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Theory for Optically Created Nonequilibrium in Covalent Solids

Harald Jeschke

ISBN 978-3-89722-503-9
171 Seiten, Erscheinungsjahr: 2000
Preis: 40.50 €
A theory for the response of covalent solids and clusters to intense laser pulses (1010-1013 W/cm2) 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 C60 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 C60 and carbon nanotubes have been studied because of their bonding which takes an intermediate position between sp2 and sp3 hybridization. An important difference between the thermal and nonthermal fragmentation of C60 molecules was found. While the heating of C60 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.

Keywords:
  • Theoretical solid state physics
  • Covalent materials
  • fullerenes and carbon nanotubes
  • Tight-binding and molecular dynamics calculation
  • Laser-induced ablation and phase transitions
  • Ultrafast excitation and relaxation processes

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