman scattering signal obtained experimentally using the produced structures as active substrates.     

飞秒脉冲激光作用下电子振荡导致的辐射空间分布特性

 采用单电子模型研究了不同偏振、不同强度飞秒脉冲激光作用下电子振荡导致的辐射的空间分布特性。研究发现:随着激光强度的增加,对圆偏振激光脉冲,电子辐射的空间分布由全空间分布变为前向双叶型结构,而对线偏振激光脉冲,电子辐射的空间分布由类似于偶极天线辐射特性的四重对称双叶结构变为双重对称三叶型结构,这可以为实验研究电子的辐射提供空间分布的相关依据。     

等束宽超短脉冲光束的远场特性

利用角谱法和复解析信号法推导出了等束宽超短高斯脉冲光束在远场的解析传输公式及其傅里叶谱.研究了若干个甚至一个光周期数的等束宽超短高斯脉冲光束的远场特性，并与等衍射长度超短高斯脉冲光束作了比较.研究表明，等束宽超短高斯脉冲光束在远场存在脉冲展宽、光谱变窄、离轴光谱红移、轴上光谱蓝移等特性. 关键词： 角谱法 复解析信号法 等束宽超短高斯脉冲光束     

脉冲激光在液体中激发的声波特性研究

理论上分析了脉冲激光在液体中产生的声波波阵面随光声源形状的变化,以及不同激发机制下光声脉冲波形的差别,并从实验中得出了脉冲CO2激光光声脉冲频谱特性.结果表明光声波波阵面为球面或柱面,热弹机制激发双极性的光声脉冲,汽化机制激发单极性的光声脉冲,CO2脉冲激光在水中激发的声波频谱峰值主要在100 kHz以下.通过选择光声源的形状和激发机制可以获得所需的光声信号.     

Laser-induced photoacoustics influenced by single-cycle terahertz radiation

Laser-induced plasma acoustic waves are enhanced under the illumination of single-cycle terahertz (THz) radiation, making THz-enhanced acoustics (TEA) a useful method for THz wave detection. During a single-cycle THz pulse with its peak field of 100 kV/cm, a pressure enhancement of 10% is observed throughout the acoustic spectrum up to 140 kHz, and the TEA signal is found to increase linearly with THz wave intensity. By using dual-color laser excitation to manipulate free electron drift, it is possible to modulate the enhanced acoustic signal and recover a coherent THz time-domain waveform by simply "listening" to the plasma.