Sergei I. Kudryashov

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.

Structural transitions in silicon induced by a femtosecond laser pulse: The role of an electron-hole plasma and phonon-phonon anharmonicity

It is shown by the methods of time-resolved self-reflection and linear reflection that irradiation of a silicon target by a 100-fs laser pulse induces successive structural transitions of the target material to new crystal and liquid metal phases, which can occur during the laser pulse or 0.1–103 ps after the pulse termination, depending on the excitation conditions. The thresholds of these structural transitions are determined, and “’soft” phonon modes involved in them are identified, which represent “hot” short-wavelength LA modes. The dynamics of the structural transitions in silicon in the time interval from 0.1 to 103 ps is described using the model of instability of phonon modes caused by an electron-hole plasma and intra-and intermode phonon-phonon anharmonic interactions

Enhanced relativistic laser–plasma coupling utilizing laser-induced micromodified target

The interaction of slighly relativistic femtosecond laser radiation with microstructured Si targets was studied. The microstructuring was performed by nanosecond pulse laser ablation with additional chemical etching of the target material. An analysis was made of the optical damage under the action of femtosecond radiation near the ablation threshold. It was experimentally demonstrated that the hot electron temperature increases appreciably in the laser-driven plasma (from ~370 to almost 500 keV) as well as radiation yield in the MeV range at the interaction of a high power femtosecond laser pulse with a microstructured surface in comparison with a flat surface. Numerical simulations using 3D3V PIC code Mandor revealed that the charged particle energy growth is caused by the stochastic motion of electrons in the complex field formed by the laser field and the quasistatic field at the sharp tips of micromodifications.

Femtosecond laser-induced ablation of graphite

The dynamics of fs-laser ablation of graphite has been investigated experimentally and theoretically. The experimental observation of two different ablation mechanisms is supported by molecular dynamics calculations, which incorporate the changes of the interatomic potentials due to electronic excitation.

Microscopic characterization of ablation craters produced by femtosecond laser pulses

The formation of well-defined craters is a general feature of laser ablation with ultrashort laser pulses, indicative of a sharp ablation threshold. Results of a microscopic characterization of ablation craters on semiconductors after irradiation with single intense ultrashort laser pulses are presented.

Ablation dynamics of solids heated by femtosecond laser pulses

Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern. Optically sharp surface occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials.

Non‐linear Absorption and Ionization of Gases by Intense Femtosecond Laser Pulses

Recent experiments on applications of high‐intensity femtosecond pulses for studying multi‐photon and tunnel ionization of different gases, including air, are discussed. Mechanisms of non‐linear absorption and ionization of pure atomic argon and molecular nitrogen gases by UV femtosecond laser pulses were studied using photogalvanic and photoacoustic technique. The effect of the intermediate Rydberg resonance, its dynamic Stark perturbation and ponderomotive up‐shift of the first ionization potential of argon atoms and nitrogen molecules by the intense laser pulses has been revealed by observing an increase of a power slope of ion yield from three to four at increasing laser intensity. The photoacoustic technique was also applied for studying the effect of tunnel ionization of air by IR femtosecond laser pulses with sub‐critical peak power in the range of intensity ∼0.5–20u2009PW/cm2. Saturation of ultrasonic signals at near‐atomic laser fields, which is well described by ADK model, is observed.

Laser ablation doping of poly crystalline tin dioxide films with palladium

Laser ablation of palladium was studied and velocity (energy) distributions of palladium ions evaporated by an Kr-F laser in a vacuum were obtained. The optimum values of energy fluence (fluence rate) of laser radiation for doping tin dioxide films, at which neither multiply charged PdN+ ions nor ionized clusters PdN+, occur in a plasma, were determined. From time-of-flight probe measurement data, Pd+ implantation depths in SnO2 films were calculated, which qualitatively agree with the results obtained by secondary neutral mass spectrometry. Electric conductivity measurements on the obtained films in a gas phase showed that introduction of palladium into polycrystalline SnO2 films by laser ablation significantly enhanced their gas sensitivity to hydrogen.

Probe detection of charged coarsely dispersed carbon particles upon laser ablation of pyrolytic graphite

The mass distribution of charged particles from the plasma plume produced by laser ablation of pyrolytic graphite was studied by an electrostatic probe technique. The velocities of motion of the charged opposite parts of the plume double-electric layer and the potential of its internal self-consistent field were determined. Using a proposed procedure for the treatment of probe signals, the amounts of detectable positive and negative ions were evaluated. The negatively charged large carbon particles with a size of up to 106 atoms were assumed to directly appear from explosive bulk effervescence and spinodal separation of the overheated carbon phase on the graphite surface.

Modification of semiconductor surfaces irradiated by single intense fs-laser pulses

Summary form only given. Material modification by ultrashort laser pulses represents a promising tool for a variety of technological applications. Nevertheless the mechanisms of femtosecond laser ablation are still not well understood. A striking feature of femtosecond laser ablation is the extremely sharp threshold; i.e. an increase of the laser fluence by less than one percent leads to the removal of a macroscopic amount of the material. Trying to understand the physical nature of this process we have conducted a detailed investigation of the final ablation morphology produced on GaAs[100] and Si[100] surfaces by single intense fs-laser pulses using optical interference, differential interference contrast (DIC), atomic force (AFM) and scanning electron (SEM) microscopies. Our craters are different from those produced under very tight focusing conditions (6 /spl times/ 6 /spl mu/m FWHM). We have also observed similar change in the crater shape when focusing very tightly. The influence of the focusing conditions on the mechanisms of final crater formation is the subject of further investigations.