Physicists at ETH Zürich (Switzerland) have for the first time resolved the response of electrons in gallium arsenide (GaAs) at the attosecond (10-18 s) time scale, gaining unexpected insights for future ultrafast optoelectronic devices with operation frequencies in the petahertz regime. Gallium arsenide is a technologically important narrow-bandgap semiconductor, in which the excitation of electrons from the valence into the conduction band produces charge carriers that can transport electrical current through electronics components. In addition to this so-called interband transition, carriers can also be accelerated within the individual bands (intraband motion) as the electrons interact with the laser light. Which of the two mechanisms dominates the response to a short intense laser pulse, and how their interplay effects the carrier injection into the conduction band, is far from obvious.
Fabian Schlaepfer and his colleagues in Ursula Keller’s group at the Institute for Quantum Electronics have studied these processes for the first time at the attosecond timescale, combining transient absorption spectroscopy with state-of-the-art first-principles calculations. They found that intraband motion has indeed an important role, as it significantly enhances the number of electrons that get excited into the conduction band. This finding was unexpected because intraband motion alone is unable to produce charge carriers in the conduction band. These results therefore represent an important step forward in understanding the light-induced electron dynamics in a semiconductor on the attosecond timescale, which will be of practical relevance for future electronics and optoelectronics devices, whose dimensions become ever smaller, and the electric fields involved ever stronger and the dynamics ever faster. Reference: F. Schlaepfer et al., Nat. Phys. (2018); doi:10.1038/s41567-018-0069-0.
