In the preceding decades, High Harmonic Generation (HHG) has evolved into a rich scientific field. In the HHG process, the frequency of the driving laser pulse is converted into a much higher output frequency. For HHG in gases, the output frequency can reach more than a thousand times that of the input. More recently, HHG has been observed in solid-state systems, where the presence of band structures adds additional richness. HHG in solid targets has emerged as a powerful spectroscopic tool to map ultrafast electronic processes during optical excitation to a generated spectrum. However, challenges remain in utilizing the HHG to unambiguously characterize the underlying electronic dynamics.

In this study, Orlando and co-authors present a theoretical model to address one of these ambiguities, disorder within the crystalline system. Disorder, such as impurities or vacancies in the crystal lattice, leads to dephasing effects in the excited wave packet not modeled in many theoretical treatments. These effects alter the HHG spectrum and need to be included to recover spectroscopic information. Here, the authors use a two-band model, with a tunable disorder parameter, to calculate the induced intraband currents and thus retrieve the HHG spectrum.

They find that their results more closely match that of many experiments, especially the observation that the radiated spectrum is more concentrated around the odd harmonics. This is opposed to simulations that show many non-harmonic spectral components when using a perfect crystal model. These results demonstrate progress in accurately modeling and predicting the underlying dynamics of HHG in real-world, solid-state systems.

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