The Peer review has evaluated this group as Excellent
Photon counting is the technique of choice for attaining the ultimate sensitivity in measurements of optical signals. It is a completely digital approach starting right from the detector, hence it completely avoids the limitations set by the noise of electronic circuits in analogue measurements. It requires, however, special detectors with an internal amplification mechanism, which generates in response to single photons macroscopic electrical signals, much higher than the circuit noise. Time-Correlated Single-Photon Counting (TCSPC) makes possible to measure optical waveforms with high sensitivity on very fast time scale. Photon counting and timing was introduced and developed with photomultiplier tubes (PMT), but it received new impulse from new solid-state detectors, the Single-Photon Avalanche Diodes SPAD. SPADs exploit the avalanche phenomenon in a junction with an approach conceptually similar to that of Geiger-Mueller counters of ionizing radiation, drastically different from the linear amplification of ordinary Avalanche Photo Diodes (APD). They produce in response to a single photon a standard current pulse with macroscopic (milliampere) size and fast (subnanosecond) rise, which marks the arrival time of the photon with precision of a few tens of picoseconds. Besides the general advantages of semiconductor devices versus vacuum tubes (reduced size and cost, higher reliability, ruggedness, suitability to integrated systems), SPADs offer improved basic performance in terms of higher photon detection efficiency, lower dark-counting rate and higher resolution in photon timing, capability of efficient operation up to high photon counting rate (various Mc/s). The group led by S. Cova carried out pioneering activity and attained worldwide acknowledged leadership in the development of SPADs and associated electronic systems. It introduced the active-quenching circuit concept (AQC), which opened the way to practical applications of SPADs, and has been the first to developed monolithic integrated AQCs. It developed new epitaxial silicon SPAD devices in various generations, in collaboration with tehnological laboratories of industry and academy (CNR-IMM, Bologna; ST-Microelectronics Milano/Catania; UCC-NMRC, Cork, Ireland). SPADs are nowadays employed in a wide range of emerging applications in chemistry, biology, medicine, material science and physics. Typical examples are: fluorescence microscopy; single molecule spectroscopy; DNA fragment separation and protein analysis; quantum key distribution for cryptography; astronomy; laser ranging in space applications and telemetry; photon correlation techniques in particle sizing and laser velocimetry; laser diode characterization; optical fiber testing in communications and in sensor applications. A main focus of the research group activity is the development of detectors and associated electronics for applications in lifesciences, namely for micro and nano-analytical techniques in biomedical, genetic and diagnostic applications. The activity is driven by the widespread interest to rapid and efficient detection of fluorescent emission from minimal quantities of biological material, i.e., from extremely small samples down to single molecules of DNA and proteins. The goal is the development of new miniaturized detection systems that overcome disadvantages and limitations of existing systems, which rely on bulky, discrete and costly sensing equipment. The activity under way is addressed to both single point sensors (for capillary electrophoresis in microchips and single molecule spectroscopy) and linear and matrix array detectors for DNA and protein microarrays (molecular recognition in ordered sets of micro-samples on glass substrate). 298 Consistent effort is addressed to detectors and electronics for adaptive optics systems of advanced telescopes, in collaboration with the european organization for astrophysics ESO (European Southern Observatory). Relying on previous experience in the project STRAP “System for Tip-tilt Removal with Avalanche Photodiodes”, which produced the advanced adaptive optical systems for the VLT (Very Large Telescope, Cerro Paranal, Chile), a new monolithic detector has been designed and implemented for detecting the distortions due to atmospheric turbulence. This Single- Photon Array Detector SPADA integrates 60 element-detectors for sampling in a 60-point array the optical wavefront. Remarkable effort is devoted also to the extension of single photon techniques to the infrared (IR) spectral range, exploiting at best Silicon SPADs and developing SPADs in Ge and III-V semiconductors for longer wavelength up to 1.6 ?m. The driving force comes from various application fields, such as: quantum key distribution for cryptography in optical fiber communication systems; non-invasive (contactless) measurement of signals in ultra large scale integrated circuit chips (ULSI); eye-safe laser ranging. A direct outcome of this theme is the foundation in 2004 of Micro Photon Devices (MPD, www.microphotondevices.com), a spin-off company of Politecnico di Milano. The mission of MPD is production and commercialization of photon counting and timing modules. After the exploratory phase, in 2006 a positive net result was obtained with about 250 modules produced. 95% of these products of Italian technology were exported world-wide.
Sergio Cova (full professor)
Massimo Ghioni (full professor)
Franco Zappa (associate professor)
Ivan Rech (assistant professor)
Alberto Tosi (assistant professor)