title: Real-time observation of interfering crystal electrons in high-harmonic generation.
abstract: Acceleration and collision of particles has been a key strategy for exploring the texture of matter. Strong light waves can control and recollide electronic wavepackets, generating high-harmonic radiation that encodes the structure and dynamics of atoms and molecules and lays the foundations of attosecond science. The recent discovery of high-harmonic generation in bulk solids combines the idea of ultrafast acceleration with complex condensed matter systems, and provides hope for compact solid-state attosecond sources and electronics at optical frequencies. Yet the underlying quantum motion has not so far been observable in real time. Here we study high-harmonic generation in a bulk solid directly in the time domain, and reveal a new kind of strong-field excitation in the crystal. Unlike established atomic sources, our solid emits high-harmonic radiation as a sequence of subcycle bursts that coincide temporally with the field crests of one polarity of the driving terahertz waveform. We show that these features are characteristic of a non-perturbative quantum interference process that involves electrons from multiple valence bands. These results identify key mechanisms for future solid-state attosecond sources and next-generation light-wave electronics. The new quantum interference process justifies the hope for all-optical band-structure reconstruction and lays the foundation for possible quantum logic operations at optical clock rates.
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Hohenleutner
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Langer
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: O
authors.initials: O
authors.lastname: Schubert
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Knorr
authors.affiliation: Department of Physics, University of Marburg, 35032 Marburg, Germany.
authors.forename: U
authors.initials: U
authors.lastname: Huttner
authors.affiliation: Department of Physics, University of Marburg, 35032 Marburg, Germany.
authors.forename: S W
authors.initials: SW
authors.lastname: Koch
authors.affiliation: Department of Physics, University of Marburg, 35032 Marburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Kira
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Huber