Floquet-Engineered Electronic States Directly Observed in Magnesium Oxide via High-Harmonic Generation
Researchers have demonstrated, through combined experiment and theory, that high-harmonic generation spectroscopy (HHGS) can directly probe Floquet Bloch states (FBSs) in magnesium oxide driven by few-cycle near-infrared laser pulses. Floquet Bloch states are photon-dressed electronic states that emerge under strong oscillating laser fields and have previously been studied mainly via time- and angle-resolved photoemission spectroscopy. The finding establishes HHGS as a new, powerful tool for probing ultrafast light-induced band hybridization in solids, with broad implications for non-equilibrium condensed matter physics.
A team of researchers has presented experimental and theoretical evidence that Floquet Bloch states (FBSs) — transient photon-dressed electronic states arising under intense oscillating laser fields — can be detected through high-harmonic generation spectroscopy (HHGS) in the wide-bandgap solid magnesium oxide (MgO). The experiments revealed a distinctive feature in the HHG yield's dependence on crystal orientation, which the authors attribute to nonadiabatic coupling between FBSs and conduction bands near the Brillouin zone edge, where the strong laser field transiently breaks time-reversal symmetry. Numerical solutions of the time-dependent Schrödinger equation reproduced this feature and confirmed its Floquet origin, showing that the coupling induces local band structure renormalization and Floquet-like hybridization. The theoretical analysis further demonstrates that FBS nonadiabatic dynamics persist even in the strong-field regime, a finding that extends understanding of how light manipulates electronic structure on ultrafast timescales. Prior investigations of FBSs relied predominantly on time- and angle-resolved photoemission spectroscopy, making this the first direct evidence of their role in HHG emission. The work advances the field of Floquet engineering and positions HHGS as a complementary, all-optical probe of non-equilibrium quantum states in solids.
What's missing
The study is a preprint posted on arXiv and has not yet undergone formal peer review, so its findings await independent validation. The work focuses on a single wide-bandgap material (MgO); whether the observed Floquet signatures in HHG are generalizable to other solids, including narrow-bandgap or topological materials, remains an open question.
What different sources said
- arXiv physicsCenter
Floquet-engineering unveiled by high-harmonic generation
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