Abstract
We review the recent two topics of optical excitation and relaxation dynamics, newly unveiled by the time- and momentum-resolved photo-electron emission from the conduction band of GaAs. One is the real-time collective relaxation dynamics, resulting in the Fermi degeneracy formation in the Γ valley. We show that it takes almost infinite time to realize the exact Fermi degeneracy, due to a restricted selection rule for the intravalley transition of the photo-excited electrons. The other is the spontaneous and instantaneous intervalley transition from the Γ valley to the L one. By considering the electron-phonon coupling before the photo-excitation, such spontaneous intervalley transition is realized within the framework of the Franck–Condon principle of the photo-excitation.
Highlights
The dynamics of photo-excited states is one of the most fascinating topics in materials science.One reason is that the control of functionalities by light, such as a photoinduced phase transition, directly connects with the development of optical devices [1,2,3]
The intervalley transition of the photo-excited electrons can be observed by 2PPES. We suggest that this intervalley transition can be caused by the electron-phonon coupling in the initial electronic states before photo-excitation within the framework of the Franck–Condon principle
In this paper, following the state-of-the-art 2PPES measurements, we review the recent progress on the theoretical study of the real-time dynamics of the photo-excited electrons in the conduction band of semiconductors
Summary
The dynamics of photo-excited states is one of the most fascinating topics in materials science.One reason is that the control of functionalities by light, such as a photoinduced phase transition, directly connects with the development of optical devices [1,2,3]. The recent remarkable progress of timeand energy-resolution in pump-probe spectroscopy helps the precise understanding of transient states after the photo-excitation [4,5]. Information on the precise time-evolution of the photo-excited states with fine energy and momentum resolutions is important for their application. Such precise information is giving another stage for investigation of more fundamental problems. The states below the Fermi energy are completely occupied, but those above the Fermi energy are unoccupied at the zero temperature. This is a well-established phenomenon called Fermi degeneracy. It is not clear how the fermions go to Fermi degeneracy and how long it takes
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