The harmonic emission from thin solid carbon and aluminum foils, irradiated by 150 fs long frequency-doubled Ti:sapphire laser pulses at $\lambda =395\mathrm{nm}$ and peak intensities of a few ${10}^{18}\mathrm{W}/{\mathrm{cm}}^2$, has been studied. In addition to the harmonics emitted from the front side in the specular direction, we observe harmonics up to the 10th order, including the fundamental from the rear side in the direction of the incident beam, while the foil is still strongly overdense. The experimental observations are well reproduced by particle-in-cell simulations. They reveal that strong coupling between the laser-irradiated side and the rear side occurs via the nonlocal electron current driven by the laser light.

• Received 25 November 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.185001