Abstract
By irradiation of solid targets with intense femtosecond pulses it is possible to create microplasmas of very small gradient scale length. 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. However diffraction effects limit the accuracy to a value of the order of the incident wavelength. From time-resolved measurements of the absolute reflectivity of a short probe pulse as a function of angle and polarisation one can extract information on the plasma density gradient. This method requires series of independent measurements and yields information on the expansion of the plasma only indirectly. A more direct method relies on the spectral analysis of the reflected probe beam at different delays. Expansion velocities can be then inferred from Doppler shifts. However, the large Fourier spectrum Δω = l/At 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
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