Quantum dots based on InAs/InP hold the promise to deliver entangled photons with wavelength suitable for the standard telecom window around 1550 nm, which makes them predestined to be used in future quantum networks applications based on existing fiber optics infrastructure. A prerequisite for the generation of such entangled photons is a small fine structure splitting (FSS) in the quantum dot excitonic eigenstates, as well as the ability to integrate the dot into photonic structures to enhance and direct its emission. Using optical spectroscopy, we show that a growth strategy based on droplet epitaxy can simultaneously address both issues.
Contrary to the standard Stranski-Krastanow technique, droplet epitaxy dots do not rely on material strains during growth, which results in a drastic improvement in dot symmetry. As a consequence, the average exciton FSS is reduced by more than a factor 4, which in fact makes all the difference between easily finding a dot with the required FSS and not finding one at all. Furthermore, we demonstrate that droplet epitaxy dots can be grown on the necessary surface (001) for high quality optical microcavities, which increases dot emission count rates by more than a factor of five. Together, these properties make droplet epitaxy quantum dots readily suitable for the generation of entangled photons at telecom wavelengths.
In this paper we report on the multi-section gain and absorption analysis of strain engineered molecular beam epitaxy
(MBE) grown GaAs and InGaAs capped bilayers. The InGaAs capped bilayer quantum dot (QD) lasers extends the
room temperature lasing wavelength to 1.45 μm. The spectral measurement of gain demonstrates that net modal gain is
achieved beyond 1.5 μm at room temperature. Analysis of the temperature and current density dependence gain
characteristics of a GaAs capped bilayer sample indicate that the temperature sensitivity of threshold current around
room temperature is due to phonon assisted thermal escape of carriers from the QDs.
Quantum Dot lasers exhibit the novel phenomenon of dual state lasing where population inversion can be achieved on
two optical transitions within the dots. In principle this might occur if a phonon bottleneck exists to impede relaxation of
carriers from the higher energy state. Here we present an alternative explanation whereby different lasing modes
compete for carriers and are spatially separable. Evidence comes from a comparison of electrical and optical
measurements made on the devices. The evolution of a particular lasing mode depends on diffusion of carriers between
dots and we show how, using an equivalent circuit model, this is consistent with our measurements.