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Application of picosecond and femtosecond laser pulses to the controlled ablation of materials represents a relatively unexplored yet important topic in laser processing. Such ultrashort pulses are of potential value in areas of thin-film deposition, micromachining, and surgical procedures. We report here some early results of systematic studies being done from the femtosecond to the nanosecond regime, as an assessment of the problems and benefits associated with various laser pulse durations and their use in processing optically absorbing media. Experimental data and theoretical results of computer simulations are presented and compared for the threshold energies of ablation in gold as a function of pulse width from 10 ns to 100 fs. This work is then extended to include further numerically computed results for gold and silicon on ablation rates, threshold surface temperatures, liquid thicknesses, and vaporization rates as a function of pulse duration throughout the ultrafast regime from tens of femtoseconds to a few hundred picoseconds.

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An ultrafast (100 fs) Ti sapphire laser (780 nm) was used for the deposition of SnO2 thin films. The laser-induced plasma generated from the SnO2 target was characterized by optical emission spectroscopy and electrostatic energy analysis. It was found that the ionic versus excited-neutral component ratio in the plasma plume depends strongly on the amount of background oxygen introduced to the deposition chamber. Epitaxial SnO2 films with high quality and a very smooth surface were deposited on the (1̄012) sapphire substrate fabricated at 700 °C with an oxygen background pressure of ∼0.1 mTorr. The films are single crystalline with the rutile structure, resulting from the high similarity in oxygen octahedral configurations between the sapphire (1̄012) surface and the SnO2 (101) surface. Hall effect measurements showed that the electron mobility of the SnO2 film is lower than that of bulk single crystal SnO2, which is caused by the scattering of conduction electrons at the film surface, substrate/film interface, and crystal defects.

Time resolved emission spectroscopy coupled with a secondary time-delayed femtosecond pulse technique has been used to study laser–matter interaction that occurs within ablation processes from a solid target, in the 1012–1014W∕cm2 energy range. It allows an examination of the emitted optical signals that characterize the species escaping from the target, namely ions, neutrals, and nanoparticles. Size distributions of nanoparticles are deduced from an analysis of the deposition substrate. The newest result concerns the huge drop of emission signal from the nanoparticles, which occurs at a delay (0.8<Δt<1ns) and has been attributed to a fragmentation process.

Hydrogen-free diamond-like carbon ͑DLC͒ films have been deposited with a 100 fs ͑FWHM͒ Ti:sapphire laser beam at intensities I in the 10 14-10 15 W/cm 2 range. The films were studied with scanning probe microscopy, variable angle spectroscopic ellipsometry, Raman spectroscopy, and electron energy loss spectroscopy. DLC films with good scratch resistance, excellent chemical inertness, and high optical transparency in the visible and near infrared range were deposited at room temperature. As the laser intensity was increased from 3ϫ10 14 to 6ϫ10 15 W/cm 2 , the films showed an increased surface particle density, a decreased optical transparency (85%˜60%), and Tauc band gap (1.4˜0.8 eV), as well as a lower sp 3 content (60%˜50%). The time-of-flight spectra recorded from the laser plume exhibited a double-peak distribution, with a high energy suprathermal ion peak preceding a slower thermal component. The most probable ion kinetic energy showed an I 0.55 dependence, increasing from 300 to 2000 eV, when the laser intensity was varied from 3ϫ10 14 to 6ϫ10 15 W/cm 2 , while the kinetic energy of suprathermal ions increased from 3 to over 20 keV and showed an I 0.33 dependence. These high energy ions are believed to have originated from an electrostatic acceleration field established by suprathermal electrons which were formed by resonant absorption of the intense laser beams.

Abstract-As ultrafast lasers achieve ever higher focused intensities on target, the problem of ensuring a clean laser-solid interaction becomes more pressing. In this paper, we give concrete examples of the deleterious effects of low-contrast interactions, and address the problem of subpicosecond laser intensity contrast ratio on both characterization and control fronts. We present the new technique of high-dynamic-range plasma-shuttered streak camera contrast measurement, as well as two efficient and relatively inexpensive ways of improving the contrast of short pulse lasers without sacrificing on the output energy: a double-pass Pockels cell (PC), and clean high-energy-pulse seeding of the regenerative amplifier.

Unhydrogenated diamondlike carbon ͑DLC͒ thin films have been deposited by laser ablation of graphite, using a high power Ti: sapphire solid state laser system. DLC films were deposited onto silicon substrates at room temperature with subpicosecond laser pulses, at peak intensities in the 4ϫ10 14-5ϫ10 15 W/cm 2 range. A variety of techniques, including scanning and transmission electron microscopy ͑SEM and TEM͒, Raman spectroscopy, spectroscopic ellipsometry ͑SE͒, and electron energy loss spectroscopy ͑EELS͒ have been used to analyze the film quality. Smooth, partially transparent films were produced, distinct from the graphite target. Sp 3 volume fractions were found to be in the 50%-60% range, with Tauc band gaps ranging from 0.6 to 1.2 eV, depending on laser intensity. Kinetic energies carried by the carbon ions in the laser induced plasma were measured through time-of-flight ͑TOF͒ spectroscopy. Their most probable kinetic energies were found to be in the 700-1000 eV range, increasing with laser intensity.