A SPAD array detector for spectrally and lifetime resolved microscopy

Time-resolved single molecule detection techniques are state-of-the-art. In order to obtain as much information as possible, parallel detection of several polarization and spectral channels is necessary. In addition, parallelization of the measurement overcomes many of the limitations due to the inherently long measurement times. Fluorescence lifetime measurements are indispensable for many single molecule measurements. The optimal detector type, combining an excellent timing resolution together with very high quantum efficiency, is the Single Photon Avalanche Diode (SPAD). Therefore SPAD detector arrays would be an ideal detection tool for either spectral or parallelized time-resolved single molecule measurements. We investigated a prototype of a silicon based SPAD array featuring eight detection channels. The array, designed by the Politecnico in Milano, Italy, achieves a detection efficiency of around 30% measured at 470 nm and 670 nm. The timing resolution is 170 ps and 80 ps respectively. Due to a novel electronic architecture, the crosstalk is reduced and reaches a value below 1%. The sensitive areas have a diameter of 50 µm and there is a pitch of 250 µm between each detection pixel. A special optical layout was developed in order to illuminate the different SPAD areas after spectral splitting minimizing the loss of fluorescence light. To this aim a set of relay lenses and an array of micro-lenses were designed, which focus the light after the spectral separation onto the different sensitive areas of the SPAD array. Preliminary results demonstrate the feasibility of the planned setup.