  We theoretically study high-order harmonic generation (HHG) from solid-phase
systems in spatially inhomogeneous strong laser fields originated by resonant
plasmons within a metallic nanostructure. The intensity of the second plateau
in HHG may be enhanced by two-three orders and be comparable with the intensity
of the first plateau. This is due to bigger transition probabilities to higher
conduction bands. It provides us a practical way to increase the yields of HHG
with laser intensity below the damage threshold. It presents a promising way to
triple the range of HHG spectra in experimental measurements. It also allows to
generate intense isolated attosecond pulse from solids driven by few-cycle
laser fields.
