Plasma density gratings induced by intersecting laser pulses in underdense plasmas

Electron and ion density gratings induced by two intersecting ultrashort laser pulses at intensities of 10¹⁶ W/cm² or lower are investigated. The ponderomotive force generated by the inhomogeneous intensity distribution in the intersecting region of the interfering pulses produces deep electron and ion density modulations at a wavelength less than a laser wavelength in vacuum. Dependence of the density modulation on the plasma densities, temperatures, and the ion mass, as well as the laser pulse parameters are studied analytically and by particle-in-cell simulations. It is found that the density peaks of such gratings can be a few times that of the initial plasma density and last as long as a few picoseconds. It is also demonstrated that the scattering of signal pulses by such a bulk density grating results in high-harmonic generation. The density gratings may be incorporated into ion-ripple lasers [K.R. Chen and J.M. Dawson, Phys. Rev. Lett. 68, 29 (1992)] to generate ultrashort X-ray pulses of a few angstroms by using electron beams at only a few tens of MeV only. PACS 52.35.Mw; 42.65.Ky; 52.25.Os     

Ultrashort-pulse laser ablation of indium phosphide in air

Ablation of indium phosphide wafers in air was performed with low repetition rate ultrashort laser pulses (130 fs, 10 Hz) of 800 nm wavelength. The relationships between the dimensions of the craters and the ablation parameters were analyzed. The ablation threshold fluence depends on the number of pulses applied to the same spot. The single-pulse ablation threshold value was estimated to be φ_th(1)=0.16 J/cm². The dependence of the threshold fluence on the number of laser pulses indicates an incubation effect. Morphological and chemical changes of the ablated regions were characterized by means of scanning electron microscopy and Auger electron spectroscopy. Received: 30 May 2000 / Accepted: 31 May 2000 / Published online: 23 August 2000     

Femtosecond laser ablation characteristics of nickel-based superalloy C263

Femtosecond laser (180 fs, 775 nm, 1 kHz) ablation characteristics of the nickel-based superalloy C263 are investigated. The single pulse ablation threshold is measured to be 0.26±0.03 J/cm² and the incubation parameter ξ=0.72±0.03 by also measuring the dependence of ablation threshold on the number of laser pulses. The ablation rate exhibits two logarithmic dependencies on fluence corresponding to ablation determined by the optical penetration depth at fluences below ∼5 J/cm² (for single pulse) and by the electron thermal diffusion length above that fluence. The central surface morphology of ablated craters (dimples) with laser fluence and number of laser pulses shows the development of several kinds of periodic structures (ripples) with different periodicities as well as the formation of resolidified material and holes at the centre of the ablated crater at high fluences. The debris produced during ablation consists of crystalline C263 oxidized nanoparticles with diameters of ∼2–20 nm (for F=9.6 J/cm²). The mechanisms involved in femtosecond laser microprocessing of the superalloy C263 as well as in the synthesis of C263 nanoparticles are elucidated and discussed in terms of the properties of the material.     

Laser imprinting of 3D structures in gel-based titanium oxide organic-inorganic hybrids

First results on femtosecond laser 3D-microstructuring are reported in a novel class of organic-inorganic hybrid materials based on titanium oxide gels. Transparent optical-grade polished hybrid samples demonstrate strong photosensitivity assigned to electrons trapped as Ti³⁺ centers. Two different regimes of the microstructuring are observed: reversible and irreversible. In tight-focusing conditions using “on-the-fly” technique the single-pulse microstructuring is achieved with nanojoule laser pulses. The process thresholds have been studied on the surface and into the bulk of the material with irradiation by 1, 2, 10, and 100 laser pulses. A reduction of the damage threshold fluence by a factor of 2.5 is observed when increasing exposure from 1 pulse (1.2 J/cm²) to 100 pulses (0.5 J/cm²). PACS 42.62.-b; 40.70.-a; 78.66.Sq     

Investigation on femtosecond laser irradiation energy in inducing hydrophobic polymer surfaces

