TECH FOCUS SINGLE PHOTON COUNTING


LASER COMPONENTS FEATURED PRODUCT


limiting the performance of thick-absorber Spads to about 10 million counts per second or fewer. It has generally been assumed that thick-absorber Spads are unsuited to higher rate counts. To overcome those problems


Count Single Photon Counting Modules


LASER COMPONENTS offers a range of single photon counting modules featuring our VLoK* avalanche photodiode (APD) design with minimal excess noise factor, which enables market leading dark count rates (<10 counts/sec), low afterpulsing probability (0.2%), and no bi-stability issues. Several variants of the Count modules are offered, which are optimised for different application requirements, from NIR optimised modules typically utilised for quantum cryptography to high timing precision variants optimised for applications such as time correlated


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which is not enough to meet the increasing needs of modern communications. Raising the rate, however, has been problematic because of the many trade-offs involved. For example, the thickness


of the absorption layer that the incoming photon hits, determines how likely the device is to catch that incoming photon: thick absorbers (around 0.1mm) have a higher probability of photon capture because of their greater depth; thinner layers have a greater chance that the photon will pass through undetected. But the thicker the absorber,


34 Electro Optics July 2021


single photon counting and fluorescence lifetime imaging. The extremely high single photon detection efficiency of >70% can be offered with peak sensitivity tuned to UV, visible or NIR wavelengths though a combination of APD design and anti-reflection coating optimisation.


Our Count modules are competitively priced and available with relatively short lead times. Requests for customised versions are also welcomed. *VLoK = Very Low K factor, which determines the excess noise factor, inherent to APDs.


Further information: www.lasercomponents.com/ uk/photon-counter/single- photon-counting-modules


the higher the applied voltage needs to be. And higher voltages can produce larger avalanches – large enough to overheat the device, reducing detection efficiency as well as increasing the risk of spurious ‘afterpulses’, in which left- over electrons trapped in the semiconductor set off a secondary avalanche after the Spad is reset. To reduce afterpulses, it is necessary to reset the system in two nanoseconds or less. Conventional modules that detect the current and then apply the quench cannot operate that fast, historically


in a thick-absorber device, the NIST team – which reported its results in March in Applied Physics Letters – began experimenting with an advanced electronics system for a commercially available thick-absorber Spad. Like many such systems,


the Spad is ‘gated’ off and on repeatedly – that is, it is reset continuously by an applied alternating voltage at some frequency. As a result, the longest time period during which the Spad can produce an avalanche is the gate interval. ‘Typical gating frequencies for these types of Spads has been limited to no higher than 150 megahertz,’ said NIST associate Michael Wayne, first author on the journal article. ‘That means that the Spad


is able to avalanche for six or seven nanoseconds,’ Wayne said. ‘While this may not seem like a long time, it is long enough for the device to both become fully saturated with charge – which increases unwanted afterpulsing – and to become hot enough at high count rates to lower its detection efficiency. Gating at a higher frequency – thus shortening the maximum duration of an avalanche – would lessen both of these effects. But because the avalanche is not allowed to grow for as long, it can become too small to detect over the “noise” caused by opening and closing the gate.’ To overcome that problem,


the team developed a method similar to noise-cancelling


headphones: applying a radio frequency signal that exactly offsets the noise. That allowed them to operate the Spad at one billion cycles per second (1GHz). Subtracting out the noise,


said project leader Joshua Bienfang, ‘we are able to reveal extremely small avalanches. Additionally, the high frequency means that the gate is open for only 500 picoseconds. This results in a reduction in average avalanche current by about a factor of 500, lowering both the afterpulsing and self-heating effects, and allowing us to count at rates up to 100 million per second.’


“This novel design is likely to impact several quantum applications”


The new Spad design


could find practical uses in the applications of quantum communication and quantum computation. ‘Both offer capabilities not possible with conventional communication and computation. And both applications would benefit from faster, lower-noise single photon detectors,’ Migdall said. ‘This novel design is likely to impact a number of quantum applications. They range from single-photon sensing, where faster count rates and lower noise reduce the time for existing measurements, to the emerging quantum internet, which relies critically on single-photon detection for quantum communication and quantum computation. Both of those can be expected to have a very substantial impact on our society and economy,’ said Migdall. EO


@electrooptics | www.electrooptics.com


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