Transient effects in pulsed laser irradiation

Theoretical analysis of the influence of the temporal profile (rectangular, triangular, Gaussian) of the laser pulse on heating/cooling and phase transition velocities and quantity of ablated material was performed on the basis of a multifront Stephan problem. Modeling showed that material removal under stationary conditions (that correspond to long pulses) is entirely controlled by specific heat and material density, while in the case of transient regimes (short pulses) thermal conductivity and heat capacity play a predominant role. Interaction of the melting and evaporation fronts characterized by an evaporation front velocity far exceeding the melting front one is one of the examples of the transient nature of the phenomena influenced by the laser pulse parameters.     

Electrical behaviour of laser-damaged silicon photodiodes

A measurement of the electrical parameters degradation of Si photodiodes irradiated by laser visible light has been performed. The laser is a Q-switched Nd:YAG, frequency doubled, operated in single pulse mode of 4 ns duration. The applied fluence levels range up to 90 J/cm². Two kinds of irradiation process were applied: either a part of the detector active area was irradiated in single pulse mode, or a scanning of the whole detector active area was performed with successive identical pulses. It has been shown that the fluence necessary to induce significant changes (local decrease of 35%) in responsivity is several times the surface melting threshold fluence (0.5 J/cm²). Conversely, the dark current is the most sensitive parameter, increasing by about four times for high irradiation. The in-depth dopant distribution is altered by high fluence irradiation in a way that cannot be explained by simple thermal modelling.     

Constraints of two-colour TiRe-LII at elevated pressures

The main objective of this work is to investigate the influence of high-pressure conditions on the determination of primary particle size distributions of laser-heated soot particles using pyrometrically determined temperature decays. The method is based on time-resolved laser-induced incandescence measurements carried out at two different wavelengths (two-colour TiRe-LII). The LII signals are transferred into a particle ensemble averaged (effective) temperature using Planck’s thermal radiation formula. Assuming that all particles within the size distribution possess a unique temperature at the end of the laser pulse, the size distribution can be determined by numerically simulating the measured temperature decay. From our investigations, for pressures up to a few bars it is obvious that this strategy can be successfully applied if standard laser pulses of nano-second duration are used as an LII-excitation source. At higher pressures the time scales of heat conduction are decreased to such an extent that a unique temperature for all particles within the ensemble cannot be assumed at the end of the nano-second laser pulse. However, further investigations show that the presented two-colour TiRe-LII technique can be successfully adopted under technical high-pressure conditions as well, if the pulse duration of the TiRe-LII-excitation source is reduced into the pico-second range.     

Experimental and numerical investigation on structural effects of laser pulses for modal parameter measurement

In this paper it has been described part of the research devoted to the development of a complete non-intrusive experimental modal analysis procedure based on laser techniques both for excitation and for measurement. In particular, attention has been focused on the thermal effects generated by laser pulses on the excited structure. An analytical model of the energy exchange between the light pulse and the target surface is proposed together with a finite element model of thermal and mechanical behaviour of the structure under excitation. Both the models (analytical and numerical) have been experimentally validated by measuring the thermal and the vibration responses induced by the laser pulses. The experimental part of the study has been performed on a cantilever beam excited with laser pulses from an Nd : YAG source (532 nm, 100 mJ/pulse) using an high-speed infrared camera and a scanning laser Doppler vibrometer. Results from this work can be used to improve understanding concerning the features of laser excitation and to establish a mechanical equivalent system of forces and moments, useful in order to increase the accuracy in the measurements of modal parameters when laser pulses are used as excitation sources.     

Dynamics of phase transitions induced by femtosecond laser pulse irradiation of indium phosphide

The structural transformation dynamics of single-crystalline indium phosphide (InP) irradiated with 150 fs laser pulses at 800 nm has been investigated by means of time-resolved reflectivity measurements covering a time window from 150 fs up to 500 ns. The results obtained show that for fluences above a threshold of 0.16 J/cm² thermal melting of the material occurs on the timescale of 1–2 ps. The evolution of the reflectivity on a longer timescale reveals the reflectivity of the liquid phase and shows resolidification times typically around 10–30 ns after which an amorphous layer several tens of nanometers thick is formed on the surface. This amorphous layer significantly alters the optical properties of the surface and finally leads to a reduced ablation threshold for subsequent laser pulses. Single-pulse ablation at higher fluences (>0.23 J/cm²) is preceded by an ultrafast phase transition (non-thermal melting) occurring within 400 fs after the arrival of the pulse to the surface. PACS 79.20.Ds; 78.47.+p; 64.70.-p     

