New Insights into the Behavior of Excitons in Semiconductors
An exciton in a semiconductor is a quasiparticle composed of a negatively charged electron and a positively charged hole. Understanding how large numbers of excitons behave in a semiconductor is a current research question in quantum mechanics. This exciton dynamics plays a key role in various contexts—for instance, in energy transport in optoelectronic semiconductor devices or in the development of novel quantum technologies. Until now, scientists have used specialized spectroscopic techniques to study the characteristic linear responses of excitons. These methods, for example, show that the observed optical properties scale directly with the strength of excitation.
The team in Dortmund has now succeeded in deciphering strong and sensitive nonlinear responses in exciton dynamics. Nonlinear effects also play an important role in acoustics. One example is the sound of an electric guitar, which changes dramatically when played through an amplifier—especially when the amplifier is turned up loud. In this case, additional overtones called higher harmonics are generated—vibrations that occur at multiples of the guitar string’s original frequency. These additional frequencies are the result of nonlinear distortion in the amplifier.
To observe such nonlinear effects in exciton dynamics—which can arise, for example, at high exciton densities or under intense excitation—the physicists used a special time-resolved optical spectroscopy technique. They applied a terahertz field analogous to a guitar signal: the team used an optical pump beam to generate free electrons and holes, then sent a short terahertz pulse through the sample to analyze how much it was distorted. It turned out that the distortion caused by excitons looked completely different from the distortion caused by free electrons. This allowed them to observe the dynamics of a large number of excitons in the semiconductor compound cuprous oxide (Cu₂O), despite the strong interactions between excitons, electrons, and holes.
“We were able to show that excitons form very shortly after the optical generation of free electrons and holes—namely, within just a few picoseconds, or 10⁻¹² seconds,” emphasized Prof. Zhe Wang. “This is particularly interesting because it means we have found especially simple experimental criteria to reliably distinguish excitons from free electrons and holes.” The findings from the Dortmund team are valuable for researchers who aim to study exciton dynamics in various contexts in the future.