The interaction of 100-fs Ti:sapphire laser pulses with thin foil targets was experimentally investigated at intensities exceeding 10<sup>18</sup> W/cm<sup>2</sup>. High harmonics were observed in transmission through an overdense plasma in the direction of the incident beam. From the cutoff frequency of the harmonics an electron density of 1 × 10<sup>24</sup> cm<sup>-3</sup> is inferred, indicating a compression of the plasma by the ponderomotive force of the laser pulse.
© 1997 Optical Society of America
R. Hässner, W. Theobald, S. Niedermeier, H. Schillinger, and R. Sauerbrey, "High-order harmonics from solid targets as a probe for high-density plasmas," Opt. Lett. 22, 1491-1493 (1997)