  Effective light extraction from optically active solid-state spin centres
inside high-index semiconductor host crystals is an important factor in
integrating these pseudo-atomic centres in wider quantum systems. Here we
report increased fluorescent light collection efficiency from laser-written
nitrogen vacancy centers (NV) in bulk diamond facilitated by micro-transfer
printed GaN solid immersion lenses. Both laser-writing of NV centres and
transfer printing of micro-lens structures are compatible with high spatial
resolution, enabling deterministic fabrication routes towards future scalable
systems development. The micro-lenses are integrated in a non-invasive manner,
as they are added on top of the unstructured diamond surface and bond by
Van-der-Waals forces. For emitters at 5 micrometer depth, we find approximately
2x improvement of fluorescent light collection using an air objective with a
numerical aperture of NA = 0.95 in good agreement with simulations. Similarly,
the solid immersion lenses strongly enhance light collection when using an
objective with NA = 0.5, significantly improving the signal-to-noise ratio of
the NV center emission while maintaining the NV's quantum properties after
integration.
