Silicon nanoparticles generated by femtosecond laser ablation in a liquid environment

Silicon nanoparticles were generated by femtosecond laser [387 nm, 180 fs, 1 kHz, pulse energy = 3.5 μJ (fluence = 0.8 J/cm²)] ablation of silicon in deionized water. Nanoparticles with diameters from ~5 up to ~200 nm were observed to be formed in the colloidal solution. Their size distribution follows log-normal function with statistical median diameter of ≈20 nm. Longer ablation time leads to a narrowing of the nanoparticle size distribution due to the interaction of the ablating laser beam with the produced nanoparticles. Raman spectroscopy measurements confirm that the nanoparticles exhibit phonon quantum confinement effects and indicate that under the present conditions of ablation they are partially amorphous.     

Adaptive control of femtosecond laser ablation plasma emission

The influence of temporal pulse shaping on plasma plume generated by ultrafast laser irradiation of aluminum is investigated. Time resolved plasma emission spectroscopy is coupled with a temporal shaping procedure in a closed loop. The ionic emission is enhanced relative to the neutral one via an adaptive optimization strategy. The plasma emission efficiency in case of optimized and ultrashort temporal shapes of the laser pulses are compared, evidencing an enhancement of the ionization degree of the plasma plume. Simplified temporal shapes of the femtosecond laser pulses are extracted from the optimized shape and their corresponding effect on laser induced plasma emission is discussed.     

Ultraviolet conical emission produced by high-power femtosecond laser pulse in transparent media

In this work, supercontinuum conical emission (SC CE) accompanying the filamentation of powerful ultrashort laser pulse in BK₇ glass and fused silica is studied. The SC CE is controlled by the laser power density and the sample thickness, and the minimum SC CE cut-off wavelength is about 309?nm in the BK₇ glass and 237?nm in the fused silica. The angular distributions of the SC CE in the wavelength range less than 510?nm are measured by using a new method, and it cannot be explained by the Cerenkov emission theory but the unabridged X-Waves solution theory. Meanwhile numerical simulations of the propagation of femtosecond laser pulse in sample are performed to provide theoretical support to our results.     

飞秒激光脉冲和液体等离子体相互作用产生的高能电子特性研究

文章研究了单脉冲和多脉冲飞秒激光和水等离子体相互作用产生的高能电子特性 ,发现多脉冲激光构型可以大幅度地增强超热电子的产生和提高其温度 .实验观测到在激光偏振面内 ,沿与激光轴反向夹角 4 6°的方向 ,对称地喷射出两束能量大于 2 5keV的高能电子 .二维粒子模拟结果和实验符合很好 .实验和理论都表明 ,这些超热电子是通过后续脉冲与前面脉冲形成的球形液滴相互作用产生的 ,具体机制为共振吸收 .     

Investigation of plumes produced by material ablation with two time-delayed femtosecond laser pulses

We experimentally investigated and herewith reported the results of laser ablation of copper and gold with two time-delayed femtosecond laser pulses at 800 nm in vacuum. The ablation plume dynamic was monitored by fast plume imaging and time- and space-resolved optical emission spectroscopy. Optical microscopy was used to follow the ablation depth as a function of the delay between the two laser pulses. Nanoparticles deposition on mica substrates was analysed by atomic force microscopy.We estimate roughly the plume's atomization degree - that is the mass fraction of atomized material over the total ablated mass - from the relative intensities of radiation emitted from atoms and nanoparticles. It is shown that the atomization degree depends critically on the time delay between both laser pulses and on the characteristic time of electron-lattice relaxation. The increase of the atomization degree is accompanied by the decrease of the ablation depth. Atomic force microscopy measurements confirm the partial atomization of nanoparticles, as the analyses of particle deposition on mica substrates show a large decrease of the number of nanoparticles for large delay between the two pulses.     

Superresolved femtosecond laser ablation

The determinist behavior of the femtosecond ablation process allows morphing features well under the diffraction limit by utilizing the thresholding effect, down to the nanometer scale. Because there are a vast range of applications where scaling down the size of the features is a major concern, we investigate the use of superresolving pupil plane filters. As is well known, these filters redistribute the focused optical intensity for a narrower bright spot and, as a trade-off, increase the sidelobes. However, this drawback can be rendered insignificant if all the outer optical power is kept under the determinist threshold value. Two types of pure absorbing binary filter have been tried, giving credence to a size reduction of the ablations in fused silica.     

Hard X-ray emission by clusters in an intense femtosecond laser field during collective recombination

We consider the new mechanism of X-ray generation by clusters under irradiation by femtosecond laser pulses, the so-called collective photorecombination. We develop the theory of the photo-recombination of electrons that pass from atomic clusters at the outer ionization to the ground level of a homogeneously charged cluster. Such a cluster is considered to be a quantum potential well. The dipole approximation is inapplicable for this process. We conclude that X-ray photons in collective photorecombination on a charged cluster as a whole have an energy that is much larger than that for photorecombination on separate atomic ions inside the cluster. For a typical cluster of 2.25 × 10⁶ electrons, with a radius R = 300 Å, and a number density of plasma electrons n _e = 2 × 10²² cm^?3, we find that at a 5% outer ionization of this cluster, the energy of hard X-ray photons is 7.2 keV.     

