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.
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.