Presidential Young Investigators Award: Ultrashort Laser- Matter Interactions

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The advent of focusable subpicosecond laser pulses of energy greater than about 1 mJ has made possible a wide range of studies of laser- matter interactions at extremely high intensity and under very fine time resolution. The high repetition rate of these lasers allows the use of low noise correlation techniques such as pump-probe methods. The combination of high pulse energy and short duration makes possible the measurement of the electrical properties of solid materials over their transition from quantum solids to high temperature solid density plasmas. Recently, the resistivity of solid aluminum up to 106 oK has been inferred from reflectivity measurements. Onset toward and passage through a resistivity maximum at 200 -cm was observed as the material disorder and electron temperature increased, the first direct observation of this phenomenon. This research is being extended to materials exhibiting band absorption, such as copper, to study the effects on plasma behavior of the condensed state. Preliminary results indicate that considerable band absorption is still present even at elevated electron temperatures ( 50 eV). Short pulse continuum soft X-ray emission from these plasmas will be used to perform time resolved studies of semiconductor and metal conduction band plasmas, scattering from crystals with large atomic plane spacing (such as KAP), time resolved EXAFS, and preliminary experiments on pumping of soft X-ray laser transitions. The effect of ultrahigh fields on gases will be investigated by studying time-of-flight electron spectra and ion mass spectra from gases irradiated by energetic ultrashort pulses. Of particular interest is the transition of gas "breakdown" from the multiphoton ionization limit at low density to avalanche breakdown at high density.