Effect of low-threshold air breakdown on material ablation by short laser pulses

Ablative formation of channels in steel by picosecond and nanosecond pulses of Nd lasers was studied. It was found that significant screening of the incident energy (up to 80–90%) in this pulse duration range is caused by breakdown of air contaminated with ablated microparticles. The breakdown threshold, size of particles, and time of their settling down were estimated. It was shown that this kind of plasma screening results in a decrease in the ablation rate and significant channel widening. Practical approaches to eliminate the low-threshold breakdown induced by microparticles were proposed and implemented. These approaches are based on experimental results of the study of the dependences of laser ablation on the pressure and repetition rate. It was shown that a moderate decrease in the pressure below 300–400 mbar makes it possible to avoid screening. In high-repetition-rate ablation, it was found that values above several kilohertz correspond to quasi-vacuum conditions in the ablation spot.     

Target effect on laser breakdown plasma

The influence of the evaporation of a little amount of the target material on the air breakdown is sometimes mainly due to an initial change in density. The estimations are in agreement with the performed experiments on Li, K, Sr targets with CO₂-TEA laser.     

Generation of terahertz radiation by the optical breakdown induced by a bichromatic laser pulse

Experimental results are presented on generation of terahertz radiation by the air breakdown induced by a high-intensity laser pulse having not only a fundamental component, but also a second-harmonic one. A theoretical explanation of the experimental data is proposed, based on a model of field ionization of a gas by a bichromatic laser.     

辐射加热不同材料靶的X光再发射研究

该课题研究辐射加热不同材料再发射Ｘ光特性。实验利用星光Ⅱ三倍频激光，以１×１０１４Ｗ／ｃｍ２激光强度辐照金盘形成辐射加热源，加热不同材料，利用多种诊断设备组合，并采取时、空、谱关联测量，给出辐射加热不同材料的再发射时间延迟，再发射效率，再发射光谱结构以及滞后等离子体碰撞发射的时、空特性。     

Dual-wavelength laser induced breakdown spectroscopic technique for emission enhancement in vacuum

A novel approach of dual-wavelength LIBS with a single Nd³⁺:YAG laser is proposed and demonstrated for lunar-simulant analysis in high vacuum conditions. Laser ablation was performed at 355 nm/532 nm wavelength, and subsequently, the plasma was reexcited with the fundamental (1064 nm) wavelength. The interpulse delay was adjusted by varying the optical path length. A significant line intensity enhancement up to a factor of 3 was observed for many of the dominant emission lines of the lunar simulant sample. A theoretical model for understanding the mechanism behind the intensity improvements of dual-wavelength configurations is also discussed. Experimentally observed plasma temperature was comparable with theoretically estimated plasma temperature of silicon, which is the major constituent of lunar simulant.     

Electrical breakdown of the space vacuum

The insulating properties of the vacuum of space are examined in this paper. Generic electrode configurations are introduced to calculate the leakage current to the electrodes and the breakdown potential of the “space-vacuum” as a function of altitude above ground. The condition that defines the breakdown threshold is dependent on the size of the sheath, r_L-B, relative to the interelectrode separation, r_c. Results for the leakage currents and breakdown voltage are presented for both cases r_L-B>r_e and r_L-Be, and discussed in the context of space power systems and the SPEAR Program in particular     

Photothermal effect of laser radiation on the electrical properties of cartilage impregnated with magnetite nanoparticles

A photothermal study of laser effects on the electrical conductivity of an articular cartilage impregnated with magnetite nanoparticles is conducted. The behavior of the electric conductivity of cartilage impregnated with magnetite nanoparticles from solutions of different concentrations (0.4 to 10 mg/mL) and depending on the dynamics of laser heating from 20 to 40°C is studied. The growth kinetics of the electrical conductivity induced by laser heating in samples of intact cartilage tissue of animals and those of the same tissue impregnated with magnetite nanoparticles are compared.     

Determination of the material properties of microstructures by laser based ultrasound

In most applications of MEMS the mechanical properties of the used materials are key parameters for the perfect working of the microsystems. Measuring bulk acoustic waves excited in MEMS structures with ultra-short laser pulses is a powerful method for the accurate and non-destructive evaluation as well as for the characterization of material properties. The pump-probe laser-based acoustic method generates bulk acoustic waves in a thermo-elastic way by absorbing the pump laser pulses. The acoustic waves are partly reflected at any discontinuity of the acoustic impedance. At the surface of the specimen the reflected acoustic pulses cause changes of the optical reflection coefficient, which are measured with the probe laser pulses. Thin membranes are part of numerous microelectromechanical systems (MEMS) like sensors, activators and bulk acoustic wave (BAW) filters for example. The described non-destructive and non-contact method is the right approach for testing such thin and brittle structures like membranes. Results of measurements on freestanding aluminium-silicon nitride multi-layer membranes with total thicknesses in the order of several hundred nanometers are presented and compared with thermo-elastic models and with measurements of the supported case. The measured results are used for the determination of the moduli of the membranes.     

