Expansion-limited aggregation of nanoclusters in a single-pulse laser-produced plume

Formation of carbon nanoclusters in a single-laser-pulse created ablation plume was studied both in vacuum and in a noble gas environment at various pressures. The developed theory provides cluster radius dependence on combination of laser parameters, properties of ablated material, and type and pressure of an ambient gas in agreement with experiments. The experiments were performed on carbon nanoclusters formed by laser ablation of graphite targets with 12 picosecond 532 nm laser pulses at MHz-range repetition rate in a broad range of ambient He, Ar, Kr, and Xe gas pressures from 2× 10-2 to 1500 Torr. The experimental results confirmed our theoretical prediction that the average size of the nanoparticles depends weakly on the type of the ambient gas used, and is determined exclusively by the single laser pulse parameters even at the repetition rate as high as 28 MHz with the time gap 36 ns between the pulses. The most important finding relates to the fact that in vacuum the cluster size is mainly determined by hydrodynamic expansion of the plume while in the ambient gas it is controlled by atomic diffusion in the gas. We demonstrate that the ultrashort pulses can be used for production of clusters with the size less than the critical value, which separates the particles with properties drastically different from those of a material in a bulk. The presented results of experiments on formation of carbon nanoclusters are in close agreement with the theoretical scaling. The developed theory is applicable for cluster formation from any monatomic material, such as silicon for example

Excitation of Coherent Phonons in Crystalline B: theory for driving atomic vibrations by femtosecond pulses

In this paper we present experimental and theoretical studies of reflectivity oscillations of an optical probe beam reflected from a single-crystal of bismuth excited by 35 fs laser pulses at deposited energy density above the melting temperature. Coherent and incoherent lattice dynamics as well as electrons dynamics were investigated starting from the reflectivity changes, measured with high accuracy ΔR/R < 10. The complex behaviour of the reflectivity could not be explained in the light of the existing theories. Therefore, we developed a new theory, starting from the very basic principles of lasermatter interaction, which shows good agreement with experimental results. We establish a direct dependence of the transient reflectivity on atomic motions driven by electron temperature gradient through electron-phonon coupling

Cluster formation through the action of a single picosecond laser pulse

We demonstrate experimentally and describe theoretically the formation of carbon nanoclusters created by single picosecond laser pulses. We show that the average size of a nanocluster is determined exclusively by single laser pulse parameters and is independent of the gas fill (He, Ar, Kr, Xe) and pressure in a range from 20mTorr to 200 Torr. Simple kinetic theory allows estimates to be made of the cluster size, which are in qualitative agreement with the experimental data. We conclude that the role of the buffer gas is to induce a transition between thin solid film formation on the substrate and foam formation by diffusing the clusters through the gas, with no significant effect upon the average cluster size

Picosecond high-repetition-rate pulsed laser ablation of dielectrics: the effect of energy accumulation between pulses

We report experiments on the ablation of arsenic trisulphide and silicon using high-repetition-rate (megahertz) trains of picosecond pulses. In the case of arsenic trisulphide, the average single pulse fluence at ablation threshold is found to be >100 times lower when pulses are delivered as a 76-MHz train compared with the case of a solitary pulse. For silicon, however, the threshold for a 4.1-MHz train equals the value for a solitary pulse. A model of irradiation by high-repetition-rate pulse trains demonstrates that for arsenic trisulphide energy accumulates in the target surface from several hundred successive pulses, lowering the ablation threshold and causing a change from the laser-solid to laser-plasma mode as the surface temperature increases

Unconventional magnetism in all-carbon nanofoam

We report production of nanostructured carbon foam by a high-repetition-rate, high-power laser ablation of glassy carbon in Ar atmosphere. A combination of characterization techniques revealed that the system contains both sp2 and sp3 bonded carbon atoms. The material is a novel form of carbon in which graphite-like sheets fill space at very low density due to strong hyperbolic curvature, as proposed for ?schwarzite?. The foam exhibits ferromagnetic-like behaviour up to 90 K, with a narrow hysteresis curve and a high saturation magnetization. Such magnetic properties are very unusual for a carbon allotrope. Detailed analysis excludes impurities as the origin of the magnetic signal. We postulate that localized unpaired spins occur because of topological and bonding defects associated with the sheet curvature, and that these spins are stabilized due to the steric protection offered by the convoluted sheets.Comment: 14 pages, including 2 tables and 7 figs. Submitted to Phys Rev B 10 September 200

Momentum transfer in a standing optical vortex

A field superposition of singular beams incident on, and then reflected from a mirror has been investigated. It was demonstrated that the standing optical wave, which contains a vortex, possesses an orbital angle momentum where the energy flux circulates only in the azimuth direction of the beam. We show in this paper that the standing light wave containing the optical vortex transfers angular momentum to a substance located in the field of the vortex without moving the substance in the azimuth or radial directions. This property of the standing vortex present an opportunity to form the three-dimensional optical traps, gasdynamic and hydrodynamic vortices, in a localised volume by a direct transfer of the orbital angular momentum from the optical vortex.Comment: English has been corrected; a second address entered; Eq.(13) and (14) corrected. 11 pages, including 1 figur

Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics

The mechanism of ablation of solids by intense femtosecond laser pulses is described in an explicit analytical form. It is shown that at high intensities when the ionization of the target material is complete before the end of the pulse, the ablation mechanism is the same for both metals and dielectrics. The physics of this new ablation regime involves ion acceleration in the electrostatic field caused by charge separation created by energetic electrons escaping from the target. The formulae for ablation thresholds and ablation rates for metals and dielectrics, combining the laser and target parameters, are derived and compared to experimental data. The calculated dependence of the ablation thresholds on the pulse duration is in agreement with the experimental data in a femtosecond range, and it is linked to the dependence for nanosecond pulses.Comment: 27 pages incl.3 figs; presented at CLEO-Europe'2000 11-15 Sept.2000; papers QMD6 and CTuK11