Quantum communications goal of 3D micro-optics research

Quantum communications goal of 3D micro-optics research
Technology News |
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.
By Rich Pell


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.

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.

Scanning electron microscope images of a QD
micro-lens (a) and the fully processed QD micro-lens/
micro-objective device (b).

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.

Related article:
905nm pulsed laser diode integrates micro lens

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