SUMMARY Time-of-Flight Positron Emission Tomography (TOF-PET) scanners provide better signal-to-noise ratio (SNR) and artifact reduction compared to conventional PET systems. The performance of TOF-PET scanners improves with the timing precision of its detectors: the more accuracy in the time detection of gamma photons the better the performance. The ultimate aim of TOF-PET is to reach a 10 ps full width at half maximum (FWHM) coincidence time resolution (CTR) to resolve precisely the positron-electron annihilation point in 3 dimensions. State-of-the-art PET detectors consist of scintillation crystals coupled to silicon photomultipliers (SiPM) and show timing resolutions in the order of 100-200 ps FWHM. In this project, we focus on improving dramatically the timing properties of the SiPMs, as such improvement would have a strong impact on TOF-PET as it would improve the timing performance of most detectors that use scintillation crystals and/or Cerenkov light emitters by several-fold. State-of-the-art SiPMs are optimized for a narrow range of wavelengths (λ) because of the difference in penetration depth at different wavelengths. For photons of λ=450 nm and λ=590 nm, the attenuation depth is 0.4 μm and 2 μm, respectively. The trade-off is to either a) to have a thicker depletion layer to absorb a wider range of wavelengths but to increase the time jitter, or b) to have a thinner depletion layer to reduce the time jitter but absorb only a narrow range of wavelengths. Therefore, there is not a state-of-the-art SiPM that provides, simultaneously, very fast time response, and high photon detection efficiency (PDE) across a wide range of wavelengths. We propose to develop an SiPM prototype with photon-trapping microstructures integrated in the depletion layer that disperses the light laterally and allows one to obtain high-detection efficiency for a wide range of wavelengths within a depletion layer of 1 μm. With such a thin layer, the jitter in the electron drift time decreases to 10 ps and the dark current is expected to decrease as well. This new photosensor could revolutionize TOF-PET. The utilization of periodic microstructures to bend light in a perpendicular orientation and trapping photons for enhanced interaction with materials, high detection efficiency and fast response have been recently shown for wavelengths between 800-900 nm for optical communication. In this proposal, we will develop a new SiPM based on this technology. First, we will simulate the optimum layer structure to integrate the hole-trapping microstructures and an avalanche region to provide a gain of >105. Second, we will do an electronic characterization for the different type of microcells, including a gain calibration and measure quantum efficiency for different λ for each cell. Finally, we will manufacture a wafer with full-size SiPMs (3x3 mm2) and test the SiPMs with scintillation crystals and Cerenkov emitters.