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

• M. Hohenleutner,
• F. Langer,
• O. Schubert,
• M. Knorr,
• U. Huttner,
• S. W. Koch,
• M. Kira
• & R. Huber

http://www.nature.com/nature/journal/v523/n7562/full/nature14652.html

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, 2, 3. The recent discovery of high-harmonic generation in bulk solids^4, 5, 6combines the idea of ultrafast acceleration with complex condensed matter systems, and provides hope for compact solid-state attosecond sources^6, 7, 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, 2, 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.