Single photon avalanche diodes for the visible and near-infrared
Single-photon avalanche diodes (SPADs) can provide practical and reliable solutions for diverse applications both in the visible (Si) and Telecom (InGaAs/InP) regimes.
InGaAs/InP SPADs working in the Geiger mode are the most important techniques for single-photon detection in the near-infrared. In the Geiger mode, even a single photon-excited carrier can produce a macroscopic avalanche signal, which is detected and quenched by the dedicated electronics. However, the after-pulsing effect severely limits the SPAD performance. The origins of after-pulses are that some carriers trapped by the defects during the previous avalanches can be subsequently released and create undesired avalanches. Therefore, developing high-performance quenching electronics, particularly to suppress the after-pulsing, is critical to implement good SPAD performance such as high detection efficiency, high saturated count rate, low dark count probability, low after-pulsing probability and small timing resolution.
We are working on several different approaches to suit different applications, ranging from QKD and QRNG, to telecommunication metrology and biology.
Rapid Gated Detectors
Where there is a need for very high detection rates, SPADs are generally operated in the rapid gating regime. Due to the ultra short gating time, the number of charge carriers during avalanches reduces and thus the after-pulsing effect can be significantly suppressed. We have implemented rapid gating techniques by applying sine waves to gate SPADs and appropriate radio frequency (RF) circuitry to extract weak avalanches. We have increased the gating frequency up to a world-record level — 2.23 GHz. Recently we have worked on reducing the size of such detectors so that they can be integrated into maturing technologies, such as quantum key distribution systems
For many applications, where the time of the photon arrival is unknown, operation in the free-running regime is essential. In 2007, in collaboration with IDQ, we developed the first free-running SPADs. Whilst we focus on the electronics we are also collaborating with companies such as Princeton Lightwave to study negative-feedback avalanche diodes (NFAD) for free-running operation. We are also experimenting with combining different aspects of these different electronics to better understand and improved detector performance for diverse regimes of operation. Our recent investigation of free-running NFAD operation at low temperatures (down to 160 K) has demonstrated the lowest dark count rates ever achieved - as low as 1 cps.