Enhanced high-order harmonic generation from periodic potentials in   inhomogeneous laser fields
  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. 
Tao-Yuan Du, Zhong Guan, Xiao-Xin Zhou, and Xue-Bin Bian
physics.optics physics.atom-ph