Sub-pulsewidth coherent spectroscopy in the visible: MI-FROG diagnostics for ultrafast plasma physics

Summary form only given. The study of ultrafast laser-plasma interactions, made possible by sub-picosecond chirped pulse amplified solid-state laser systems enjoys the notable advantage of being decoupled from ion motion, which occur on multi-ps to ns time scales. Ultrafast optical-pulse probing of such plasmas, though, is still complicated by electron dynamics that exist on pulsewidth or shorter time scales. Particularly interesting areas of study, such as ultrafast ionization in gases, a potentially important source of high brightness high-harmonic radiation, and large amplitude longitudinal plasma oscillations, a candidate for future high-energy particle accelerators, inherently involve dynamics on the few femtosecond time scale. We have developed a combination of FROG with multi-pulse interferometry, termed multi-pulse interferometric FROG, or MI-FROG, which is uniquely suited for pump-probe coherent spectroscopy using amplified short-pulse systems and/or emissive targets. By using double-pulse probes in a FROG apparatus, all-order spectral phase sensitivity is achieved. As an example application of MI-FROG to the study of ultrafast laser-plasma interaction, we discuss recent measurements of ultrafast ionization in gases.