基于脉冲复制的单脉冲激光对比度测量

为测量超短单脉冲激光的时间波形以及对比度信息,基于三阶强度相关原理,结合光脉冲复制技术,提出了基于脉冲复制的测量方法。对测量原理进行了详尽的理论分析。利用分步傅里叶和龙格库塔数值计算方法对测量方案做了模拟验证。基于脉冲复制的测量法能同时进行多窗口测量。通过拼接测量时间窗口,可以有效解决测量分辨率与测量时间窗口不能兼顾的问题,同时实现大时间窗口和高分辨率测量。通过将主脉冲与预脉冲分离到不同的测量窗口,避免了梯度衰减片的使用,且具有高对比度测量能力。     

基于脉冲复制的单脉冲激光对比度测量

为测量超短单脉冲激光的时间波形以及对比度信息,基于三阶强度相关原理,结合光脉冲复制技术,提出了基于脉冲复制的测量方法。对测量原理进行了详尽的理论分析。利用分步傅里叶和龙格库塔数值计算方法对测量方案做了模拟验证。基于脉冲复制的测量法能同时进行多窗口测量。通过拼接测量时间窗口,可以有效解决测量分辨率与测量时间窗口不能兼顾的问题,同时实现大时间窗口和高分辨率测量。通过将主脉冲与预脉冲分离到不同的测量窗口,避免了梯度衰减片的使用,且具有高对比度测量能力。     

Distinct magnetic properties of one novel type of nanoscale cobalt-iron Prussian blue analogues synthesized in microemulsion

One novel type of nanoscale cobalt-iron Prussian blue analogues (PBA) in the form of mixed nanorods and nanocubes were synthesized using cetyltrimethyl ammonium bromide (CTAB) as the surfactant in microemulsion at low temperature. The generated products were characterized by SQUID, XRD and IR techniques, etc. The effects of potassium contents, cobalt-to-iron ratios, reaction temperatures on the properties of the nanoscale cobalt-iron PBA were systematically explored. The results showed that the novel type of nanomaterials possessed distinct magnetic properties in that their coercivities were intensely dependent on cobalt-to-iron ratios and potassium contents. Furthermore, it was observed that low reaction temperature not only affected the morphologies of the products, but also had influence on their magnetic properties. Additionally, the cobalt-iron Prussian blue analogues were strongly influenced by CTAB around their surface, which led to higher Curie temperatures.     

Microdroplet evolution induced by a laser pulse

Interaction between high-power ultrashort laser pulse and giant clusters (microdroplets) consisting of 10⁹ to 10¹⁰ atoms is considered. The microdroplet size is comparable to the laser wavelength. A model of the evolution of a microdroplet plasma induced by a high-power laser pulse is developed, and the processes taking place after interaction with the pulse are analyzed. It is shown theoretically that the plasma is superheated: its temperature is approximately equal to the ionization potential of an ion having a typical charge. The microdroplet plasma parameters are independent of the pulse shape and duration. The theoretical conclusions are supported by experimental studies of x-ray spectra conducted at JAERI, where a 100-terawatt Ti-sapphire laser system was used to irradiate krypton and xenon microdroplets by laser pulses with pulse widths of 30 to 500 fs and intensities of 6×10¹⁶ to 2×10¹⁹W/cm².     

Pressure effect in a shock-wave-plasma interaction induced by a focused laser pulse

The effect of ambient pressure on the interaction between a laser plasma bubble and a shock wave involving a Richtmyer-Meshkov instability was experimentally studied via framing Schlieren visualization. A sharp plasma interface could be formed without any separation material that causes undesired disturbances. The fundamental vortex structure which was produced via baroclinic effects was self-similar with respect to the laser energy-ambient pressure ratio. Yet, the higher the ambient pressure, the more high-wave-number instabilities were enhanced so as to contaminate the self-similarity.     

Nanospallation induced by an ultrashort laser pulse

A femtosecond laser pulse with power density of 10¹³ to 10¹⁴ W/cm² incident on a metal target causes ablation and ejection of the surface layer. The ejected laser plume has a complicated structure. At the leading front of the plume, there is a spall layer where the material is in a molten state. The spall layer is a remarkable part of the plume in that the liquid-phase density does not decrease with time elapsed. This paper reports theoretical and experimental studies of the formation, structure, and ejection of the laser plume. The results of molecular dynamics simulations and a theoretical survey of plume structure based on these results are presented. It is shown that the plume has no spall layer when the pulse fluence exceeds an evaporation threshold F _ev. As the fluence increases from the ablation threshold F _a to F _ev, the spall-layer thickness for gold decreases from 100 nm to a few lattice constants. Experimental results support theoretical calculations. Microinterferometry combined with a pump-probe technique is used to obtain new quantitative data on spallation dynamics for gold. The ablation threshold is evaluated, the characteristic crater shape and depth are determined, and the evaporation threshold is estimated.     

