ECAMP 15

High harmonic generation (HHG) occurs when an intense infrared laser interacts with matter, producing radiation at integer multiples of the laser's fundamental frequency [1]. It was first studied in atomic and molecular gases [1] and later extended to solids [2], enabling compact ultrafast light sources. Recent interest in the quantum properties of the driving pulse [3,4], together with persistent discrepancies between theoretical and experimental HHG spectra in solids [5,6], has driven the development of new computational approaches [4]. We compare the conventional HHG observables [7] with an alternative formulation that treats the light field quantum mechanically and includes terms typically neglected in the standard approach. We find that both methods reproduce the main spectral features - a plateau and odd harmonics - but differ in other qualitative aspects. The fully quantum mechanical observable yields sharper spectral components in solids, although its broad features in the interband region remain similar to the conventional result. In the intraband region, however, the two approaches produce qualitatively different spectra. We attribute this difference to contributions from initially unoccupied electronic bands that are omitted in the conventional observable.

[1] M. Lewenstein et al., Phys. Rev. A 49, 2117 (1994)
[2] S. Ghimire et al., Nat. Phys. 7, 138 (2011)
[3] M. Lewenstein et al., Nat. Phys. 17, 1104 (2021)
[4] Gorlach, A., Neufeld, O., Rivera, N. et al. Nat Commun
11, 4598 (2020).
[5] I. Floss et al., Phys. Rev. A 97, 011401(R) (2018)
[6] Cavaletto et al. Nat. Rev. Phys. 7, 38 (2025)
[7] B. Sundaram and P. W. Milonni, Phys. Rev. A 41, 6571(R) (1990)