Vibrational excitation of diatomic molecular ions in strong field ionization of diatomic molecules

A model based on the strong-field and Born-Oppenheimer approximations qualitatively describes the distribution over vibrational states formed in a diatomic molecular ion following ionization of the neutral molecule by intense laser pulses. Good agreement is found with a recent experiment [X. Urbain et al., Phys. Rev. Lett. 92, 163004 (2004)]. In particular, the observed deviation from a Franck-Condon-like distribution is reproduced. Additionally, we demonstrate control of the vibrational distribution by a variation of the peak intensity or a change of frequency of the laser pulse.     

Influence of Initial Pulse Chirp on Rainbow-Like Supercontinuum Generation from Filamentation in Air

Supereontinuum （SC） generation from laser filamentation in air is found to depend strongly on the pulse duration. Rainbow-like SC generation is observed only for a pulse of appropriate negative chirp that agrees with the predictions put forward by Golubtsov et al. [Quantum Electron. 33 （2003） 525]. The conversion efficiency of an 800-nm laser light to rainbow-like SC is found to be the highest for 257fs pulses with an initial negative chirp. A larger chirp will lead to filamentation surviving at longer distance.     

A Kinetic Theory approach for laser pulse heating process

Pulsed lasers are being widely used in industry for their high precision and low cost in material processing. In order to understand the laser machining mechanism, a thermal analysis of the process is of utmost importance. In the present study, heat transfer mechanisms relevant to pulsed laser heating is solved numerically using the Kinetic Theory approach for different types of laser pulses. A comparison between the temperature profiles developed due to different pulses is also carried out. In the heat transfer model, conduction and convection due to melting and evaporation are considered. The laser pulse profiles introduced in the analysis are selected as appropriate to laser machining processes. Stainless steel is selected as the heat transfer medium.     

脉冲强激光破坏Hg0.8Cd0.2Te晶片材料的机理分析

 利用光学金相显微镜对TEA-CO₂脉冲强激光辐照的Hg_0.8Cd_0.2Te晶片表面进行了观察。在单脉冲能量为37.5 J，能量密度为937.5 J/cm²的强激光辐照下，晶片表面呈现出熔融迹象和大量的微裂纹，微裂纹密度从激光辐照区中心向外逐渐减少，裂纹沿晶体的(111)面扩展。随着脉冲连续作用次数的增加，晶片表面熔融更加剧烈，裂纹数目、裂纹深度和宽度都有所增加。分析认为：HgCdTe晶片的破坏与激光辐照能量、脉冲连续作用次数、激光场强分布、激光热应力、激光支持的燃烧波和物质的蒸发波等冲击波有关。     

SEM and Raman spectroscopy analyses of laser-induced periodic surface structures grown by ethanol-assisted femtosecond laser ablation of chromium

The effect of fluence and pulse duration on the growth of nanostructures on chromium (Cr) surfaces has been investigated upon irradiation of femtosecond (fs) laser pulses in a liquid confined environment of ethanol. In order to explore the effect of fluence, targets were exposed to 1000 pulses at various peak fluences ranging from 4.7 to 11.8?J?cm^–2 for pulse duration of ～25?fs. In order to explore the effect of pulse duration, targets were exposed to fs laser pulses of various pulse durations ranging from 25 to 100?fs, for a constant fluence of 11.8?J?cm^–2. Surface morphology and structural transformations have been analyzed by scanning electron microscopy and Raman spectroscopy, respectively. After laser irradiation, disordered sputtered surface with intense melting and cracking is obtained at the central ablated areas, which are augmented with increasing laser fluence due to enhanced thermal effects. At the peripheral ablated areas, where local fluence is approximately in the range of 1.4–4?mJ?cm^–2, very well-defined laser-induced periodic surface structures (LIPSS) with periodicity ranging from 270 to 370?nm along with dot-like structures are formed. As far as the pulse duration is concerned, a significant effect on the surface modification of Cr has been revealed. In the central ablated areas, for the shortest pulse duration (25?fs), only melting has been observed. However, LIPSS with dot-like structures and droplets have been grown for longer pulse durations. The periodicity of LIPSS increases and density of dot-like structures decreases with increasing pulse duration. The chemical and structural modifications of irradiated Cr have been revealed by Raman spectroscopy. It confirms the formation of new bands of chromium oxides and enol complexes or Cr-carbonyl compounds. The peak intensities of identified bands are dependent upon laser fluence and pulse duration.     

