(a) Single-atom and (c) macroscopic HHG spectra calculated by using the QRS model, and (b) returning electron wave packet calculated by the SFA. The maximum returning electron energy is fixed at 350 eV. The fundamental optimal wavelengths in the two-color waveforms are 2831, 1625 and 1350 nm for Ar, Ne and He, respectively. (d) Photorecombination cross sections (PRCSs) for the three targets over a broad photon energy region are shown. Adapted from Ref. [98].