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In the last decade the Single Photon Avalanche Diode (SPAD) has become the detector of choice in many applications that relies on Time Correlated Single Photon Counting techniques, like single molecule spectroscopy or optical radar (LiDAR). However, SPADs cannot fully address the needs of some emerging applications, which requires a combination of high detection efficiency and low-timing jitter in the near infrared (800-1000 nm). The reason is an intrinsic tradeoff that exists in the current SPAD structures: as the detector is illuminated from the top, increasing the detection efficiency requires a thicker depleted region that results in a larger timing jitter, owing to the dispersion of the carriers’ transit times. As an example, in state of the art SPADs, increasing the detection efficiency at 900 nm from 7% to 18% led to a worsening of the timing jitter from 30 to 90 ps FWHM. 
To overcome this limitation, a side-illumination approach, in which the light propagates transversally to the direction of the electric field, is proposed. This way, the absorption efficiency can be improved by increasing the detector length rather than its thickness, thus decoupling light-absorption from avalanche multiplication. More specifically, the detector will be embedded in a silicon waveguide and the photons, generated externally, will be coupled into the SPAD through an optical fiber. With this approach it is possible to envision an absorption efficiency exceeding 90% at 900 nm, while maintaining a timing jitter of about 30 ps.
Such results, combined also to the possibility to operate at room temperature, may have a dramatic impact on applications like Quantum Key Distribution (QKD), Quantum Information Processing (QIP), Non-Line-Of-Sight imaging (NLOS), etc.
Although the idea is fairly simple and the advantages are unquestionable, a waveguide-SPAD has never been implemented so far, most likely because of the challenges associated with its design and fabrication.
During the Master Thesis a preliminary study of the integration of a SPAD in a silicon waveguide has been performed. The work, whose aim was to evaluate the effectiveness of this solution, to identify possible problems, and to assess its technological feasibility, confirmed that this solution is very promising and possible.
Given these results, my research project is dedicated to transform the idea of a waveguide SPAD into a real detector available to the research and industrial community.