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.