MEASUREMENT OF ULTRAFAST DYNAMICS IN THE INTERACTION OF INTENSE LASER PULSES WITH GASES, ATOMIC CLUSTERS, AND PLASMAS

Abstract

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.