Influence of pulse duration on the doping quality in laser chemical processing (LCP)—a simulative approach

The laser chemical processing (LCP) technique for the local doping of crystalline silicon solar cells is investigated. Here, a liquid jet containing a dopant source acts as a waveguide for pulsed laser light, which results in the melting and subsequent doping of the silicon surface. Typical LCP pulse durations are in the 15 ns range, giving satisfactory results for specific parameter settings. While great potential is assumed to exist, optimization of the pulse duration has until now not been deeply investigated, because it is hard to change this parameter in laser systems. Therefore, this paper accesses the influence of the pulse duration by a simulative approach. The model includes optics, thermodynamics, and melt dynamics induced by the liquid jet and dopant diffusion into the silicon melt. It is solved by coupling our existing finite differences Matlab-code LCPSim with the commercial fluid flow solver Ansys Fluent. Simulations of axial symmetric single pulses were performed for pulse durations ranging from 15 ns to 500 ns. Detailed results are given, which show that for longer pulse durations lateral heat conduction significantly homogenizes the inhomogeneous dopant distribution caused by the speckled intensity profile within the liquid jet cross section. The melt expulsion by the liquid jet is low enough that a sufficiently doped layer remains after full resolidification for all pulse durations. Last, temperature gradients are evaluated to give an indication on the amount of laser damage induced by thermal stress.     

Multi-terawatt laser system generating 12-fs pulses at 100 Hz repetition rate

We have developed a laser system generating high peak-power ultrashort pulses based on the chirped pulse amplification of a Ti:sapphire laser. The pulse duration of the laser is reduced to 12 fs, owing to the successful compensation of gain narrowing during amplification with novel optics. The pulse energy and the repetition rate are 40 mJ and 100 Hz, respectively, as a consequence of sufficient energy extraction from a laser medium in the final stage of a multipass amplifier chain, which is designed to balance focusing with a thermal lens against defocusing with convex mirrors without cryogenic cooling for Ti:sapphire crystals. The laser pulses delivered from this laser system are suitable for generating an intense attosecond pulse train in a vacuum ultraviolet or extreme ultraviolet region.     

A short pulse, free running, Nd : YAG laser for the cleaning of stone cultural heritage

This paper presents a Nd : YAG laser operating in free running (FR) regime, with a pulse duration (20 μs) shorter than conventional systems (>200 μs), mainly developed for applications in laser cleaning of stones, especially for the restoration of cultural heritage. The system was also optimized to achieve high energy and low divergence, for easy coupling with optical fibers. The unusual pulse temporal regime induces a spiky behavior of the laser output which could also help in the application. Details on the technologies for the flashlamps power supplies, including the discharge circuits needed to achieve the short pulses, are given. Application trials on artworks and artificial samples are also discussed. Results show that the intermediate pulse duration avoids the mechanical damage induced by the photomechanical effect of Q-switch lasers and the thermal damage, as superficial melting, usually induced by long pulse FR lasers.     

Dominance of radiation pressure in ion acceleration with linearly polarized pulses at intensities of 10(21) W cm(-2)

A novel regime is proposed where, by employing linearly polarized laser pulses at intensities 10(21) W cm(-2) (2 orders of magnitude lower than discussed in previous work [T. Esirkepov et al., Phys. Rev. Lett. 92, 175003 (2004)]), ions are dominantly accelerated from ultrathin foils by the radiation pressure and have monoenergetic spectra. In this regime, ions accelerated from the hole-boring process quickly catch up with the ions accelerated by target normal sheath acceleration, and they then join in a single bunch, undergoing a hybrid light-sail-target normal sheath acceleration. Under an appropriate coupling condition between foil thickness, laser intensity, and pulse duration, laser radiation pressure can be dominant in this hybrid acceleration. Two-dimensional particle-in-cell simulations show that 1.26 GeV quasimonoenergetic C(6+) beams are obtained by linearly polarized laser pulses at intensities of 10(21) W cm(-2).     

