Broadband femtosecond OPCPA system driven by the single-shot narrow-band iodine photodissociation laser SOFIA

A two-stage optical parametric amplifier driven by a frequency-tripled beam from the high-energy iodine laser system SOFIA was built. This single-shot Optical Parametric Chirped Pulse-Amplification facility (OPCPA) and the system synchronizing the pump and signal pulses are described in detail. The chirped seed pulse of a Ti:sapphire oscillator running at the central wavelength of 800?nm is amplified in the two-stage (LBO and KDP) optical parametric amplifier over 10⁸ times. The amplified spectral bandwidth of 68?nm corresponds to the pulse duration of 14?fs when a transform-limited pulse is assumed. This implies a compressed pulse of TW power. Systematic gain measurements reveal a good match with the theoretical predictions. Signal and idler beam fluence profiles are presented. The suitability of the iodine photo-dissociation laser as a pump source for the OPCPA technique is thus proved for the first time experimentally. A distinctive feature of the iodine laser is its very narrow gain bandwidth (<0.1?cm^?1) and, therefore, the conventional chirped-pulse amplification technique does not lead to pulse durations at the femtosecond level.     

Generation and detection of superstrong shock waves during ablation of an aluminum surface by intense femtosecond laser pulses

Superstrong shock waves of multimegabar level generated during ablation of an aluminum surface by intense (<1 PW/cm²) femtosecond laser pulses have been detected by observing the propagation of a shock wave in air from the ablated surface to a broadband piezoelectric receiver. The estimated initial pressure and velocity of the shock wave (ablation plume) agree well with data obtained earlier by various methods for shock waves propagating inside ablated targets.     

Laser-induced changes in titanium by femtosecond,picosecond and millisecond laser ablation

In order to realize the qualitative control of the laser-induced changes trend and the quantitative control of the laser-induced changes range in titanium upon laser irradiation with different pulse duration, comparative ablation experiments by femtosecond, picosecond and millisecond pulsed lasers were carried out on titanium in this study. Then the final surface morphology, aspect ratio, chemical composition and microstructural state of the ablated titanium were analyzed by laser scanning confocal microscopy, X-ray photoelectron spectroscopy and transmission electron microscopy, respectively. The dependency of the morphology, size, composition and microstructure of ablated titanium on laser pulse duration variation were emphatically discussed. It is found that, as the laser pulse duration increases from femtosecond to millisecond scale, surface morphology quality of ablated titanium gets worse, aspect ratio of microgroove decreases, proportion of titanium oxides in final ablation products becomes larger and the microstructural state of ablated titanium has a higher amorphization degree, which can be attributed to the decreased laser intensity per pulse and enhanced heat conduction effect in titanium with the pulse duration increasing.     

激光间接驱动冲击波实验测量

在神光Ⅱ高功率激光装置上,利用光学条纹相机观测台阶靶背面的冲击波加热发光,对样品中冲击波的平面性和冲击波速度进行了测量。通过冲击波法测得的腔内辐射温度与软X光能谱仪的测量结果一致,验证了冲击波速度测量的可信度。实验结果表明：在8路能量各为260 J、波长为351 nm的激光注入腔靶时,激光间接驱动的冲击波阵面在288 mm范围内平面性较好,Al样品中的冲击波速度为3.15×106 cm/s,对应的冲击波压力为1.66×1012 Pa。     

激光间接驱动冲击波实验测量

 在神光Ⅱ高功率激光装置上,利用光学条纹相机观测台阶靶背面的冲击波加热发光,对样品中冲击波的平面性和冲击波速度进行了测量。通过冲击波法测得的腔内辐射温度与软X光能谱仪的测量结果一致,验证了冲击波速度测量的可信度。实验结果表明：在8路能量各为260 J、波长为351 nm的激光注入腔靶时,激光间接驱动的冲击波阵面在288 mm范围内平面性较好,Al样品中的冲击波速度为3.15×10⁶ cm/s,对应的冲击波压力为1.66×10¹² Pa。     

Probe-beam deflection diagnostics of shock waves generated during laser drilling

A probe-beam deflection technique was employed to investigate the evolution of shock waves generated during drilling processes of optical materials by a high repetition rate copper vapor laser. Experimental observations were analyzed by using a theoretical model of shock wave expansion which also furnished quantitative information on the main physical functions associated to laser-induced acoustic phenomena.     

利用单飞秒激光脉冲驱动类氖钛X射线激光的研究

提出了一种利用单飞秒激光脉冲驱动类氖钛X射线激光的物理方案.利用自相似方法研究了不同脉冲前沿的单飞秒激光辐照钛平板靶产生的类氖钛X射线激光等离子体的特性,得到了电子温度、电子密度和定标长度三者的定标律,讨论了给定输入参数下各定标律曲线的特性.研究表明,利用单个飞秒激光能够实现X射线激光的产生,而且脉冲前沿强度随时间增长平缓的飞秒激光有利于驱动X射线激光.本研究为实验上实现单飞秒激光脉冲驱动X射线激光提供了一种新的方案.     

