Abstract

By irradiation of solid targets with intense femtosecond pulses it is possible to create microplasmas of very small gradient scale length l/λ<0.1. Useful information on the early evolution of these plasmas on a picosecond or even subpicosecond time scale can be obtained via optical detection. For instance, subpicosecond time-resolved Schlieren measurements can determine the location of the critical density layer of the plasma.[1] However diffraction effects limit the accuracy to a value of the order of the incident wavelength. A method to detect plasma velocities relies on the spectral analysis of the reflected probe beam at different delays.[2] Expansion velocities can be inferred from Doppler shifts. However, the large Fourier spectrum Δω≈1/Δt of short pulses makes it difficult to estimate frequency shifts much less than Δω. Furthermore, this method is sensitive to the detrimental shot-to-shot frequency and spatial fluctuations of the lasers.

© 1994 Optical Society of America