Light is a powerful tool to non-invasively probe scattering media for clinical applications. Time-domain (TD) technique relies on the injection of fast laser pulses and on the collection of the re-emitted photons though a single-photon detector. TD technique has 2 main advantages: i) disentangling scattering from absorption; ii) encoding the mean depth reached by the photons in their arrival time (thus decoupling superficial and deep information). Those features make the technique very attractive for several applications like oncology, neurology, molecular imaging, quality assessment of food, wood and pharmaceuticals.

Recently, state-of-the-art results have improved with respect to classical detector (e.g. SPADs) by using 1 x 1 mm² silicon photomultipliers (SiPMs) in contact with the tissue under investigation, which provides single-photon sensitivity, large active area and good singlephoton time resolution. Larger area SiPM instead demonstrated insufficient performance due to a faint avalanche and high noise.

The goal of this project is to push this technique more towards the actual limits. We will develop a potentially revolutionary detector based on a high-performance largerarea SiPM. Bigger collection area increases photon harvesting, giving much better sensitivity. This would actually pave the way to completely novel spectroscopy approaches. For example, it would be possible to non-invasively study the functionality and/or the compositions of organs such as lungs, heart or even the chest in transmittance geometry. Moving to the target SiPM dimension of 10×10 mm², but keeping the same detection efficiency, it would increase >100 times the sensitivity, due to bigger area and augmented numerical aperture, allowing to explore deeper regions of the body.

The important challenge is the development of a big area detector, with enhanced sensitivity in the NIR, keeping an extremely low level of noise (1 ÷ 2 Mcps in 1 cm²) and good single-photon time resolution. We will evaluate different technological solutions to reduce the noise level and develop a compact packaging including Peltier cooler. Moreover, we will develop an innovative layout for the SiPM to enhance significantly the signal extraction, including possible segmentation of the active area.

In conclusion, we will study, develop and test an innovative very-high sensitivity optical diffuse spectroscopy probe, which will be very useful in several biomedical and medical applications, for detection, inspection, and possibly will improve patience life quality. In a future, the developed detector can be integrated together with a compact pulsed source (e.g. a VCSEL) and a time-to-digital converter thus making a powerful, compact and low-cost probe for patient’s health monitoring. Indeed, in this project not only the performance will be a breakthrough. The goal is also to pave the way to a cheaper alternative to the more common and invasive technologies (e.g. endoscopy, PET, SPECT, MRI).