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.
For their second stage of lens integration, the researchers applied a UV sensitive photoresist to the lensed QD and patterned the micro-objective using 3D femtosecond direct 780nm laser writing. “The photoresist is polymerized via two-photon absorption at 390nm in a small volume element around the laser focus, resulting in sub-micrometre resolution”, the paper reveals.
As in many 3D printing technologies, moving the focus of the laser through the photoresist (following a specific CAD path corresponding to the micro-lens geometry) exposed the optical system, accurately aligned with the monolithically integrated micro-lens.
With the finalized optics, the researchers reported a broadband photon-extraction efficiency of 40+- 4% for their quantum devices, with a high suppression of multi-photon emission events. They are convinced that further development using micro-objectives with NAs close to unity could yield highly efficient plug-and-play fibre-coupled single-photon sources for quantum communications.
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