Linking high harmonics from gases and solids

Abstract

When intense light interacts with an atomic gas, recollision between an ionizing electron and its parent ion creates high-order harmonics of the fundamental laser frequency. This sub-cycle effect generates coherent soft X-rays and attosecond pulses, and provides a means to image molecular orbitals. Recently, high harmonics have been generated from bulk crystals, but what mechanism dominates the emission remains uncertain. To resolve this issue, we adapt measurement methods from gas-phase research to solid zinc oxide driven by mid-infrared laser fields of 0.25 volts per ångström. We find that when we alter the generation process with a second-harmonic beam, the modified harmonic spectrum bears the signature of a generalized recollision between an electron and its associated hole. In addition, we find that solid-state high harmonics are perturbed by fields so weak that they are present in conventional electronic circuits, thus opening a route to integrate electronics with attosecond and high-harmonic technology. Future experiments will permit the band structure of a solid to be tomographically reconstructed.

Anisotropic high-harmonic generation in bulk crystals

“…To explain the observation of a harmonic signal for an elliptically polarized field-in particular the circularly polarized field-it is necessary to include the role of multiple atomic sites. This is an important difference from the generalized recollision model discussed recently by Vampa et al 10,27 and from atomic HHG (see Supplementary Information). The ellipticity dependence in solids is also material-dependent; for example, it is relatively weak in ZnO (ref.…”

“…We find that harmonic e ciency is enhanced (diminished) for semi-classical electron trajectories that connect (avoid) neighbouring atomic sites in the crystal. These results indicate the possibility of using materials' own electrons for retrieving the interatomic potential and thus the valence electron density, and perhaps even wavefunctions, in an all-optical setting.High-harmonic generation (HHG) in bulk crystal is attributed to the sub-cycle electronic motion driven by an intense laser field [2][3][4][5][6][7][8][9][10][11] . There has been a growing interest in utilizing HHG to probe the electronic structure of solids 8,9,11 .…”

“…High-harmonic generation (HHG) in bulk crystal is attributed to the sub-cycle electronic motion driven by an intense laser field [2][3][4][5][6][7][8][9][10][11] . There has been a growing interest in utilizing HHG to probe the electronic structure of solids 8,9,11 .…”

Bicircular High-Harmonic Spectroscopy Reveals Dynamical Symmetries of Atoms and Molecules

“…The two-dimensional nature of the electron trajectories and the wider range of recollision angles spanned by the returning electron provide new pathways for developing the self-imaging aspect of HHS, which will give access to attosecond time scales. These properties are expected to extend to BHHS of solids [60,61], where it will open up promising directions, such as the time-resolved study of symmetry and symmetry breaking in crystals.…”

Correction

“…The strong-field physics of individual atoms in the gas phase is well understood, but the different scaling of the maximum harmonic frequency with the strength of the incident field in solids had hinted at different underlying processes 5 . However, a recent comparative study 7 found that some aspects of the gas-physics picture apply to solids too, such as the importance of controlling the electrons' pathways so as to generate high-harmonic radiation. In another study 8 , researchers applied optical pulses of sub-period duration to a silicon dioxide crystal and showed that the observed high-harmonic radiation reaches frequencies of 8,500 THz (corresponding to the extreme-ultraviolet domain), a record value for intra-band currents induced in a solid.…”