154 research outputs found
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
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
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
First-principles assessment of potential ultrafast laser-induced structural transition in Ni
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