Spatial and temporal study of shock waves generated by laser ablation for Ti target

ＳｐａｔｉａｌａｎｄｔｅｍｐｏｒａｌｓｔｕｄｙｏｆｓｈｏｃｋｗａｖｅｓｇｅｎｅｒａｔｅｄｂｙｌａｓｅｒａｂｌａｔｉｏｎｆｏｒＴｉｔａｒｇｅｔＷＵＪｉａｄａ；ＷＵＬｉｎｇｈｕｉ；ＷＵＣｈａｎｇｚｈｅｎｇ；ＬＩＦｕｍｉｎｇ（ＳｔａｔｅＫｅｙＪｏｉｎｔＬａ...     

Surface ablation of lithium tantalate by femtosecond laser

We report measurements of the laser induced breakdown threshold in lithium tantalate with different number of pulses delivered from a chirped pulse amplification Ti: sapphire system. The threshold fluences were determined from the relation between the diameter D² of the ablated area and the laser fluence F₀. The threshold of lithium tantalite under single-shot is found to be 1.84 J/cm², and the avalanche rate was determined to be 1.01 cm²/J by calculation. We found that avalanche dominates the ablation process, while photoionization serves as a free electron provider.     

A detailed study through the focal region of near-threshold single-shot femtosecond laser ablation nano-holes in borosilicate glass

A detailed study of the morphology of nano-craters drilled in borosilicate glass by single shot femtosecond laser ablation near the ablation threshold has been performed by scanning electron microscopy, atomic force microscopy and scanning electron microscopy imaging after focused ion beam sectioning. The influence of the numerical aperture (NA = 0.4 and 0.8), the pulse energy (16 nJ < E_p < 600 nJ) and the position of the specimen surface into the focal region were systematically investigated, leading to nanometric or micrometric scales in every spatial dimension. The nanocrater’s size is not restricted by the diffraction limit but determined by the laser pulse stability and the material properties. If the beam is focused inside the glass, two craters are drilled, shaping very distinct morphologies. Their dimensions have been studied in details and different relationships have been proposed for the evolutions of the depths and of the various diameters of these craters as functions of the pulse energy, the numerical aperture and the position of specimen surface in the beam-material interaction region. It is suggested that the long, thin conical profile with very high aspect ratio of the secondary craters is due to a spontaneous reshaping of the beam which transforms the incoming Gaussian pulse into a Gaussian-Bessel pulse. As proposed in the developed model the geometry of the second craters seems to be connected with the one of the main craters.     

Ultrafast nanoporous silica formation driven by femtosecond laser irradiation

A type of glass modifications occurring after femto‐second laser irradiation gives rise to strong (10⁻²) from birefringence. This form birefringence is thought to be related to index nanostructure (called nanogratings). Analyzing induced tracks in fused silica using scanning electron microscopy (SEM) with nm resolution shows that nanostructures are porous nanoplanes with an average index lower than typical silica (Δn ∼ –0.20). Their origin is explained as arising from fast decomposition of the glass under localized, high‐intensity femtosecond laser radiation where strong nonlinear, multiphoton‐induced photoionization leads to plasma generation. Mechanistic details include Coulombic explosions characteristic of strong photoionization and the production of self‐trapped exciton (STE). Rapid relaxation of these STE prevents recombination and dissociated atomic oxygen instead recombines with each other to form molecular oxygen pointed out using Raman microscopy. Some of it is dissolved in the condensed glass whilst the rest is trapped within nanovoids. A chemical recombination can only occur at 1200 °C for many hours. This explains the thermal stability of such a nanostructure. Precise laser translation and control of these birefringent nanoporous structures allo arbitrarily tuning and positioning within the glass, an important tool for controlling optical properties for photonic applications, catalysts, molecular sieves, composites and more.     

Thermal melting and ablation of silicon by femtosecond laser radiation

The space-time dynamics of thermal melting, subsurface cavitation, spallative ablation, and fragmentation ablation of the silicon surface excited by single IR femtosecond laser pulses is studied by timeresolved optical reflection microscopy. This dynamics is revealed by monitoring picosecond and (sub)nanosecond oscillations of probe pulse reflection, which is modulated by picosecond acoustic reverberations in the dynamically growing surface melt subjected to ablation and having another acoustic impedance, and by optical interference between the probe pulse replicas reflected by the spalled layer surface and the layer retained on the target surface. The acoustic reverberation periods change during the growth and ablation of the surface melt film, which makes it possible to quantitatively estimate the contributions of these processes to the thermal dynamics of the material surface. The results on the thermal dynamics of laser excitation are supported by dynamic measurements of the ablation parameters using noncontact ultrasonic diagnostics, scanning electron microscopy, atomic force microscopy, and optical interference microscopy of the modified regions appearing on the silicon surface after ablation.     