Saturation of the hanle effect by intense broad-band laser radiation

We have developed a simple extension of the theory of the Hanle effect which is valid at arbitrary intensities of the optical field which produces the alignment. The optical field is limited to producing transitions only between the two excited atomic levels considered, and the results are applicable to experiments performed with broad-band laser excitation. The theoretical results predict that the Hanle effect is broadened but remains a lorentzian function of magnetic field. Also field-dependent saturation signals having the same widths as the Hanle effect appear in the total intensity of fluorescent light from the upper and lower laser levels. At high intensities the half-width of the degree of polarization signal differs from that of the Hanle signal. The results are in good agreement with recent experimental observations.     

Microwave breakdown of air initiated by a short electromagnetic vibrator

The electrical breakdown of air (under a pressure from several tens of Torr to several hundreds of Torr) is initiated in a linearly polarized quasi-optical microwave beam using a rounded-end cylindrical metallic electromagnetic vibrator placed parallel to the electric component of the microwave field. The vibrator is much shorter than the wavelength of the field. The field strength at the top of the vibrator as a function of its length is determined. An empiric expression relating the field strength at the top with the vibrator length-to-diameter ratio is found for the vibrator length range studied. The practicability of locally measuring the field in a quasioptical microwave beam is substantiated. The idea is to determine the maximal air pressure at which electrical breakdown of air initiated by a short vibrator placed at a given point of the beam takes place.     

Measurement of the Gaponov-Miller force produced in vacuum by tightly focused intense femtosecond laser radiation

A method for measuring the Gaponov-Miller force (GMF) is demonstrated based on the deflection of a picosecond photoelectron beam exposed to tightly focused intense femtosecond laser radiation. It is shown experimentally that the action of this force produced by femtosecond laser pulses linearly depends on their intensity. The method can be used to verify the correctness of measuring the duration of an ultrashort electron bunch based on the GMF.     

Photodestructive effect of IR laser radiation on the cornea

A mathematical model of the photodestructive effect of high-power IR laser radiation on cornea tissues is presented. The threshold energy exposure is calculated as a function of the wavelength and the laser pulse duration in the range 10^?5–10^?1 s under the assumption that the irreversible primary changes in the structure of tissues have a thermochemical nature. The adequacy of the model is supported by comparison of the results of calculations with a great body of experimental data available in the literature. The model is oriented for use in designing medical equipment (for example, for the refraction correction by the thermal keratoplasty method) and in refining the operating laser safety standards.     

Effect of vacuum conditions on breakdown voltage between electrodes in vacuum

The electrical breakdown in vacuum was studied in the configuration: wire —plane. Measurements were performed in two different experimental systems: with technical vacuum and with the clean one. It was found that the value of breakdown voltage decreases with increasing vacuum and that at the same pressure the insulating properties of electrodes are better in the technical vacuum than in the oilless one.     

Generation of milliwatt narrow-bandwidth vacuum ultraviolet radiation by an all-solid-state tunable high-average-power laser system

Generation of milliwatt narrow-bandwidth vacuum ultraviolet radiation by two-photon resonant four-wave mixing in Xe at 153 nm is demonstrated. The output of extreme ultraviolet radiation was at the microwatt level at 85 nm. For this demonstration, we developed an all-solid-state tunable 5-kHz Ti:sapphire laser system that produces 0.6-ns 0.7-GHz-bandwidth pulses at an average power of 32 W at the fundamental, 12 W at the second harmonic, and 6.3 W at the third harmonic.     

PZT铁电材料的总剂量辐照效应实验研究

 采用传统固相反应法制备PZT铁电材料,并制作成平行平板无源电容器结构,在ELV-8电子直线加速器上进行了总剂量效应辐照实验。结果表明：样片经过不同强度高能高速直流电子束辐照后的电滞回线随着辐照强度的增加,电滞回线所包围的面积逐渐减小,饱和极化强度、剩余极化强度和矫顽场呈线性减小。其中当辐照剂量为1×10⁸rad（Si）时,饱和极化强度、剩余极化强度和矫顽场的衰减幅度分别为14.1%,15.0%和2.7%,样片抗总剂量辐照能力可达1×10⁸rad（Si）。