This study investigates the use of ultrashort femtosecond laser pulses to induce hydrophobic properties on PMMA surfaces. The modification of surface wetting property exhibits a strong dependence on the amount of energy deposited on the PMMA surface. A simple equation has been deduced from the laser parameters to express the energy deposition. It was revealed that water contact angle (WCA) of more than 120°, with a maximum of around 125°, could be achieved when the total energy deposited per unit area on the PMMA surface ranged from 600 J/cm² to 900 J/cm² at an energy deposition rate of around 50 J/cm²/s. Beyond this range, WCA reduced with increasing amount of energy deposition. Furthermore, with higher energy deposition rate or higher laser fluence, total energy required to induce hydrophobic surfaces was reduced. Under different energy deposition, the quantity of polar groups or non-polar groups induced was responsible for the changes in WCA and thus the different surface hydrophobicity.     

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.     

Theory for the ultrafast dynamics of excited clusters: interplay between elementary excitations and atomic structure

We present a theoretical study of the short-time relaxation of clusters in response to ultrafast excitations using femtosecond laser pulses. We analyze the excitation of different types of clusters (Hg_n, Ag_n, Si_n, C₆₀ and Xe_n) and classify the relaxation dynamics in three different regimes, depending on the intensity of the exciting laser pulse. For low-intensity pulses (I<10¹² W/cm²) we determine the time-dependent structural changes of clusters upon ultrashort ionization and photodetachment. We also study the laser-induced non-equilibrium fragmentation and melting of Si_n and C₆₀ clusters, which occurs for moderate laser intensities, as a function of the pulse duration and energy. As an example for the case of high intensities (I>10¹⁵ W/cm²), the explosion of clusters under the action of very intense ultrashort laser fields is described. Received: 26 November 1999 / Published online: 2 August 2000     

Surface texturing of the carbon steel AISI 1045 using femtosecond laser in single pulse and scanning regime

Surface texturing of the metals, including steels, gained a new dimension with the appearance of femtosecond lasers. These laser systems enable highly precise modifications, which are very important for numerous applications of metals. The effects of a Ti:sapphire femtosecond laser with the pulse duration of 160 fs, operating at 775 nm wavelength and in two operational regimes - single pulse (SP) and scanning regime, on a high quality AISI 1045 carbon steel were studied. The estimated surface damage threshold was 0.22 J/cm² (SP). Surface modification was studied for the laser fluences of 0.66, 1.48 and 2.37 J/cm². The fluence of 0.66 J/cm², in both working regimes, induced texturing of the material, i.e. formation of periodic surface structures (PSS). Their periodicity was in accordance with the used laser wavelength. Finally, changes in the surface oxygen content caused by ultrashort laser pulses were recorded.     

Nonlinear photoionization and laser-induced damage in silicate glasses by infrared ultrashort laser pulses

We show that photoionization of wide band gap silicate glasses by infrared ultrashort laser pulses can occur without laser-induced damage. Two glasses are studied, fused silica and a multi-component silicate photo-thermo-refractive (PTR) glass. Experiments are performed by low numerical aperture focusing of ultrashort laser pulses (100 fsec<τ<1.5 psec) at the wavelengths 780 nm, 1430 nm, and 1550 nm. Filaments form inside both glasses and are visibly observable due to intrinsic luminescence. Keldysh’s theory of nonlinear photoionization is used to model the formation of filaments and values of about 10¹³ W cm⁻² for the laser intensity and 10¹⁹ cm⁻³ for the free electron density are estimated for stable filaments to arise. Laser-induced damage is studied by the generation of a third harmonic from an interface created between a damage site and the surrounding glass matrix. It is found that third harmonic generation occurs only after several thousands of laser shots indicating that damage is not a single-shot phenomena. The ability to photoionize PTR glass without damage by ultrashort laser pulses offers a new approach for fabricating diffractive optical elements in photosensitive glass.     

