40-nm SPAD-Array System for Ultra-Fast Raman Spectroscopy

The PoteinID (ProID) European project focuses on the development of a novel detection system that can perform ultra-fast Raman Spectroscopy (RS) with the aim of protein identification. The protein comprised of an amino acid (aa) chain is first denatured and then passed through a plasmonic nanopore. Each aa molecule is excited with a monochromatic laser and such excitation is followed by a Raman scattering process and subsequent autofluorescence. The goal of the detector developed by us, is to collect only the Raman photons while rejecting the fluorescence signal. By exploiting the fact that the fluorescence signal has a characteristic time constant in the order of few nanoseconds, while the Raman signal has a characteristic time constant in the order of few hundreds of picoseconds, we can reject the fluorescence signal by employing time-filtering techniques. The time- filtering technique used in this system is the Time-Gated Single-Photon Counting (TG-SPC). With this gating strategy we can define a temporal window (referred to as gate) where the SPADs are photosensitive and outside this window the photons are not detected. By using a sub-ns gate generated internally by the array, we can make the SPADs photosensitive only to the Raman photons. After the photons are detected and counted by each pixel, the sum of the four pixels inside a column is available at the output as an 8-bit word. The sums of the columns are outputted sequentially starting from the last one and then scanned in a shift-register pattern. Each column has a readout time of around 10 ns, so by using a 100 MHz reading clock, a full readout of 1.28 μs can be achieved. Thanks to the innovative jump readout modality, it is possible to select a specific subset of the column to read and thus reducing the overall readout time by 10 ns per each disabled column. By disabling 28 columns it is possible to reach sub-μs readout time. This technique was inspired by results shown presented in [1]. In this paper, we present the characterization of a detector developed as a preliminary version to the 128 x 4 SPAD array. The detector tested in this paper is based on a 16 x 4 SPAD array developed in the same 40 nm planar technology.