An international team of researchers from Germany, China, Israel and Vietnam have deciphered the dynamics of the attosecond collision of electrons with neighboring atoms in solids, making them the first to identify the structure and dynamics of certain information encoded in the structure of the band. Their results were recently published in the technical journal “Ultrafast Science”, a new partner journal of the famous “Science” magazine.
The scientists worked with the generation of high harmonics (HHG) in their simulations. In the field of physics, this term is used to refer to a very specific type of oscillation which is several times higher than the base frequency. The importance of HHGs for study lies in their application to the creation of attosecond pulses which are considered to be the “world’s fastest camera”, using spectroscopy to show the movement of electrons. An attosecond is a billionth of a billionth of a second, so an attosecond is to a second what a second is to the age of the universe. It is only recently that HHGs have been observed in crystalline solids, unlocking great potential for compact attosecond light sources and determining the band structure of solid materials. Band structure describes the state of electrons and holes in a crystalline solid, providing information about the state of its electronic structure, while hole refers to the positive charge carrier in semiconductors. This is where an electron was – before its energy reached a higher level, allowing it to make its way through the solid.
“Many aspects underlying HHG in solids are not yet well understood,” explains Professor Torsten Meier of the University of Paderborn. This is what the team of physicists wanted to change. Meier describes what was involved in their simulations. “To put it in layman’s terms, these are strong electric fields on the atoms of a crystal creating both moving electrons and holes. These electrons and holes are then accelerated and migrate through the crystal. Light or radiation occurs when electrons and holes meet in space – it’s collision – and destroy each other. This is what we call recombination. An important question is whether an electron-hole pair recombines with each other, or whether recombination occurs with other electrons and holes. Likewise, we are interested in where the recombination takes place in space and how the movement of electrons and holes in the crystal is changed by atoms. We have been able to show that the emitted radiation provides important information on the structure of the band, in other words, on the possibility of binding and movement inside the crystal.
By analyzing the HHGs generated in solids, scientists were able to decipher the dynamics of the collisions that are encoded in the band structure. So in principle, HHGs were the means to an end. Professor Xiaohong Song of Shantou University in China has this to say about it: “Scientists have long dreamed of obtaining information about the internal structure and ultra-fast dynamics. We are not normally able to observe this type of information directly – high resolution spectroscopy allows this. HHG has proven to be a useful tool. Electrons can be released using intensive laser fields to stimulate material, resulting in attosecond radiation. If the electron collides with its associated hole under certain conditions, high energy photons, or high harmonics, are emitted. Photons are the smallest light particles that make up electromagnetic radiation.
Professor Weifang Yang, also from Shantou University, explains, “These photons, which are usually in the UV range of the electromagnetic spectrum, store structural and dynamic information about electron-hole pairs. HHG spectroscopy exhibits a direct mapping relationship between the dynamic processes of electrons and holes. In addition, scientists have shown for the first time that the collision between electrons – holes and neighboring atoms depends on the pulse they received from the laser field. “When the electron-hole pair receives a strong pulse before the laser field reverses its direction, so that the wavelength of the pair is comparable to the size of an atom, it causes collisions with the atoms. neighbors, ”adds Ruixin Zuo, also from Shantou. University.
The team’s work proved that the collision information is encoded in the band structure. They established a clear mapping between the electron-hole band structure and the harmonic spectrum. Not only do these results provide a unifying picture for experimental observations, but the improved understanding can also be used to specifically tailor the radiation and, for example, create extremely short attosecond pulses.
Read the article in Ultrafast science here: https://doi.org/10.34133/2021/9861923
The title of the article
Collisions of neighboring atoms in the generation of high harmonics in the solid state
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