Session CI1B - Basic plasma physics.
INVITED session, Monday afternoon, November 15
Room Chatham A/B, SCC
A nonlinear theory has been developed to describe electron response and ion acceleration in dense clusters that are smaller in size than the laser wavelength. This work is motivated by high-intensity laser-cluster interaction experiments. The theory reveals that the breakdown of quasi-neutrality affects the cluster dynamics in a dramatic way. The laser creates an ion shell that expands quickly due to its own space charge whereas the central part of the cluster expands at a much slower rate under the thermal electron pressure. The developed theory also shows a trend for the electron population to have a two-component distribution function: a cold core that responds to the laser field coherently and a halo that undergoes stochastic heating. As the ion shell expands, the potential well for the electron core becomes shallower, producing a leak of the core electrons. The response of the cold electron core to the laser field is similar to that of a driven nonlinear pendulum. As a result, the third harmonic signal from an isolated cluster exhibits resonant enhancement when the laser frequency is close to 1/3 of the core eigenfrequency. It is essential that the ion background has to be non-uniform to produce a nonlinear electron response. The theory has been extended to describe coherent response of multiple clusters. This extension addresses recent fs pump-probe experiments that exhibit a narrow peak in third-harmonic emission from argon clusters under a probe pulse at time delay of 300 fs following heating by a short pump pulse, in contrast with a much broader resonance in linear absorption. The third-harmonic feature is sensitive to focus and interaction geometry, which indicates that phase matching in the evolving cluster plasma is primarily responsible for the third harmonic peak.