Fast timing performance is of paramount importance in a number of time-resolved medical applications, including TOF-PET, FLIM and Raman. The EPFL AQUA lab has explored several strategies to increase the performance of miniature all-solid-state silicon detectors in the form of single-photon avalanche diodes (SPADs), also known as Geiger-mode APDs. SPADs feature an exquisite timing performance (tens of ps), together with single-photon sensitivity. Arrays of SPADs have encountered commercial success in the form of analog SiPMs, and lately in digital form for LIDAR, with an expanding applications portfolio.
Our investigations have focused on one side on the device design itself, together with the corresponding optimised front-end electronics. This has allowed us to break the 10 ps (FWHM) single-photon timing resolution barrier at device level, while maintaining high photon detection probability in the visible, coupled to low noise and dead times as small as 3 ns (and 1.5 ns in a different technology node).
Another track has investigated the 3D-stacking of a photodetection layer, coupled to a control and processing layer implemented in deep-submicron CMOS technology. This approach is aimed at increasing the granularity of timestamps, together with timing improvements by statistical methods. Furthermore, the spatial granularity can in principle be traded off, e.g. through the clustering in mini-SiPMs, with respect to additional functionalities in the bottom tier. This can lead to architectures which differ from conventional ones, possibly even with the inclusion of on-chip artificial intelligence functionalities.
Finally, we have also explored another class of time-resolved detectors, namely superconducting nanowires, coupled to large bandwidth and low power front-end electronics based on a SiGe platform. The latter could potentially find applications in the read-out chains of analog SiPMs, as a building block for short-term timing improvements using existing detectors.