Theory for Optically Created Nonequilibrium in Covalent Solids
Harald Jeschke
ISBN 978-3-89722-503-9
171 pages, year of publication: 2000
price: 40.50 €
A theory for the response of covalent solids and clusters to intense
laser pulses
(10
10-10
13 W/cm
2)
was developed. We employ a molecular dynamics method on the basis of
an electronic tight binding Hamiltonian. This real-space calculation
takes into account all atomic degrees of freedom. Special attention
was paid to the strong nonequilibrium created in the electronic system
by the ultrashort laser pulse. A method of calculating nonequilibrium
occupation numbers for the energy levels of the system leads to a
molecular dynamics calculation on time-dependent potential energy
surfaces. This approach provides a theoretical framework for the
treatment of strong nonequilibrium situations in materials where
atomic and electronic degrees of freedom play an equally important
role. The theoretical method has been applied to covalent solids like
graphite, diamond, silicon and to large covalent clusters like
C
60 and carbon nanotubes.
The investigation of the absorbed energy in diamond and graphite as a
function of (1) offered energy and (2) laser pulse duration has
produced very interesting dependencies. It was shown that the
structural response of the material during the action of the laser
pulse can be responsible for strong changes in the absorption of the
materials. Ablation thresholds have been calculated for diamond,
graphite and silicon. For diamond and graphite, also damage thresholds
below the ablation threshold have been found. While for both carbon
materials, the thresholds are found to be nearly independent of the
laser pulse duration (between
t=20 and
t=500 fs), in silicon an increase of the
ablation threshold with pulse duration is found.
The structural changes occurring in diamond and graphite at
the damage thresholds have been investigated in detail. In diamond, an
ultrafast graphitization was discovered. The nonequilibrium
graphitization takes place for a large range of pulse durations and
intensities. It can be explained by the suppression of the diamond
minimum in the potential energy surface in the presence of a strong
electron-hole plasma. The laser-induced solid-solid phase transition
triggered by the photoinduced suppression of a minimum in the
potential energy surface may be a mechanism of general validity.
In graphite a new ablation mechanism has been observed. At energies
below the ablation threshold the laser-induced electron-hole plasma
causes an intense vibrational excitation of graphite planes. The
strong oscillation of the atoms vertical to the graphite layers causes
a collision of two adjacent planes in which momentum is
transferred. This momentum causes planes on the graphite surface to be
removed as a whole. This ablation process is very different from
ablation at higher deposited energies, since large graphite plane
segments instead of small carbon clusters (chains) are produced. This
process may play a role in the formation of carbon nanotubes by
graphite ablation.
The investigation of the nonequilibrium melting of carbon has given
interesting insight in the processes induced by an intense
electron-hole plasma. The melting of graphite under a pressure of
p=10 GPa takes place in two steps: First, disorder is generated
inside the graphite layers while the layer structure itself remains
intact. This process takes place on a time scale of 200 fs and already
yields a metallic-like density of states. The destruction of the layer
structure of graphite takes place on a larger time scale of
approximately 1 ps. The investigation of the melting process of carbon
is important for the numerous technical applications of carbon
materials and especially for the graphite to diamond transition.
Fullerenes like C
60 and carbon nanotubes
have been studied because of their bonding which takes an intermediate
position between sp
2 and
sp
3 hybridization. An important difference
between the thermal and nonthermal fragmentation of
C
60 molecules was found. While the heating
of C
60 to a temperature of
T=5600 K
leads to the emission of carbon dimers on a time scale of several
picoseconds, a laser-induced electron-hole plasma leads to the
emission of carbon monomers on a much shorter time scale of a few
hundred femtoseconds. It was shown that there are two damage
mechanisms in the response of carbon nanotubes to a laser-induced
electron-hole plasma. Carbon monomers can be emitted with clean holes
appearing in the wall of the nanotube, and the nanotubes can be torn
open with the result that graphite planes segments straighten
out.
Finally, ideas have been developed to understand crystalline-amorphous
phase transformations, taking for an example germanium-doped antimony
(GeSb). Possibilities of inducing a transition from graphite to
diamond with the help of laser-induced electronic excitations are
discussed.