Simulation modelling for the analysis and the optimal design of SPAD detectors for time-resolved fluorescence measurements Marina Repich *a,b , David Stoppa b , Lucio Pancheri b , Gian-Franco Dalla Betta a a Department of Information Engineering and Computer Science - DISI, University of Trento, Trento, Italy; b Fondazione Bruno Kessler - FBK-IRST, Trento, Italy ABSTRACT This paper describes a simulation model (implemented in MATLAB ® ) of a typical setup used for time-resolved fluorescence measurements, including: a laser source, basic fluorescence sample, optics, single-photon avalanche diode and read-out electronics. The correctness of the model has been verified by setting up a simple time-resolved fluorescence measurement using a CMOS SPAD-based detector. The solution of fluorophore (CdSe/ZnS quantum dots in toluene) in a glass capillary was placed above the detecting surface and excited by laser pulses. We have used a time- gating technique with 10-ns observation window shifted at 60-ps time steps across the appropriate time interval. The observed curve corresponds to the convolution of the fluorescence emission and the 10-ns observation window. Simulation accuracy has been verified by comparing the experimental fluorescence decay with the simulated one using chi-square test. The proposed model allows researchers to simulate the behaviour of SPAD detectors with a good accuracy and demonstrates how imperfections in the experimental system can affect the result. The model enables the design of SPAD-based detectors with the best performance for a specific application area. Keywords: single-photon avalanche diode, fluorescence measurement, time-correlated single photon counting, time- gating, simulation modelling 1. INTRODUCTION Due to their properties, single photon detectors find their applications in astronomy [1], laser ranging [2], quantum cryptography [3], single molecule detection [4], fluorescence decay [5,6], etc. Historically, photomultiplier tubes were the first detectors used in fluorescence measurements. They provide a low noise and fairly high quantum detection efficiency in the visible spectrum. At the same time they are bulky, fragile, and expensive, require high supply voltage (2-3 kV) and are sensitive to electromagnetic fields and mechanical vibrations. Single photon avalanche diodes (SPAD) have become an attractive alternative to photomultiplier tubes [7] due to the advantages of solid-state devices, such as: magnetic field immunity, robustness, long operative lifetime, small sizes, lower cost, lower bias voltage, suitability for inclusion in integrated systems. There has also been much progress in terms of improved detector time resolution, quantum efficiency and noise performance. Recent progress in CMOS technology allows SPAD detectors to be fabricated alongside integrated read out electronics, which ultimately reduces the sensitive node capacitance and allows additional processing features [8,9]. This leads reduced system costs and the ability to produce monolithic arrays for large area detection. Co-integration of a SPAD and an electronic circuit on the same substrate provides advantages in terms of time and noise characteristics. The main disadvantages of the CMOS SPAD are the high dark count rate (DCR) and low photon detection efficiency at longer wavelengths (above 800nm) [10]. At the present time, the area of SPAD detectors is being developed constantly and quickly. Many new sensors with enhanced characteristics are produced every year. Fully integrated SPAD-based detectors with extensive on-chip processing features have been recently proposed [8,9,11,12,13,14]. However, current research works focus mainly on the improvement of particular characteristics using different performance metrics without consideration of their integral usefulness for experimenters. Moreover, the importance of each SPAD characteristic depends on the application. We * E-mail: repich@fbk.eu