Presidential Young Investigators Award: Ultrashort Laser- Matter Interactions

能量大于约1mj的可聚焦亚皮秒激光脉冲的出现，使得在极高强度和非常精细的时间分辨率下进行激光与物质相互作用的广泛研究成为可能。这些激光器的高重复率允许使用低噪声相关技术，如泵浦探测方法。高脉冲能量和短持续时间的结合使得测量固体材料从量子固体到高温固体密度等离子体转变过程中的电学特性成为可能。最近，固体铝高达106 oK的电阻率已经从反射率测量推断出来。随着材料的无序性和电子温度的升高，观察到在200 -cm处开始并通过电阻率最大值，这是第一次直接观察到这种现象。这项研究正在扩展到表现出能带吸收的材料，如铜，以研究凝聚态对等离子体行为的影响。初步结果表明，即使在较高的电子温度（50 eV）下，仍然存在相当大的能带吸收。这些等离子体的短脉冲连续软x射线发射将用于半导体和金属导带等离子体的时间分辨研究，大原子平面间距晶体（如KAP）的散射，时间分辨EXAFS以及软x射线激光跃迁泵送的初步实验。通过研究高能超短脉冲辐照气体的飞行时间电子能谱和离子质谱，探讨超高场对气体的影响。特别令人感兴趣的是气体“击穿”从低密度的多光子电离极限到高密度的雪崩击穿的转变。

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.