3D micro optics on top of singled-out quantum dots

June 08, 2017 // By Julien Happich
A team of German scientists has focused its efforts on improving the extraction efficiency of quantum light sources, with the hope to be able to couple such sources directly to optical fibres for quantum communications.

Taking a quantum dot singled-out for its emission characteristics, the researchers wanted to combine a monolithically integrated micro-lens with a multi-lens micro-objective aligned and built on top.

Schematic view of the QD micro-lens/micro-objective
device (left) and the calculated ray propagation of the
micro-objective (right).

Using numerical simulations with a finite-element solver, the researchers first optimized the Quantum Dot (QD) micro-lens in order to maximize the calculated photon-extraction efficiency. At a target wavelength of 930nm, they obtained that maximum for a hemispherical lens with a base-width of 2400nm and a height of 420nm. Then, using a separate ray-tracing software and geometrical optics, they designed a lens system consisting of four aspherical surfaces, with the aim to yield a numerical aperture (NA) of 0.7.

In a paper published in the ACS Photonics journal under the title "Single quantum dot with micro-lens and 3D printed micro-objective as integrated bright single-photon source", the researchers detailed the fabrication processes involved to create such integrated optics.

The actual device they start with is a dense array of InGaAs QDs single-photon emitters grown by metalorganic chemical vapour deposition on a GaAs(001) substrate, above a distributed Bragg reflector (DBR). The QDs were then capped by a 420nm thick GaAs layer to provide material for the monolithically integrated micro-lenses. For a single, pre-selected QD, a micro-lens was shaped by 3D Electron Beam Lithography into a low-temperature electron-beam resist. The lens shapes were then transferred into the 420nm thick GaAs capping layer by inductively coupled plasma reactive-ion etching.