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

TL;DR

This paper demonstrates how to optimize high harmonic generation spectra from single atoms using a time-dependent density-functional approach, revealing the roles of electronic structure and correlations in spectral control.

Contribution

It introduces a computational scheme to enhance and control HHG spectra at the single-atom level, combining quantum mechanical modeling with pulse shaping techniques.

Findings

Significant harmonic selectivity achieved through single-atom response.

Electronic correlations influence the extent of spectral optimization.

Single-emitter enhancement complements phase-matching for better HHG control.

Abstract

High harmonic generation (HHG) provides a flexible framework for the development of coherent light sources in the extreme-ultraviolet and soft x-ray regimes. However it suffers from low conversion efficiencies as the control of the HHG spectral and temporal characteristics requires manipulating electron trajectories on attosecond time scale. The phase matching mechanism has been employed to selectively enhance specific quantum paths leading to HHG. A few important fundamental questions remain open, among those how much of the enhancement can be achieved by the single-emitter and what is the role of correlations (or the electronic structure) in the selectivity and control of HHG generation. Here we address those questions by examining computationally the possibility of optimizing the HHG spectrum of isolated Hydrogen and Helium atoms by shaping the slowly varying envelope of a 800 nm, 200-cycles long laser pulse. The spectra are computed with a fully quantum mechanical description, by explicitly computing the time-dependent dipole moment of the systems using a first-principles time-dependent density-functional approach (exact for the case of H). The sought optimization corresponds to the selective enhancement of single harmonics, which we find to be significant. This selectivity is entirely due to the single atom response, and not due to any propagation or phase-matching effect. In fact, this single-emitter enhancement adds to the phase-matching techniques to achieving even larger HHG enhancement factors. Moreover, we see that the electronic correlation plays a role in the determining the degree of optimization that can be obtained.