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Title: Interband versus intraband currents in high-harmonic generation from solids
Plain: An intense infrared laser shining on a crystal makes it emit light at many multiples of the laser frequency (high harmonics). Two pictures compete: electrons accelerating within one energy band, or electrons and holes recombining across the band gap.
Precise: Bulk or monolayer crystals (ZnO, SiO2, GaSe, MoS2) driven by a mid-infrared field with Keldysh parameter $\gamma < 1$ below the damage threshold. Quantity: the fraction of the harmonic yield in each order from the interband polarization (electron-hole recollision) versus the intraband current (nonparabolic band motion, Bloch-like oscillation), as a function of order, field strength and wavelength. An answer is an experimental observable that separates the two contributions unambiguously (harmonic phase, ellipticity or two-color response) in at least one material, matched by a parameter-free microscopic calculation.
Settled by: Phase-resolved harmonic measurements in a well-characterized crystal compared with ab initio time-dependent calculations that include propagation through the sample.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: Different materials and drive wavelengths have been reported to favor different dominant mechanisms; no unified microscopic account exists as of 2024.
Posed since: 2011