Silicon photomultipliers (SiPMs) are being increasingly used in diffuse-optics applications such as functional brain imaging, mammography, and investigations of photon migration in random (such as turbid) media, as SiPMs are more robust than traditional photomultiplier tubes (PMTs), lower in cost, and have high photon-collection efficiency because of their large (up to a few square millimeters) active area and high numerical apertures. But the use of SiPMs for time-domain (TD) optics, where the arrival of photons are timed to picosecond-scale levels, has remained unexplored due to SiPMs' high background noise, which can overwhelm the time-correlated single-photon-counting (TCSPC) electronics; their tendency to produce strong afterpulses; and the optimization of commercial SiPMs for the multiphoton, rather than single-photon, regime.
Now, a multinational group of European researchers has combined targeted selection of commercial SiPMs with custom dedicated electronics, leading to a 1 mm2 device that has a timing resolution of better than 80 ps (measured at a 690 nm wavelength), a fast tail with a mere 90 ps decay time constant, and high photon-collection efficiency. They mounted the SiPM detector and electronics, forming an "optode," into a compact probe about 10 × 20 mm in size, avoiding the bulk, complexity, and loss of light from the use of a fiber bundle. The researchers experimentally verified its capabilities by measuring the optical properties of homogeneous solid phantoms (prepared dark diffuse media) and for inhomogeneous problems with a localized black solid inclusion placed into a liquid phantom. Inserting time-gating capability into the circuitry is a future task.
Reference: A. Dalla Mora et al., Opt. Express (2015); doi:10.1364/OE.23.013937.
