Abstract

Previous studies^1,2 have demonstrated the utility of ionization-induced self-phase modulation of femtosecond laser pulses for time- and space-resolved measurements of moderately high field (10¹⁴ < I < 10¹⁵ W/cm²) ionization dynamics in gases at pressures p ≥ 1 atm. At such high gas/plasma densities, strong defocusing of the pulses² and participation of collisions in ionization dynamics¹ degrade the beam quality of the transmitted blueshifted pulses and complicate quantitative interpretation of the results. In this paper, we examine spectral blue shifts induced on single 1.5 mJ., 80 fs., 800 nm. pulses and their third harmonics (3ω) upon ionizing much lower pressure (0.5 < p < 500 torr) gases at higher focused intensity (I ~ 10¹⁶ W/cm²) than used in previous quantitative studies of this effect. At tens to hundreds of torr pressure, we find that channeling effects³ in the interaction volume give rise to collimated, pressure- and intensity-tunable 3rd harmonic pulses of good beam quality. At still lower pressures (< 10 torr), the effect of channeling and plasma-induced defocusing becomes negligible, as the interaction region simplifies to a near-vacuum Gaussian focal geometry. In this regime, we reliably measure ω and 3ω blueshifts Δω/ω ≤ 10^-4, thus demonstrating the feasibility of quantitative, sensitive time- and space-resolved blueshift diagnostics of collisionless high field ionization fronts or small amplitude Langmuir waves (e.g. laser wakefields)⁴ without the complication of severe beam distortion.

© 1994 Optical Society of America