  We present a scheme for amplifying an extreme-ultraviolet (XUV) seed isolated
attosecond pulse via stimulated Raman scattering of a pulse-train pump. At
sufficient seed and pump intensity, the amplification is nonlinear, and the
amplitude of the seed pulse can reach that of the pump, one order of magnitude
higher than the initial seed amplitude. In the linear amplification regime, we
find that the spectral signature of the pump pulse train is imprinted on the
spectrum of the amplified seed pulse. Since the spectral signature is imprinted
with its frequency downshifted by the plasma frequency, it is possible to
deduce the electron density in the region of interaction. This region can be of
micrometer length scale longitudinally. By varying the delay between the seed
and the pump, this scheme provides a local electron-density measurement inside
solid-density plasmas that cannot be probed with optical frequencies, with
micrometer resolution.