Lithographical laser ablation using femtosecond laser

Lithographical laser ablation was demonstrated using a femtosecond laser with a lithographical optical system. In this method, a femtosecond laser beam passes through a mask and the pattern is imaged on a film by a coherent optical system. As a result, the film is lithographically ablated, and a micron-sized pattern can be generated in a single shot. The resolution of generation was 13 m, and the narrowest width of a generated line was about 4 m. Moreover, the system was applied to transmission gratings as masks, and nano-sized periodic structures such as nano-sized hole matrices and nano-meshes were generated in a single shot. PACS 52.38.Mf; 42.25.Hz; 42.82.Cr; 81.16.-c     

Time-resolved emission spectra of plasma produced by excimer laser ablation of Pb-Bi-Sr-Ca-Cu-O

Time-resolved emission spectroscopy was employed to detect excited species formed in the laser ablation of superconductor Pb-Bi-Sr-Ca-Cu-O by an excimer laser at various background gas pressures. A simple model has been created to explain the chemical kinetics of reactions in a plasma plume by its expansion. We explained the time dependence of plasma emission on the wavelengths of lead, strontium and their oxides. For the purpose of this paper, we determined reaction rates of lead deexcitation and the rate of recombination of Pb⁺ ion. This work was supported by the Grant Agency of Charles University (Grant No. 28/97) and Grant Agency of Academy of Sciences of the Czech Republic (Grant No. A4040708).     

Low-threshold field electron emission of Si micro-tip arrays produced by laser ablation

Low-threshold field electron emission (FEE) is reported for periodic arrays of micro-tips produced by laser ablation of Si wafers. The best samples show emission at threshold fields as low as 4–5 V/μm for n-type Si substrates and of 1–2 V/μm for p-doped Si substrates, as measured with a flat-screen technique. Auger electron spectroscopy and X-ray electron spectroscopy reveal island-like deviation of the SiO₂ stoichiometry on the tip surfaces, with lateral dimensions of less than 100 nm. Microscopic studies using a special field-emission STM show that the emission originates from well-conducting regions of sub-micron size. The experimental data suggest FEE from the tip arrays by a geometric field enhancement of both the individual micro-tip and the narrow conducting channels in the tip body. Received: 3 May 2002 / Accepted: 1 July 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/135-82-34, E-mail: shafeev@kapella.gpi.ru     

Spatial distribution of high-energy electron emission from water plasmas produced by femtosecond laser pulses

High energy electrons emitted by water plasmas produced by a single or a multiple laser pulse are investigated. The multipulse mode greatly enhances the generation and the temperature of hot electrons. Directional emission of high energy electrons over 25 keV is observed in two symmetric directions with respect to the laser axis and at 46 degrees from the directions of the laser electric field. Two-dimensional particle-in-cell simulations reproduce well the experimental results and indicate that the acceleration mechanism of the high energy electrons is due mainly to the resonance absorption at the edge of the spherical droplets formed by the leading pulse.     

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.     

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.     

Phase transitions in femtosecond laser ablation

In this study we simulate an interaction of femtosecond laser pulses (100 fs, 800 nm, 0.1-10 J/cm²) with metal targets of Al, Au, Cu, and Ni. For analysis of laser-induced phase transitions, melting and shock waves propagation as well as material decomposition we use an Eulerian hydrocode in conjunction with a thermodynamically complete two-temperature equation of state with stable and metastable phases. Isochoric heating, material evaporation from the free surface of the target and fast propagation of the melting and shock waves are observed. On rarefaction the liquid phase becomes metastable and its lifetime is estimated using the theory of homogeneous nucleation. Mechanical spallation of the target material at high strain rates is also possible as a result of void growth and confluence. In our simulation several ablation mechanisms are taken into account but the main issue of the material is found to originate from the metastable liquid state. It can be decomposed either into a liquid-gas mixture in the vicinity of the critical point, or into droplets at high strain rates and negative pressure. The simulation results are in agreement with available experimental findings.     

Time evolution of emission spectra from plasmas produced by irradiation of seawater surfaces by a femtosecond laser

The spectral and temporal characteristics of plasmas produced by femtosecond laser pulses acting on seawater surfaces are studied. The time dependence of the decay in the intensities of emission lines and the continuum are determined and the electron density is estimated using the three-body recombination time. It is shown that an atomic line with a low excitation potential is the most sensitive for femtosecond laser induced breakdown spectroscopy.     

Silicon micro-hemispheres with periodic nanoscale rings produced by the laser ablation of single crystalline silicon

We describe the fabrication of silicon micro-hemispheres by adopting the conventional laser ablation of single crystalline silicon in the vacuum condition without using any catalysts or additives. The highly oriented structures of silicon micro-hemispheres exhibit many periodic nanoscale rings along their outer surfaces. We consider that the self-organized growth of silicon micro-structures is highly dependent on the laser intensity and background air medium. The difference between these surface modifications is attributed to the amount of laser energy deposited in the silicon material and the consequent cooling velocity.     

ZnO nanoparticles produced by reactive laser ablation

The paper presents the results of theoretical and experimental researches of the analysis of nanopowder ZnO and ZnO-based structures formation mechanisms by means of pulse laser reactive technology (λ = 1.06 μm, τ = 10⁻⁷ to 10⁻⁵ s). The developed 2D model combines non-stationary heat transfer and fluid motion along with the calculated profile of surface deformation. The characteristics of the dispersive and chemical compositions and structural parameters of the synthesized nanopowder together with the influence of the energy of laser impulse evaporation, its duration and gas pressure in the reaction chamber have been studied using X-ray diffractrometry (XRD), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM). Particle size distribution analysis of ZnO has shown that the majority of them range from 5 to 60 nm in size. The photoluminescence emission spectra of the initial ZnO nanopowder at room temperature have been identified.