The main part of this project concentrates on the interaction of intense laser pulses with single size selected nanoparticles. Specifically, experiments will investigate (1) a new type of laser absorption resonance in nanoparticles whereby a subset of hot electrons is driven through the particle in phase with the laser, where the resonance is strongly particle size-sensitive, and (2) the effects of multiple phased intense laser beams on controlling the explosion/expansion of intense laser-heated nanoparticles. For smaller nanoparticles, strong ponderomotive force-induced distortion and compression is predicted by both fluid and particle-in-cell (PIC) models. For larger particles (larger than 50 nanometers) and longer pulses in the hundreds of femtoseconds, ablative compression appears to play a role. These effects take place in a unique regime, the near field limit, where the laser wavelength is much larger than the cluster size. Issues such as the Rayleigh-Taylor instability do not enter the picture and the effects may be insensitive to low quality laser beams with hot spots. The intriguing possibility is that the dynamics of hot dense matter on a sub-wavelength spatial scale could be controlled with appropriately directed and phased intense laser pulses. To do these experiments, previous methods are limited in several ways. First, particle size-dependent effects are likely to be completely masked by the wide size distribution of van der Waals clusters from gas jets. Second, the high cluster density in jets limits results to ensemble averages. Beam skimmers and geometry can be used to limit the cluster density, but the problem of the cluster size distribution remains. The methods described will allow the generation of very low densities of single-size nanoparticles, allowing consistent laser interactions with single particles of predetermined fixed size.
