MEASUREMENT OF ULTRAFAST DYNAMICS IN THE INTERACTION OF INTENSE LASER PULSES WITH GASES, ATOMIC CLUSTERS, AND PLASMAS
Milchberg, Howard M
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We have investigated the time resolved dynamics of intense, ultrashort pulse laser interactions with gases, nanometer-size clusters, and plasma waveguides. To probe the ultrafast dynamics in these interactions, we developed a new femtosecond optical diagnostic, single-shot supercontinuum spectral interferometry (SSSI), which measures ultra-rapid transients induced by an intense laser pulse in the complex index of refraction. The measurement of the transient refractive index in intense laser-heated materials provides a direct view of how the laser-produced perturbation evolves in time and space. Our SSSI diagnostic is capable of ~10 fs temporal resolution on a temporal window ~1.5 ps long, along with ~7 mm one-dimensional (1D) spatial resolution. SSSI was first applied to probe the ionization dynamics of helium gas under the irradiation of high intensity (~10^17 W/cm2) laser pulses. It revealed a characteristic stepwise transition process He - He+ - He2+, in agreement with the optical field ionization model. This measurement was used as a test case to demonstrate that finite laser-target interaction lengths can strongly affect the interpretation of all measurements involving extraction of transient phases. The time-resolved explosion dynamics of intense (~10^15 W/cm2) laser-heated clusters was also studied with SSSI and additional ultrafast optical diagnostics. Here, the ultrafast processes are ionization and rapid cluster plasma explosion. The measurement strongly supports our laser-cluster interaction scenario in which laser-heated clusters explode layer-by-layer, and the laser is strongly coupled at critical density. For the cluster sizes and laser intensities of this experiment, the measured several hundred-femtosecond evolution timescale of laser-heated clusters can be understood in terms of plasma hydrodynamics. A major implication of our understanding of microscopic cluster dynamics was the prediction and observation of self-focusing in clustered gases. Finally, using SSSI, we have explored the interaction of intense laser pulses with preformed plasma waveguides. This measurement revealed the presence of guided laser-induced distortions such as ionization, which can lead to degraded waveguide performance. To overcome this problem, a funnel-mouthed plasma waveguide was developed and diagnosed. In addition, a new plasma waveguide generation method has been demonstrated, which uses the unique features associated with the laser-cluster interaction self-focusing and strong absorption.