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Spectral signatures of high-symmetry quantum dots


In short: Quantum dots with at least three symmetry planes provide a very promising route for the generation of entangled photons for quantum information applications. The great challenge to fabricate nanoscopic dots of high symmetry is complicated by the lack of characterization techniques able to resolve small symmetry breaking. Here, we present an approach for identifying and analyzing the signatures of symmetry breaking in the optical spectra of quantum dots.

Semiconductor quantum dots that trap and confine electrons on a nanoscopic length scale have been a subject of investigation for more than two decades. Recent research on quantum dots is focused towards their application as quantum light emitters in the area of optical quantum communication and information processing. In particular, quantum dots have proven to be excellent emitters of single and time-correlated photons. Currently there is a significant effort to develop symmetric quantum dots as emitters of entangled photons.

The most widely studied quantum dots are so far highly strained and asymmetric InGaAs dots. However, alternative quantum dots systems have recently attracted a great interest due to their potentially high symmetry. Unlike the conventional quantum dots, the dot formation mechanisms for many of the alternative systems does not rely on any strain. Therefore, many of the models developed for understanding the conventional InGaAs quantum dots are not applicable to the alternative unstrained quantum dots.

In this work, we present a complete analysis of the intrinsic spectral features of InGaAs quantum dots grown in inverted tetrahedral micropyramids. Unlike the conventional quantum dots, the effects of strain in this class of pyramidal quantum dots are small or negligable.

The quantum dots in this study are of high technological and scientific interest as they ideally possess three symmetry planes and currently is the only system providing a high density of emitters of polarization entangled photons. Moreover, the pyramidal dots are site-controlled with extreme dot-to-dot uniformity in the spectral features, enabling rigorous analysis of very complex optical spectra, including the spectral patterns of quantum dots with various degrees of symmetry breaking. Our general experimental and theoretical approach is here expanded with full coherence to a great variety of exciton complexes, without the need to introduce a more complex model.

We demonstrate that our approach allows a confident identification of all spectral features as well as strict prediction of missing peaks hidden due to spectral overlap with other high intensity components. Analysis  based on symmetry arguments alone and some general knowledge about the electronic structure of the quantum dots enables extraction of all the relevant interaction energies from the experimental spectroscopic data. Effects of symmetry breaking are analyzed in terms of energy level splitting and relaxed optical selection rules. Most importantly, the spectral signatures of high symmetry quantum dots are identified. The methods presented in this work are general and they should be applicable to any quantum dot system, and, in particular, the symmetry based approach for analyzing the fine structure patterns is very effective when dealing with high symmetry quantum dots.

Details of the research are described in New Journal of Physics 17 103017 (2015)


K Fredrik Karlsson, Danel Y. Oberli, Marc-André Dupertuis, Valentina Troncale, Martin Byszewski, Emanuelle Pelucchi, Alok Rudra, Per Olof Holtz and Eli Kapon.


Fredrik Karlsson, assoc. professor
Phone: +46 (0)13 28 25 33
E-mail: freka@ifm.liu.se


  • Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University.


Responsible for this page: Fredrik Karlsson
Last updated: 10/14/15