Condensed Matter & Surface Sciences

COLLOQUIUM

Michael Scheibner

Naval Research Laboratory

“Optically Active Artificial Molecules”

Quantum dots (QDs), so called artificial atoms, are a versatile platform for studying physics on the quantum level and for developing novel ideas in nanotechnology. Fostered by the knowledge of the various properties of QDs gained during the past two decades, a new field is currently emerging that explores artificial molecules—structures formed by coupling two or more QDs together. This solid state molecular physics is to a large extent driven by the prospect of scalable quantum information technologies. Beyond that, it provides fertile ground for the discovery of exciting physical effects on a fundamental level.

We study quantum dot molecules (QDMs) by optical spectroscopy. A highly successful approach to obtain optically active QDMs is the molecular beam epitaxial growth of two closely spaced layers of self assembled QDs. Strain in the material causes the QDs in the second layer to nucleate preferably on top of QDs in the first layer, forming ‘diatomic’ QDMs. In addition we embed the QDMs in field effect structures. This electrical control combined with the technologies of single QD spectroscopy enables us to controllably create molecular superposition states and explore the properties of individual QDMs [1-4].

QDMs are characterized by many new features in the optical transition spectrum such as multiplets of optical transitions, interdot optical transitions, and avoided crossings of optical transitions. With the underlying properties the coupling which is based on quantum mechanical tunneling emerges as a versatile tool, steadily expanding the functionality of QDs. Examples of this functionality include the widened spectral tuning range of optical transitions [4], the in situ characterization of the electronic structure of the QDs forming a QDM [5], and the control of a single spin’s properties [6], [7].

[1] E. A. Stinaff, et al. Optical Signatures of Quantum Dot Molecules. Science 311, 636 (2006).

[2] A.S. Bracker, et al. Engineering of electron and hole tunneling with asymmetric InAs quantum dot molecules. Appl. Phys. Lett. 89, 233110 (2006).

[3] M. Scheibner, et al. Spin fine structure of optically excited quantum dot molecules. Phys. Rev. B 75, 245318 (2007).

[4] M.Scheibner, et al. Photoluminescence Spectroscopy of the Molecular Biexciton in InAs-GaAs Quantum Dot Pairs. Phys. Rev. Lett. 99, 197402 (2007).

[5] M. Scheibner, et al. Optically Mapping the Electronic structure of Quantum dot Molecules. Nature Physics 4, 291 (2008).

[6] M. F. Doty, et al. Electrically Tunable g Factors in Quantum Dot Molecular Spin States. Phys. Rev. Lett. 97, 197202 (2006).

[7] D. Kim, et al. A quantum dot spin qubit with simultaneous optical initialization and non-destructive readout. (submitted for publication).

Thursday, October 9, 2008 -- 4:10 p.m.

Walter Lecture Hall 245