Design of freeform beam shaping micro-optics for single photon sources

Efficient coupling of single photons into single-mode fibers (SMFs) is critical for scaling many quantum technology applications, including quantum key distribution and quantum repeater networks. In such systems, single photons must be extracted from solid-state emitters and transmitted via SMFs while minimizing loss and decoherence. However, the far-field radiation patterns emitted by bare single-photon sources (SPSs), such as molecular or semiconductor-based emitters, do not inherently match the spatial and angular properties of SMF-supported modes, limiting achievable coupling efficiencies. Common strategies to overcome this mismatch include the use of evanescent coupling, semiconductor-based cavities, and grating structures, where coupling is mediated through near-field interactions between the emitter and the surrounding nanostructures. In this work, we explore an alternative approach based on far-field shaping using continuous freeform micro-optical surfaces. The adopted design workflow combines near-field modeling of SPS emission via finite-difference time-domain (FDTD) simulations with far-field shaping techniques traditionally used in non-imaging illumination optics. Our results demonstrate that non-imaging design methods, which are commonly associated with LED-based illumination problems, can be effectively repurposed for designing coupling micro-optical interfaces to point-like single photon source emitters.