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

• Published in: Nature (London) 2015-07, Vol.523 (7562), p.572-575
• Main Authors: Hohenleutner, M., Langer, F., Schubert, O., Knorr, M., Huttner, U., Koch, S. W., Kira, M., Huber, R.
• Format: Article
• Language: English
• Subjects: 140/125; 639/624/400/3923; 639/624/400/584; 639/766/119; 639/766/483; Bandwidths; Collisions (Nuclear physics); Electrons; Humanities and Social Sciences; letter; multidisciplinary; Optics; Particle physics; Properties; Science
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature14652

The generation of high harmonics in the solid phase is studied with time-resolved measurements and a quantum many-body theory; the underlying motion of electrons is found to differ from that observed during high-harmonic generation in atomic gases, and involves quantum interference between electrons from multiple valence bands. High-harmonic radiation in bulk solids A new field of ultrafast photonics in solids has emerged with the demonstration, first reported a few years ago, of short wavelength radiation in bulk crystals produced via high-harmonic generation. The underlying mechanisms are involved in this phenomenon are under intense investigation. Matthias Hohenleutner et al . study in particular the generation of high harmonics in bulk with time-resolved measurements. They find that the underlying motion of electrons differs from the mechanism of high-harmonic generation in atomic gasses and involves quantum interference between electrons from multiple valence bands. This observed effect suggests new directions for the development of solid-state ultrafast light sources and light-controlled electronics. 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 , 6 combines 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