Femtosecond Coherent Spectroscopy at 800nm: Ml-FROG Measures High-Field Ionization Rates in Gases

We report the first quantitative phase-sensitive measurement of ultrafast ionization rates in gases. Ultrafast probe depletion via frequency mixing in the ionization front is observed. The detailed dynamics of an atom in a strong laser field is rich in both interesting physics and potential applications. The source of both high-harmonic WVW-XW radiation [1] and ionization-based sources of tunable radiation [2,3], successful “mode-locking” of ultrafast ionization may soon yield the first attosecond pulses [4]. Previous studies [2,3] of ultrafast ionization have relied on spectral power density measurements using 100 fs pulses. As the detailed structure of the ionization-front blue-shifted spectra depends on a complicated interplay of the input pulse intensity/phase structure and the ionization dynamics [5], the conclusions made from such experiments are at best well-thought-out and limited inferences. To achieve a quantitative mess structure, we used an experiment configuration which, by utilizing three power spectra channels and one optically gated frequency resolved channel per each of four CCD images (pumped or unpumped cases with one or two probe pulses, see figure 1), provides simultaneous and self-consistent dual-channel power spectra and frequency domain interferograms, Multi-pulse Interferometric Freauencv Reiul:e of the ionization rate independent of probe