High-Performance Single Photon Avalanche Diode Array and Integrated Electronics for Time Correlated Single Photon Counting

Time Correlated Single Photon Counting (TCSPC) is a very effective measurement technique to perform the analysis of extremely weak and fast, periodical light signals. Based on the detection of single photons and on the measure of their arrival time within the period of the signal, TCSPC technique is increasingly widespread in a large number of fields, from medicine to chemistry and biology, and in a large number of applications such as single molecule fluorescence spectroscopy, fluorescence imaging, and laser scanning microscopy.

Nowadays, most of the high-performance TCSPC systems are focused on single channel applications and the use of multiple parallel acquisition chains is necessary to obtain a multidimensional system, , with very high costs and large occupied areas. It is therefore necessary to develop new acquisition systems based, on the one hand, on detectors with high quantum efficiency and suitable for parallel operation, and, on the other hand, on electronics intrinsically designed to simultaneously acquire data from multiple channels. Concerning the first aspect, Single Photon Avalanche Diodes (SPADs) proved to be a valid alternative to PMTs in many applications, from quantum cryptography to astrophysics and biology.

Recent systems for single-photon timing applications typically present a trade-off between number of channels and performance: the higher the number of channels, the poorer the obtained performance on each channel.

In order to overcome this trade-off, in this work we present the development of the detector and the electronics necessary to develop photon timing systems featuring a large number of channels in conjunction with high performance. Starting from the SPAD developed in our lab in the last years, it is important to develop all the integrated electronics to extract and elaborate the timing information. First of all, a circuit capable of reading directly the avalanche current coming from the detectors is of the utmost importance in order to achieve a very high temporal resolution while minimizing the crosstalk between adjacent channels. Thus, a fully integrated trans-impedance stage has been designed in 180nm Si-Ge with a gigahertz bandwidth in order to obtain few tens of picoseconds full width at half maximum resolution and negligible crosstalk between different channels. Secondly, a fully integrated array of high performance time-to-amplitude converters (TAC) has been designed. In fact, in any TCSPC application one of the most important section in the acquisition chain is the time measurement block. Three main performances are required in the time measure to reconstruct the analyzed signal without introducing distortions: a high temporal resolution (in the order of few tens of picoseconds), a differential nonlinearity (DNL) of few percent of the histogram channel width and a high measurement rate (in the Megahertz order).

Finally, in order to develop a fully parallel and compact TCSPC system, resource sharing is mandatory. To achieve this

goal an integrated chip capable of routing the signal coming from the trans-impedance stage towards the time

measurement circuit has been designed in 180nm Si-Ge technology. Since the probability of detecting a photon during one excitation cycle in any TCSPC measurement is far less than one, we have designed the routing logic to connect the

SPAD to one of the TAC converters only if a photon has been detected, thus allowing the system to have a number of

TAC converters lower than the detectors one.

The design of these three main blocks in conjunction with fully custom high efficiency SPAD arrays opens the way to the development of a TCSPC acquisition system that can feature both a very high number of parallel channels and very high performance.