Strong Field Theory

Forest fires and cluster ionisation in strong fields. Prof P L Knight, Lucien Gaier and Luke Chipperfield.

In a collaboration with the group of Paul Corkum at NRC Ottawa, we investigated the ionisation of clusters by short, intense laser pulses. Traditionally, the dynamics of laser-induced breakdown is divided into several stages. First, conventional multiphoton ionisation provides a seed population of free electrons. Second, these electrons take energy from the field through laser-assisted collisions. Third, when the electron energy exceeds the bandgap, electrons are promoted into the conduction band. Clearly, this picture should change when dealing with extremely short - few cycle laser pulses: at some point the pulse duration should become too short to enable significant heating of the electrons in the conduction band. Where this boundary lies depends on the laser wavelength, intensity, bandgap etc. Very large atomic clusters are similar to dielectrics for short laser pulses, as long as the expansion of the cluster is negligible during the pulse. Mechanisms of damage in dielectrics are thus similar to ultrafast cluster ionisation, and act at very early time. They are complimentary to the ionisation and absorption mechanisms induced by cluster expansion.

Figure 1 Numerical modelling of electron dynamics driven by a single cycle pulse

We have concentrated on one such damage mechanism which becomes operative when the laser pulses are too short (and the intensities and/or energies are too low) to induce traditional avalanche ionisation. Mathematically, this mechanism can be described as the propagation of "forest fires". The core effect responsible for laser-induced "forest fires" in clusters and dielectrics is known as "enhanced ionisation" in molecules and "ionisation ignition" in clusters. When electrons localise on the nuclei (e.g. during dissociation of a molecule, or in a rare gas cluster), removal of such an electron leaves an uncompensated positive charge - a hole. We describe the dynamics of ultrafast damage in dielectric materials irradiated by light below the conventional breakdown threshold. The damage occurs on a sub-laser-wavelength scale. It starts with the formation of nano-droplets of plasma which grow like forest fires, without any need for heating of the electrons promoted to the conuction band. The dimensionality of the damaged area can be fractal and changes during the laser pulse. This mechanism is operative on both rare gas clusters and dielectrics interacting with ultrashort, moderately intense laser pulses which include only a few periods of the driving field, too fast for traditional avalanche mechanisms.