Attosecond Pulses from a Solid Driven by a Synthesized Two-Color Field at Megahertz Repetition Rate

Probing coherent quantum dynamics in light-matter interactions at the microscopic level requires high-repetition-rate isolated attosecond pulses (IAPs) in pump-probe experiments. To date, the generation of IAPs has been mainly limited to the kilohertz regime. In this work, we experimentally achieve attosecond control of extreme-ultraviolet (XUV) high harmonics in the wide-bandgap dielectric MgO, driven by a synthesized field of two femtosecond pulses at 800 and 2000 nm with relative phase stability. The resulting quasi-continuous harmonic plateau with ∼9 eV spectral width centered around 16.5 eV photon energy can be tuned by the two-color phase and supports the generation of an IAP (∼700 attoseconds), confirmed by numerical simulations based on the three-band semiconductor Bloch equations. Leveraging the high-repetition-rate driver laser, the moderate intensity requirements of solid-state high-harmonic generation, and band-structure-induced spectral enhancement, we achieve IAP production at an unprecedented megahertz repetition rate, paving the way for compact all-solid-state XUV sources for IAP generation.