Feasibility of experimental investigation of nonlinear QED processes—photon emission by electrons and pair creation by photons in strong laser field

On the basis of the theory of nonlinear QED processes occurring in collisions of high-energy electrons and γ-quanta with a strong laser pulse [1], the feasibility of their experimental investigation is discussed. The use of existing ⋍100 GeV electron beam accelerators and terawatt lasers of ultrashort pulses enables one to attain optimal values for the parameters and to have fields ⋍F_o=m²c³/ħe act on the electron. The choice and organization of laser system, synchronization of accelerator and laser operation, distributions of created γ-quanta and e⁺e⁻-pairs in different variables and parameters, their dependence on polarization of laser target and incident particles, background problems, and other questions connected with carrying out the experiments are discussed. Translated from preprint No. 11, 1993 of the Lebedev Physics Institute, Moscow, Russia.     

Short-pulse UV laser ablation of solid and liquid metals: indium

2 to 2.5 mJ/cm² when a 0.5 ps pulse is used instead of a 15 ns laser pulse. Measurements on liquid indium show a different behavior. With 15 ns laser pulses the threshold fluence is lowered by a factor of ∼3 from 100 mJ/cm² for solid indium to 30 mJ/cm² for liquid indium. In contrast, measurements with 0.5 ps laser pulses do not show any change in the ablation threshold and are independent of the phase of the metal at 2.5 mJ/cm². This behavior could be explained by thermal diffusion and heat conduction during the laser pulse and demonstrates in an independent way the energy lost into the material when long laser pulses are applied. Time-of-flight measurements to investigate the underlying ablation mechanism show thermal behavior of the ablated indium atoms for both ps and ns ablation and can be fitted to Maxwell-Boltzmann distributions. Received: 2 December 1996/Accepted: 11 December 1996     

Microfabrication techniques used in Japan for VLSI and other devices

Lead films of thickness 100 Å, 250 Å. and 350 Å were vacuum deposited on AI and laser treated in air using single pulses (7 ns FWHM) from a Nd: glass laser operating in TEM₀₀ mode, at peak energy densities of 1.5-5.0 J/cm². Rutherford back-scattering of 1.8 MeV He⁺ ions was employed to determine the depth profiles of Pb in Al. Up to about 1.4 J/cm², we observe only evaporation loss of Pb and formation of Pb-rich cells on the surface. At higher energies, liquid phase diffusion of Pb is observed up to 4 J/cm², beyond which convection effects are seen. A quantitative analysis of data for 350 Å film at 3.0 J/cm² shows evidence of nonequilibrium segregation effects.     

Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF _thres = 172 mJ/cm². The ablation rate per pulse is measured as a function of intensity in the range of 5 × 10⁹ to 2 × 10¹¹ W/cm² and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.     

Analysis of the nonlinear absorption mechanisms in ablation of transparent materials by high-intensity and ultrashort laser pulses

The mechanisms of nonlinear absorption in transparent materials under irradiation with ultrashort laser pulses are considered theoretically. Nitride semiconductor, sapphire and others transparent dielectrics were investigated. The ablation threshold for these materials is within multi-TW/cm² range. The model was used based on the tunneling absorption under the irradiation by high-intensity ultrashort pulses in terms of the theory of ionization of solid in a field of strong electromagnetic wave. The effect of the energy gap of material on the threshold of laser ablation was adequately explained.     

Electron radiography using hot electron jets from sub-millijoule femtosecond laser pulses

Electron jets produced in the intermediate intensity range of 10¹⁵ to 10¹⁷ W/cm² from submillijoule 120 fs Ti:Sapphire laser pulses focused to spots of a few microns in diameter have been characterized. The experimental results show strong emission of hot electrons with energies from 80 keV to above 250 keV from microplasmas created with both p- and s-polarized 250 μJ laser pulses. The electron jets with energies above 250 keV are observed to be highly directional. The electron jets are observed in the plane of polarization of the laser electric field for both p- and s-polarized laser pulses. The hot electrons emitted from these femtosecond laser plasmas have also been used for radiographic imaging. It is expected that the short initial duration of these electron pulses would make them useful for time resolved applications. PACS 41.75. Fr; 52.38.Kd; 52.70.Nc     

Sub-surface damage in indium phosphide caused by micromachining of grooves with femtosecond and nanosecond laser pulses

