Ultrafast optical spectroscopy for depth-resolved optical property estimation

Reflectance spectroscopy is a well-established, non-invasive diagnostic tool for early malignancy detection and intra-operative tumour margin assessment, yet lacks the ~2mm spatial resolution many applications require. Knowledge of the sample’s refractive index and the time-of-flight of remitted light allows estimation of photon path length and sampling depth. However, hardware limits in temporal resolution and the absence of suitable light-transport models have so far prevented depth-resolved reflectance spectroscopy in a clinically practical form factor. We address this using superconducting nanowire single-photon detectors from Single Quantum (Delft), with 10–15 ps temporal resolution enabling ~1–2 mm sampling depth. With this setup, we measured tissue-mimicking phantoms to obtain time-resolved subdiffuse reflectance spectra. Optical properties were retrieved via our lab’s finite-element light-transport platform, bridging subdiffuse and diffuse regimes for deterministic time-resolved light-transport simulation in turbid media. We demonstrate that this approach enables accurate recovery of depth-resolved optical properties.