(a) Comparison of a single-color (SC) sinusoidal wave and an optimized waveform (Opt. WF) over one optical cycle (o.c.) of the fundamental. Open and filled circles show the tunnel ionization and recombination times, for an electron with kinetic energy of 2U_p following the “long” or “short” trajectories, respectively. The inset depicts the electric fields at ionization time versus the kinetic energies of the returning electrons for “short”- and “long”-trajectory electrons, labeled as “S” and “L” in the figure, respectively. The yields of harmonic emission versus electron recombination times are shown for (b) the two-color and (c) the single-color waves, respectively. Adapted from Ref. [97].