When InGaAs/GaAs quantum dots are excited with high-power density laser beams (of several kW/cm2), the entire photoluminescence spectrum becomes dominated by biexcitonic emission patterns. This emission originates from a state in which two electrons and holes are confined at once in a quantum dot, explain team members Sven Höfling of the Technische Physik at the University of Würzburg in Germany and Grzegorz Sek at Wroclaw University of Technology in Poland. The spectrum then transforms into a broad emission band that noticeably shifts towards longer wavelengths. “Such strange behaviour has never been observed in a quasi-zero-dimensional system (like a quantum dot) before and reminds us of the so-called Mott transition from an insulating to conducting phase that sometimes occurs in one or two dimensional systems,” said Höfling.

“Wetting layer” states

The team believes that these multiexcitonic features arise from interactions of excitons confined in the dots with the quantum well like “wetting layer” states occupied with electrons and holes at the high optical excitation energies employed during the experiments.

The researchers looked at how light was emitted from single InGaAs/GaAs quantum dots in micro-photoluminescence experiments at liquid helium temperatures. The intensity of the emitted light was detected using a liquid nitrogen cooled InGaAs CCD camera combined with a single-grating monochromator and the photoluminescence spectra were then examined as a function of laser excitation power density.

“The multiexcitons we observed, which are actually biexcitons interacting with excitons in their surroundings, are formed in these elongated dots and as such emit light with characteristic features,” Höfling told nanotechweb.org. “Most importantly, the wetting layer states create opportunities for efficient Coulomb interactions between excitons confined in the quantum dots and the wetting layer. This shows how important excitons confined in this layer are for the formation of quantum dot-wetting layer multiexcitonic states.”

Novel photonics applications

The result will be important for better understanding many cavity quantum electrodynamics experiments in which such quantum dots are employed as quantum emitters. Biexciton and multiexciton states need to be clearly distinguished in such experiments that aim at producing single photon or entangled photon pairs. “Our work is an important step in understanding the emission properties of large InGaAs/GaAs quantum dots, which are in themselves important structures for constructing nanolasers and other novel photonic devices thanks to the fact that they allow strong single exciton-single photon coupling in a microcavity,” said Höfling.

The researchers now plan to perform more sophisticated experiments, such as time-resolved microphotoluminescence at the single dot level and photon correlation spectroscopy. “We hope that such investigations will give us better and more detailed insights into the physics of large and elongated quasi-zero dimensional structures,” he added.

The current work is reported in the Journal of Applied Physics.