  To implement quantum light sources based on quantum emitters in applications,
it is desirable to improve the extraction efficiency of single photons. In
particular controlling the directionality and solid angle of the emission are
key parameters, for instance, to couple single photons into optical fibers and
send the information encoded in quantum light over long distances, for quantum
communication applications. In addition, fundamental studies of the radiative
behavior of quantum emitters, including studies of coherence and blinking,
benefit from such improved photon collection. Quantum dots grown via
Stranski-Krastanov technique have shown to be good candidates for bright,
coherent, indistinguishable quantum light emission. However, one of the
challenges associated with these quantum light sources arises from the fact
that the emission wavelengths can vary from one emitter to the other. To this
end, broadband light extractors that do not rely on high-quality factor optical
cavities would be desirable, so that no tuning between the quantum dot emission
wavelength and the resonator used to increase the light extraction is needed.
Here, we show that metallic nano-rings combined with gold back reflectors
increase the collection efficiency of single photons and we study the
statistics of this effect when quantum dots are spatially randomly distributed
within the nano-rings. We show an average increase in the brightness of about a
factor 7.5, when comparing emitters within and outside the nano-rings in
devices with a gold back reflector, we measure count rates exceeding 7 x 10^6
photons per second and single photon purities as high as 85% +/- 1%. These
results are important steps towards the realisation of scalable, broadband,
easy to fabricate sources of quantum light for quantum communication
applications.
