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

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

Gallium transformation under femtosecond laser excitation: Phase coexistence and incomplete melting

The reversible phase transition induced by femtosecond laser excitation of Gallium has been studied by measuring the dielectric function at 775 nm with ~ 200 fs temporal resolution. The real and imaginary parts of the transient dielectric function were calculated from absolute reflectivity of Gallium layer measured at two different angles of incidence, using Fresnel formulas. The time-dependent electron-phonon effective collision frequency, the heat conduction coefficient and the volume fraction of a new phase were restored directly from the experimental data, and the time and space dependent electron and lattice temperatures in the layer undergoing phase transition were reconstructed without ad hoc assumptions. We converted the temporal dependence of the electron-phonon collision rate into the temperature dependence, and demonstrated, for the first time, that the electron-phonon collision rate has a non-linear character. This temperature dependence converges into the known equilibrium function during the cooling stage. The maximum fraction of a new phase in the laser-excited Gallium layer reached only 60% even when the deposited energy was two times the equilibrium enthalpy of melting. We have also demonstrated that the phase transition pace and a fraction of the transformed material depended strongly on the thickness of the laser-excited Gallium layer, which was of the order of several tens of nanometers for the whole range of the pump laser fluencies up to the damage threshold. The kinetics of the phase transformation after the laser excitation can be understood on the basis of the classical theory of the first-order phase transition while the duration of non-thermal stage appears to be comparable to the sub-picosecond pulse length.Comment: 28 pages, including 9 figs. Submitted to Phys. Rev. B 14 March 200

Origin of magnetic moments in carbon nanofoam

A range of carbon nanofoam samples was prepared by using a high-repetition-rate laser ablation technique under various Ar pressures. Their magnetic properties were systematically investigated by dc magnetization measurements and continuous wave (cw) as well as pulsed EPR techniques. In all samples we found very large zero-field cooled-field-cooled thermal hysteresis in the susceptibility measurements extending up to room temperature. Zero-field cooled (ZFC) susceptibility measurements also display very complex behavior with a susceptibility maximum that strongly varies in temperature from sample to sample. Low-temperature magnetization curves indicate a saturation magnetization MS ≈0.35 emu g at 2 K and can be well fitted with a classical Langevin function. MS is more than an order of magnitude larger than any possible iron impurity, proving that the observed magnetic phenomena are an intrinsic effect of the carbon nanofoam. Magnetization measurements are consistent with a spin-glass type ground state. The cusps in the ZFC susceptibility curves imply spin freezing temperatures that range from 50 K to the extremely high value of >300 K. Further EPR measurements revealed three different centers that coexist in all samples, distinguished on the basis of g -factor and relaxation time. Their possible origin and the role in the magnetic phenomena are discussed

Three-dimensional femtosecond laser nanolithography of crystals

Nanostructuring hard optical crystals has so far been exclusively feasible at their surface, as stress induced crack formation and propagation has rendered high precision volume processes ineffective. We show that the inner chemical etching reactivity of a crystal can be enhanced at the nanoscale by more than five orders of magnitude by means of direct laser writing. The process allows to produce cm-scale arbitrary three-dimensional nanostructures with 100 nm feature sizes inside large crystals in absence of brittle fracture. To showcase the unique potential of the technique, we fabricate photonic structures such as sub-wavelength diffraction gratings and nanostructured optical waveguides capable of sustaining sub-wavelength propagating modes inside yttrium aluminum garnet crystals. This technique could enable the transfer of concepts from nanophotonics to the fields of solid state lasers and crystal optics.Comment: Submitted Manuscript and Supplementary Informatio

Ultrafast laser micro-nano structuring of transparent materials with high aspect ratio

Ultrafast lasers are ideal tools to process transparent materials because they spatially confine the deposition of laser energy within the material's bulk via nonlinear photoionization processes. Nonlinear propagation and filamentation were initially regarded as deleterious effects. But in the last decade, they turned out to be benefits to control energy deposition over long distances. These effects create very high aspect ratio structures which have found a number of important applications, particularly for glass separation with non-ablative techniques. This chapter reviews the developments of in-volume ultrafast laser processing of transparent materials. We discuss the basic physics of the processes, characterization means, filamentation of Gaussian and Bessel beams and provide an overview of present applications