UV and blue picosecond pulse generation by a nitrogen-laser-pumped distributed feedback dye laser

Tunable picosecond pulse generation in the 362–420 nm spectral range is reported. The laser is simple in construction and can be easily constructed from relatively inexpensive optical and mechanical parts.     

Method for single-shot measurement of the carrier envelope phase of a few-cycle laser pulse

The interference between different harmonics of a few-cycle optical pulse in the region of the spectral overlap is sensitive to the phase of the optical carrier inside the pulse envelope. Near-surface third-harmonic generation from Si(001) combined with second-harmonic generation in a 10-mum -thick beta-barium borate crystal produces sufficiently strong harmonic emission for single-shot measurement. We propose using this technique to measure the carrier envelope phase of high-energy 5-fs pulses.     

Formation of amorphous sapphire by a femtosecond laser pulse induced micro-explosion

We report on structural characterization of void-structures created by a micro-explosion at the locus of a tightly focused femtosecond laser pulse inside the crystalline phase of Al₂O₃ (R3c space group). The transmission electron microscopy (TEM), micro-X-ray diffraction (XRD) analysis, and Raman scattering revealed a presence of strongly structurally modified amorphous regions around the void-structures. We discuss issues of achieving the required resolution for structural characterization and assignment of newly formed phases of nano-crystallites by TEM, XRD, and Raman scattering from micro-volumes of modified materials enclosed inside the bulk of the host phase.     

Spectral attenuation of a 400-nm laser pulse propagating through a plasma filament induced by an intense femtosecond laser pulse

The spectral attenuation of a 400-nm probe laser propagating through a femtosecond plasma in air is studied. Defocusing effect of the low-density plasma is an obvious effect by examining the far-field patterns of the 400-nm pulse.Besides, the energy of 400-nm pulse drops after interaction with the plasma, which is found to be another effect leading to the attenuation. To reveal the physical origin behind the energy loss, we measure fluorescence emissions of the interaction area. The fluorescence is hardly detected with the weak 400-nm laser pulse, and the line spectra from the plasma filament induced by the 800-nm pump pulse are clearly shown. However, when the 400-nm pulse propagates through the plasma filament, the fluorescence at 391 nm from the first negative band system of N_2~+ is enhanced, while that from the second positive band of neutral N_2 at 337 nm remains constant. Efficient near-resonant absorption of the 400-nm pulse by the first negative band system occurs inside the plasma, which results in the enhanced fluorescence. Furthermore, the spectral attenuation of the 400-nm probe laser is measured as a function of the pump–probe time delay as well as the pump-pulse energy.     

Nuclear fusion induced by a super-intense ultrashort laser pulse in a deuterated glass aerogel

A SiO₂ aerogel with absorbed deuterium is proposed as a target for the fusion reaction d + d → He³ + n induced by a superintense ultrashort laser pulse. The multiple inner ionization of oxygen and silicon atoms in the aerogel skeleton occurs in the superintense laser field. All the formed free electrons are heated and removed from the aerogel skeleton by the laser field at the front edge of the laser pulse. The subsequent Coulomb explosion of the deuterated charged aerogel skeleton propels the deuterium ions up to kinetic energies of ten keV and higher. The neutron yield is estimated at up to 10⁵ neutrons per laser pulse for ～200–500 ps if the peak intensity is 10¹⁸ W/cm² and the pulse duration is 35 fs.     

Near-field modification of femtosecond laser beam to enhance single-shot pulse filamentation in glass medium

In this paper, a study of the effect of a transparent dielectric medium (soda-lime glass placed after an objective lens) on femtosecond pulse filamentation in Corning Eagle²⁰⁰⁰ glass is presented. The dielectric medium placed after the objective lens increases the effective numerical aperture of the focusing lens and tightly focuses the beam to elevate the power density in a tiny focal volume. A single-shot narrow filamentary void of diameter 3.8 μm is observed to propagate as long as 700 μm in Corning Eagle²⁰⁰⁰ glass. Elevated optical intensity confined in the small focal volume and low ionization losses per unit length in the narrow plasma column collectively enhance the filamentary propagation of the ultra-short pulse in bulk glass. The applied technique also offers an approach to control the length and width of a filamentary void structure in a transparent dielectric medium.     

Ultrafast semiconductor-metal phase transition in vanadium dioxide induced by a femtosecond laser pulse

A model is proposed for a photoinduced Peierls-type semiconductor-metal phase transition that makes it possible to determine the time dependence of the bandgap width in the electronic spectrum of vanadium dioxide subjected to a light field and the dependence of the time at which a photoinduced semiconductor-metal phase transition occurs on the laser pulse duration. The theoretical results obtained are consistent with experimental data on the illumination of a VO₂ film with an intense laser pulse.     

