Real-time observation of interfering crystal electrons in high-harmonic generation

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 (1-3). The recent discovery of high-harmonic generation in bulk solids (4-6) combines the idea of ultrafast acceleration with complex condensed matter systems, and provides hope for compact solid-state attosecond sources (6-8) and electronics at optical frequencies (3, 5, 9, 10). 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 (1-3, 9, 11), 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.
Ultrafast time resolution in the few-femtosecond or attosecond regime has provided systematic insight into quantum control of individual atoms (12), molecules (13), and solids (14). A spectacular example has been [...]