High-harmonic generation from crystalline solids has become one of the most active fields in the ultrafast phenomena. Electrons driven by strong and slow field are expected to be free particles in the sense of particles obeying band dispersion like the parabolic dispersion in the vacuum, because kinetic energy part of the Hamiltonian dominates compared to interelectron interaction. The band dispersion is potentially encoded in the high-harmonic spectra through the free particle motion. Since band dispersion is a key ingredient to understand how electrons respond to perturbations in a matter, measuring and describing band dispersion has been a central issue in condensed matter physics. The high-harmonic spectroscopy is a promising candidate to reveal band structure by all-optical procedure [1-3].
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