Ultrahigh-strain-rate plastic deformation of a stainless-steel sheet with TiN coatings driven by laser shock waves

In this paper, a novel dynamic ultrahigh-strain-rate forming method driven by laser impact is reported. The technique is based on a mechanical, not thermal, effect. It is found that the ultrahigh-strain-rate is the most important feature for laser shock forming. Usually it is about 10⁷–10⁹ s^-1, two or more orders of magnitude higher than that of explosive forming, a method with the largest strain rate previously. Studies on the hardness and residual stress of the surfaces indicate that laser shock forming has some peculiarities other forming methods lack. It introduces strain hardening and compressive residual stresses on both surfaces of the metal sheet, resulting in an obvious improvement in fatigue and corrosion resistance. We also discover some non-linear plastic deformation characteristics in laser shock forming. PACS 42.62.Cf; 81.70.C; 62.20.-x; 81.40.Vw; 62.50.+p; 81.65.-b     

飞秒超强激光驱动太赫兹辐射特性的实验研究

强太赫兹源是太赫兹科学技术发展的关键,其中大能量强场太赫兹脉冲源在超快物态调控、新型电子加速器等领域具有重要的应用前景.超快超强激光与等离子体相互作用是近年来发展起来的一种新型的强场太赫兹辐射产生途径.本文报道了利用超强飞秒激光脉冲与金属薄膜靶作用产生太赫兹辐射的实验结果,研究了激光能量和离焦量对靶后太赫兹辐射能量的影响,并通过监测激光背向散射光谱,定性揭示了其变化规律与不同光强下的电子加热机制的相关性.实验表征了太赫兹辐射的频谱、偏振及聚焦光斑情况.测量结果表明,实验产生了脉冲能量达458μJ、聚焦场强高达GV/m量级的超宽带太赫兹辐射,为开展极端太赫兹脉冲与物质相互作用研究提供了一种新的强场太赫兹光源.     

Interferometric diagnostics of femtosecond laser microplasma in gases

The laser plasma formed in gaseous media due to their optical breakdown under tightly focused femtosecond laser pulses has been experimentally investigated. Pump-probemicrointerferometry is chosen to perform spatial and temporal diagnostics of the plasma. Time dependences of the laser plasma electron density are obtained. It is shown that in breakdown of different gases (air, nitrogen, argon, and helium) at different pressures (in the range from 1 to 10 atm) the electron concentration continues to increase during ??1 ps when the laser irradiation is over. This effect is related to the impact ionization of the plasma by the hot electrons formed in interaction of intense femtosecond laser pulses with matter. The results of theoretical simulation of the post-ionization processes are presented.     

Comparison between geometrically focused pulses versus filaments in femtosecond laser ablation of steel and titanium alloys

Kerr self-focusing of high-power ultrashort laser pulses in atmosphere may result in a structure or structures of high intensity that can propagate over long distances with little divergence. Filamentation has garnered significant interest in the nonlinear optics community due to its unique properties. Salient features of filaments include a central region of intense laser power (greater than the ionization threshold of the propagation medium) and a low temperature plasma column that lasts up to nanoseconds in duration after the passage of the laser pulse. Steel and titanium samples are ablated by filaments and by sharply focused sub-picosecond laser pulses. We then performed metrology on the samples to compare the ablation features in addition to modeling of the plasma ablation process. Ablation with filaments leads to a wider range of material responses as compared to ablation with sharply focused pulse. This results in potential complications for applications of filament ablation that depends on the rate of material removal and spectroscopic analysis.     

Ultra-intense femtosecond super-heavy ion beams driven by a multi-PW laser

The paper reports the results of numerical studies on the laser-driven acceleration of super-heavy ions by a multi-PW laser pulse of ultra-relativistic intensity attainable with the Extreme Light Infrastructure lasers currently being built in Europe. Using a multi-dimensional (2D3V) particle-in-cell code, it is shown that multi-GeV super-heavy (thorium) ion beams with an intensity of $～{10}^{21}\text{–}{10}^{22}\text{W}/{\text{cm}}^2$, fluence $～{10}^{17}\text{–}{10}^{18}{\text{cm}}^{?2}$ and time duration $～20\text{–}100\text{fs}$ can be produced from a sub-μm thorium target irradiated by a 150-J, 20-fs laser pulse with an intensity of ${10}^{23}\text{W}/{\text{cm}}^2$. Such ion beams are impossible to obtain presently with the use of conventional RF-driven accelerators, so they can open the door to new areas of research in both nuclear and high energy-density physics.     

Surface ablation of congruent and Mg-doped lithium niobate by femtosecond laser

Lithium niobate is praised as silicon in nonlinear optics for its wide usage both in fundamental science and applications in optics. But its photorefractive effect hinders it from more extensive application in optics. The deadlock has been broken by doping some metallic elements such as magnesium into the congruent lithium niobate crystal for its high damage resistance. The single-shot and multi-shot surface ablation experiments by femotosecond laser with the wavelength of 800 nm and the duration of 80 fs were conducted in the same condition and the ablation threshold fluence was gained for congruent lithium niobate crystal and 6 mol % Mg-doped lithium niobate crystal. The band gap widening after doping is responsible for the discrepancy of the ablation thresholds between the two samples. The laser threshold fluence dependence of lasershot number demonstrates the difference of the accumulation effect. The large reduction of trapping cross section for electrons after heavy doping of magnesium is focused more to interpret this distinction.