Time- and energy-resolved measurements of spontaneous emission from ordered quantum dots

The confinement of electrons in semiconductor quantum dots (QDs) enables the realisation of many quantum optical phenomena in semiconductors including quantum infonnation processing. Self-assembled QDs grown by the Stranski Krastanov (SK) method can produce QDs with high optical quality, which is needed for such experiments.

They are, however, randomly distributed on the surface on which they are grown. Many quantum optics experiments require lateral ordering of the QDs. This can be achieved by growing the QDs on a strained surface made by a superlattice whereby the QDs nucleate in small clusters in a well defined lattice [lj.

An intriguing aspect of these clusters is the formation of coupled QD molecules that will enable new functionalities such as generation of entangled photons and two qubit gates. An open question has been to what extent the structuring influences the radiative properties of the QDs. This question is addressed here using time- and energy-resolved spectroscopy on QDs [21.

We conclude that the radiative properties are preserved for the ordered QDs. To investigate the optical quality of ordered QDs detailed tinne-resolved measurements of the spontaneous emission of ordered InAs/GaAs QDs grown on a (In,Ga)As/GaAs superlattice have been carried out. The results are compared with the regular SK grown InAs/GaAs QDs that are randomly placed and known to have a high optical quality.

An AFM image of the ordered QDs is shown in the left part of figure 1. Time- and energy-resolved spontaneous emission has been measured using a streak camera and the resulting decays are modelled well by a single exponential decay. The resulting decay rates are shown in the right part of figure 1 together with measurements on randomly distributed QDs.

The emission spectra of the QDs (not shown) are centered at —1.15 eV for the ordered QDs and at —1.22 eV for the randomly positioned QDs corresponding to the excitonic ground states. The decay rate of the ground state of the ordered QDs levels off at the same value as for the randomly distributed QDs.

At higher emission energies the decay rates increase due to non-radiative contribution from the excited state for both types of QDs - except for the decrease in decay rate at high emission energies, which is originating from the superlattice. We model the ground state decay rate by a simple model using Fenni’s "golden rule".

The decay rates of tho ordered QDs demonstrate that the optical quality has not been deteriorated by growing the QDs on the superlattice, i.e. the quantum efficiency of the ground state is high. This establishes the capabilities of ordered QDs for quantum optics experiments.