Grooves laser-micromachined in InP using 130 fs and 8 ns pulses with fluences 2 and 0.7 J/cm² are investigated by cross-sectional transmission electron microscopy. At the fluence of 2 J/cm², irradiation with both femtosecond and nanosecond laser pulses yield substantial resolidified layers with a maximum thickness of 0.5 m. In contrast, at the fluence of 0.7 J/cm², irradiation with nanosecond pulses leads to a layer of similar thickness, while femtosecond irradiation produces laser induced periodic surface structures with minimal resolidified material. For both fluences, femtosecond pulses generate substantial densities of defects extending over a few microns in depth, while nanosecond laser irradiation leads to no observable damage beneath the resolidified layer. The high peak power density and the stress confinement obtained from femtosecond pulses, along with incubation effects, are identified as the major factors leading the observed plastic deformations. PACS 61.80.Ba; 68.35.Gy; 79.20.Ds     

KrF excimer laser dry and steam cleaning of silicon surfaces with metallic particulate contaminants

KrF excimer laser-assisted dry and steam cleaning of single-crystal silicon wafers contaminated with three different types of metallic particles was studied. The laser fluence used was 0.3 J/cm². In the dry process, for samples cleaned with 100 laser pulses the cleaning efficiency was 91, 71 and 59% for Au, Cu and W particles, respectively, whilst in steam cleaning the efficiency is about 100% after 5 laser pulses, independently of the type of contaminant. The effects of laser irradiation on the Si surface are investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Laser processing at 0.3 J/cm² does not deteriorate the Si-wafer surface, either in dry or steam cleaning. However, the measured XPS intensity coming from the metallic component is greater on the cleaned surfaces than in the initial condition. Quantification of the XPS results, assuming a stratified overlayer model for the detected species and accounting for the presence of the metallic particles on the surface, showed that the obtained results can be explained by the formation of a fractional metallic monolayer on the cleaned surfaces, due to partial vaporisation of small particles initially present on the sample surface. This contamination of the substrate could be considered excessive for some applications and it shows that the process requires careful optimisation for the required efficiency to be achieved without degradation of the substrate. Received: 14 January 2001 / Accepted: 19 February 2001 / Published online: 20 June 2001     

Using IR laser radiation for backside etching of fused silica

Laser-induced backside etching of fused silica with gallium as highly absorbing liquid is demonstrated using pulsed infrared laser radiation. The influences of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography were studied and the results are compared with these of excimer laser etching. The high reflectivity of the fused silica-gallium interface at IR wavelengths results in the measured high threshold fluences for etching of about 3 J/cm² and 7 J/cm² for 18 ns and 73 ns pulses, respectively. For both pulse lengths the etch rate rises almost linearly with laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm², respectively. The etching process is almost free from incubation processes because etching with the first laser pulse and a constant etch rate were observed. The etched surfaces are well-defined with clear edges and a Gaussian-curved, smooth bottom. A roughness of about 1.5 nm rms was measured by AFM at an etch depth of 0.95 μm. The normalization of the etch rates with respect to the reflectivity and the pulse length results in similar etch rates and threshold fluence for the different pulse widths and wavelengths. It is concluded that etching is a thermal process including the laser heating, the materials melting, and the materials etching by mechanical forces. The backside etching of fused silica with IR-Nd:YAG laser can be a promising approach for the industrial usage of the backside etching of a wide range of materials. PACS 81.65.C; 81.05.J; 79.20.D; 61.80.B; 42.55.L     

Parametric transformation and spectral shaping of supercontinuum by high-intensity femtosecond laser pulses

The cascaded nonlinear-optical transformation of high-power ultrashort light pulses in an ionizing gas medium involving supercontinuum generation, followed by a frequency conversion of this radiation in the field of femtosecond laser pulses with an intensity of 10¹⁴–10¹⁵ W/cm² has been demonstrated. Parametric four-wave mixing is shown to allow a highly efficient spectral transformation and shaping of supercontinuum radiation. The maximum efficiency of a parametric frequency conversion of femtosecond laser pulses in an ionizing gas medium achieved under the conditions of our experiments is estimated as 1%.