Generation and detection of incoherent phonons in picosecond ultrasonics

In picosecond ultrasonics experiments the absorption of a femtosecond laser pulse in a thin metallic transducer is used to generate very short acoustic pulses. These pulses are made of coherent longitudinal waves with a frequency spectrum that can reach 100-200 GHz. The laser pulse absorption gives rise to a heating of the film of a few Kelvin within a typical time of 1 ps. Later on, the heat goes in the substrate through an interface thermal resistance and is diffused by thermal conduction. At very low temperature and in pure crystals the thermal phonons emitted by the heated metallic film can propagate ballistically over large distances and produce a so-called heat pulse. We report on the experimental evidence of the coexistence of the coherent acoustic pulse and the incoherent heat pulse generated and detected by laser ultrasonics.     

Skin-Depth Theory Explaining Anomalous Picosecond-Terawatt Laser Plasma Interaction II

The anomaly of ion emission at laser irradiation of targets measured by Badziak et al. was analyzed on the background of the extensive research in the past. In contrast to the irradiation with lasers of longer than 100 ps pulse duration, a drastic decrease of the maximum ion energies was measured with ps pulses. Very strange was the observation that the number of emitted fast ions was intensity independent. The usual ponderomotive or relativistic self-focusing theory and related processes could not explain the results. Instead a direct interaction within the skin depth of the irradiated target was concluded. This model confirms the plane geometry nonlinear force interaction in the ps range producing fast plasma blocks moving perpendicular off or into the target. The block moving into the target opens a new scheme of laser fusion by modifying the experiments of Norreys et al. The use of relatively low subrelativistic laser intensities for the new scheme of laser fusion is evaluated on the background of the long years studies of nonlinear force driven plasma blocks and earlier interpenetration fusion reactions for providing the parameters for expected fusion gains much higher than the experiment of Norreys et al. for a fusion power scheme.     

Picosecond pulse laser ablation of yttria-stabilized zirconia from kilohertz to megahertz repetition rates

Thin films of yttria stabilized zirconia were deposited onto silicon substrates using high repetition rate picosecond pulse lasers. The applied lasers covered the repetition rate range from 10 kHz to 4 MHz. We found that the laser pulse overlapping which results from increased repetition rates led to considerable changes in the ablation process. Defect formation and local heating lead to lower ablation thresholds and, with sufficiently high repetition rates, to melting of the target and even to thermal evaporation. We found that yttria-stabilized zirconia (YSZ) films deposited with picosecond pulses at 1064 nm wavelength below repetition rates of 2 MHz have rough, nanostructured morphology and the same atomic ratio of yttrium and zirconium as the target. Films deposited with 2 MHz and higher repetition rates with high number of overlapping pulses are very smooth, but are yttrium deficient, providing evidence of the importance of the thermal processes.     

激光高精度细胞微手术机理的研究

激光微手术的精度通常受到热作用或者热机械效应间接损伤的限制。把激光脉宽调整到与吸收颗粒的热弛豫时间一致可以避免热副作用,可对所选细胞产生选择性处理。这里胶体金与牛肠硷性磷酸脂酶（Alkaline phosphatase,AP）结合物作为分析强吸收颗粒附近的蛋白质变性的模型系统,利用皮秒和纳秒激光对结合体进行照射,在一定的条件下可以使蛋白质失活,并可以使细胞膜的通透性提高。通过计算可知,采用脉宽为皮秒或者纳秒级的激光照射纳米吸收物（胶体金）时,颗粒很容易被加热到几百开,而且热效应仅限于几十纳米的空间范围内。利用纳米吸收颗粒进行激光微手术的方法．在蛋白质失活的情况下对其机弹讲行了分析．     

Variable square pulse vs conventional PFN pumping of Er:YAG laser

The influence of the flashlamp pump current pulse shape on Er:YAG laser efficiency and laser rod thermal focusing was studied theoretically and experimentally. Two pulse shapes, PFN (Pulse Forming Network) and VSP (Variable Square Pulse), were considered. Theoretical modeling and experimental measurements show that the pump pulse shape itself does not have a significant influence on the Er:YAG laser efficiency or thermal focusing. Instead, the major parameter influencing Er:YAG laser efficiency and thermal focusing was found to be the overall pulse duration. For PFN pulses, rise and fall times directly define the overall pulse duration, and therefore do have influence on thermal focusing. By contrast, VSP pulse duration is defined by the externally controlled on-time of the switching transistor. For square shaped pulses, short rise and fall times do not have a direct beneficial influence on thermal lensing.