Amorphous-polycrystal transition induced by laser pulse in self-ion implanted silicon

Reflection high energy electron diffraction has been used to investigate the amorphous to polycrystalline structure transition in silicon induced by laser pulse. The power density of the ruby laser pulse, in the free generation mode, has been maintained below the threshold to induce surface damage. Depth analysis has been carried out in 〈100〉 silicon crystal using the channeling effect technique.     

Dynamics of ablation plasma induced by a femtosecond laser pulse in electric fields

Direct observations of ablation plasma dynamics in electric field is presented. A time-resolved spatial profile of the ablation plasma induced by femtosecond laser ablation (fsLA) with high fluence is visualized using a planar-laser-induced fluorescence (P-LIF) method. The external electric field is produced by installing a mesh electrode at 6 mm from a Samarium solid target. The Sm ion plasma created by the fsLA showed collective motion regardless of the external electric field, until they reached close to the electrode. When the accelerating and decelerating field was applied, the ions almost disappeared behind the electrode from the field of view. The observations are understood utilizing a SIMION simulation with a conceivable potential gradient caused by Debye shield effect, which is that the ablation plasma keeps the same potential as the target voltage and follows electric potential gradient near the mesh electrode. It is also revealed that this effect degrades time-of-flight resolution at high fluence irradiation. This work gives a new direction for further developments of a fsLA time-of-flight spectrometer.     

Adjusting pulse-front tilt and pulse duration by use of a single-shot autocorrelator

We present a method of adjusting the pulse duration and eliminating the pulse-front tilt of an ultrashort pulse in real time by use of a specially configured single-shot autocorrelator. Pulse-front tilt, or a temporal delay across the pulse front, is a common ultrashort-pulse phenomenon when dispersive elements are being used. We show the design of an autocorrelator that can be used to eliminate the pulse-front tilt and simultaneously adjust the pulse duration in real time by adjustment of the pulse compressor of a chirped-pulse amplified laser system.     

Microscopic spallation mechanisms induced by a pulse laser at the solid-state interface

This paper presents a study of the transient behavior of structural dynamics and the associated innovatory microscopic spallation mechanism at the solid-state interface, induced by an incident femtosecond pulse laser. By detailed structural dynamic analysis, using the technique of molecular dynamics simulation, the spallation mechanism at the solid–solid interface is observed. The occurrence of structural spallation is mainly characterized by extraordinary expansion dynamics and tensile stress that induces interior structural void defect coalescence, eventually leading to cracking. The microscopic phenomenon of moderate ductile fracturing at the solid–solid interface is identified. A high strain rate in the order of 10⁹ s^-1 is observed. Both aforementioned phenomena are analogous to the experimental results of metal-film spallation excited by a pulse laser. Moreover, it is also shown that the critical value of the stain rate is one of the dominant factors that influences the occurrence and mechanism of structural spallation. The results of simulations reveal that the thin-film structure is safe if the strain rate is below certain critical values. The critical damage threshold is evaluated and technical suggestions to avoid interfacial fracture are also presented. PACS 02.70.Ns; 42.62.-b; 64.60.Ht; 61.72.Cc; 64.60.-i     

单脉冲纳秒激光诱导硅表面微结构

利用Nd:YAG纳秒脉冲激光(波长532 nm)在空气中对单晶硅表面进行单脉冲辐照,研究了激光能量密度和光斑面积变化对微结构的影响。通过场发射扫描电子显微镜和原子力显微镜(AFM)对样品表征,并对纳秒激光辐照硅的热力学过程进行分析。结果显示：当脉冲激光的能量密度接近硅的熔融阈值且光斑直径小于8 m时,形成尖峰微结构;随着能量密度或光斑面积增大,尖峰结构消失,形成边缘隆起和弹坑微结构。通过流体动力学模型得到微结构形貌的解析解,模拟得到的微结构形貌与实验测得的AFM数据一致。研究表明微结构的形成主要是由于表面张力引起的熔融硅流动。表面张力与表面温度和表面活性剂的质量浓度有关。温度梯度引起的热毛细流作用和表面活性剂浓度引起的毛细作用共同影响下形成尖峰、边缘隆起和弹坑微结构。     

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.     

重复频率脉冲激光作用下膜内包裹物对损伤阈值的影响

 利用介质薄膜中包裹物的热理论模型,结合S.Papernov 等利用电子束蒸发技术在熔融石英上沉积含Au包裹物的HfO2薄膜实验,得出Au的吸收截面。以包裹物Au为例,计算了脉冲激光作用下不同包裹物半径对损伤阈值的影响,分析了重复率脉冲激光作用下薄膜损伤阈值的变化及重复频率与激光损伤阈值的关系。结果表明：随着包裹物半径的增加,激光损伤阈值先减小,接着增加而后再减小。激光损伤阈值与脉冲宽度的0.4次方成正比。随着脉冲重复频率的增加,激光损伤阈值单调下降,产生损伤所需的最小脉冲数则单调上升。