  Hybrid photonic integration exploits complementary strengths of different
material platforms, thereby offering superior performance and design
flexibility in comparison to monolithic approaches. This applies in particular
to multi-chip concepts, where components can be individually optimized and
tested on separate dies before integration into more complex systems. The
assembly of such systems, however, still represents a major challenge,
requiring complex and expensive processes for high-precision alignment as well
as careful adaptation of optical mode profiles. Here we show that these
challenges can be overcome by in-situ nano-printing of freeform beam-shaping
elements to facets of optical components. The approach is applicable to a wide
variety of devices and assembly concepts and allows adaptation of vastly
dissimilar mode profiles while considerably relaxing alignment tolerances to
the extent that scalable, cost-effective passive assembly techniques can be
used. We experimentally prove the viability of the concept by fabricating and
testing a selection of beam-shaping elements at chip and fiber facets,
achieving coupling efficiencies of up to 88 % between an InP laser and an
optical fiber. We also demonstrate printed freeform mirrors for simultaneously
adapting beam shape and propagation direction, and we explore multi-lens
systems for beam expansion. The concept paves the way to automated fabrication
of photonic multi-chip assemblies with unprecedented performance and
versatility.
