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High harmonic generation (HHG) provides a flexible framework for thedevelopment of coherent light sources in the extreme-ultraviolet and soft x-rayregimes. However it suffers from low conversion efficiencies as the control ofthe HHG spectral and temporal characteristics requires manipulating electrontrajectories on attosecond time scale. The phase matching mechanism has beenemployed to selectively enhance specific quantum paths leading to HHG. A fewimportant fundamental questions remain open, among those how much of theenhancement can be achieved by the single-emitter and what is the role ofcorrelations (or the electronic structure) in the selectivity and control ofHHG generation. Here we address those questions by examining computationallythe possibility of optimizing the HHG spectrum of isolated Hydrogen and Heliumatoms by shaping the slowly varying envelope of a 800 nm, 200-cycles long laserpulse. The spectra are computed with a fully quantum mechanical description, byexplicitly computing the time-dependent dipole moment of the systems using afirst-principles time-dependent density-functional approach (exact for the caseof H). The sought optimization corresponds to the selective enhancement ofsingle harmonics, which we find to be significant. This selectivity is entirelydue to the single atom response, and not due to any propagation orphase-matching effect. In fact, this single-emitter enhancement adds to thephase-matching techniques to achieving even larger HHG enhancement factors.Moreover, we see that the electronic correlation plays a role in thedetermining the degree of optimization that can be obtained.

Enhancing and controlling single-atom high-harmonic generation spectra: a time-dependent density-functional scheme