Physica E 9 (2001) 124–130 www.elsevier.nl/locate/physe Optical emission from single, charge-tunable quantum rings R.J. Warburton a ; * , C. Sch aein a , D. Haft a , F. Bickel a , A. Lorke a , K. Karrai a , J.M. Garcia b , W. Schoenfeld b , P.M. Petro b a Center for NanoScience and Sektion Physik, Ludwig-Maximilians-Universit at, Geschwister-Scholl-Platz 1, 80539 M unchen, Germany b Materials Department and QUEST, University of California, Santa Barbara, CA 93106, USA Abstract We have succeeded in preparing excitons with a specic charge in single semiconductor quantum rings. Buried InAs quantum rings are loaded with electrons from a reservoir through a tunneling barrier and an additional electron–hole pair is generated by optical excitation. Single rings are addressed with nano-optical techniques. We observe abrupt shifts in the emission energy as electrons are added one by one. Furthermore, the experiments provide unique insights into the interaction of electrons in semiconductor nano-islands with their environment. ? 2001 Elsevier Science B.V. All rights reserved. Keywords: Quantum dots; Photoluminescence; Tunneling 1. Introduction Semiconductor quantum dots conne electrons and holes in all three directions and have therefore atomic-like properties. For instance, when a quantum dot is lled sequentially with electrons, the charg- ing energies are pronounced for particular electron numbers [1,2], exactly as in atomic physics where the shell lling is described by Hund’s rules. Semi- conductor quantum dots have also a valence band with strong optical transitions to the conduction band. These interband transitions form the basis for much * Correspondence address: Department of Physics, Heriot-Watt University, Edinburgh EH14 4AS, UK. Tel.: +44-131-4518069: fax: +44-131-4513136. E-mail address: r.j.warburton@hw.ac.uk (R.J. Warburton). of the interest in the optical properties of quantum dots [3], and in their application as the gain medium in lasers [3], as storage elements [4 –7], and as uo- rescent optical markers [8]. The issue we address here is how the interband transitions of a single quantum dot change as electrons are added one by one. Our motivation is two-fold. First, the shifts in emission energies on charging, and also the appear- ance of satellites for highly charged dots, are direct measures of the Coulomb interactions between the particles. This is a model system for the investigation of Coulomb correlations because we have a known small number of conned particles. Secondly, by weakly coupling a dot to a reservoir of charge, we can explore the interactions between the electrons in the quantum dot and their environment. In particular, we show here how we can use the optical emission of 1386-9477/01/$ - see front matter ? 2001 Elsevier Science B.V. All rights reserved. PII:S1386-9477(00)00186-7