Researchers reveal information on the collision dynamics of electron/hole with neighboring atoms encoded in the band structure by analyzing high-order harmonic generation in solids

High-order harmonic generation (HHG) provides a main route to generate attosecond (10^-18 s) pulses, which have been considered “the world’s fastest film camera” for imaging the movement of electrons. Recently, HHG has been observed from crystalline solids with great potential for compact attosecond light sources and determination of the band structure of solid-state materials. However, the underlying mechanism of HHG in solids remains highly debated due to the complexity of the dynamics in periodic arrangements of atoms. An international research team of scientists from China, Germany, Israel and Vietnam has elucidated the attosecond collision dynamics of electron/hole with neighboring atoms behind the HHG in solids. The results have been published in the journal Ultrafast Science, a new title in the Science Partner Journal group of scientific journals, distributed by the American Association for the Advancement of Science in partnership with Xi'an Institute of Optics and Precision Mechanics. The study strongly improves the understanding of the underlying mechanism of HHG from solids, which will pave the way for controlling the HHG and visualizing the structure-dependent electron dynamics in solids.

Acquiring detailed information on the inner structure and ultrafast dynamics of microscopic particles has been a longstanding dream for scientists. However, usually, such information cannot be observed directly and is instead accessed by high-resolution spectroscopy. HHG has also turned out to be a very useful tool. Prof. Torsten Meier, co-author of the paper, explains, “By exciting matter with an intense laser field, electrons can be liberated and then move in the combined laser and Coulomb potentials. When anelectron recombines with its associated hole under certain conditions, high-energy photons are emitted.” Co-author Prof. Weifeng Yang continues, “These photons, typically in the UV/XUV range of the electromagnetic spectrum, store information about the structure and electron/hole dynamics. As a result, HHG spectroscopy provides a direct mapping relationship between the electron/hole dynamic processes and the high-harmonic spectroscopy is essential to retrieve the information on the dynamics.”

Schematic representation of collision of electron/hole with neighboring atoms. If the wave nature of electron/hole plays a dominant role, there would be no collision with neighboring atom. The re-combination of electron and hole would be driven by the laser field, which is similar to that in atoms. When the particle nature dominates, collision with neighboring atoms may facilitate/destroy the re-combination of electron with its associated hole, which would enhance/suppress the HHG. All the collision information of electron/hole with neighboring atoms are mapped into the HHG.

In their Ultrafast Science article, the researchers report on the investigation of angular dependence of the HHG in MgO by an intense laser field. The authors clearly demonstrate that collision of electron/hole with neighboring atoms depends on the momentum acquired by electron/hole from the laser field. “If the acquired momentum is small and the electron and hole move only within a small fraction of the Brillouin zone, the wavelength of electron/hole is much larger than the size of atoms, so the wave-like behavior dominates, and the electron/hole would directly cross neighboring atoms as if they were absent,” explains Ruixin Zuo, first author of the paper. Zuo continued, “Thus, no scattering with these atomic sites occurs and this behavior explains the weak anisotropy of low order of harmonics. Whereas when the electron/hole gain a large momentum before the laser field reverses, so that the electron/hole wavelength is comparable to the atomic size, collision with neighboring atoms would occur.” Co-author Prof. Xiaohong Song concludes, “By our joint work we demonstrate that the collision information of electron/hole with neighboring atoms has already been encoded in the band structure from which it is dynamically mapped into HHG.” The authors thus build a clear mapping between the electron/hole band structure and the harmonic spectrum, and pave the way towards a more complete understanding of the wave-particle duality in collision dynamics within sub-cycle attosecond time scale in solids.

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