The November 2011 issue of Laser Focus World contains an interesting artilce on single photon avalanche photodiodes.
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Apparently Sergio Cova and his colleagues at the Milan Polytechnic have devised a new planar structure that works rather well.
Only part of the planar surface is available to absorb photons, but by covering the device with an array of microlenses, most of the incoming photons can be directed to the absorbing patches.
It looks cute. It doesn't seem to be commercially available yet, and my reading of the article is that they are fishing for potential customers rich enough to fund the next stage of development, but it is the kind of thing that Phil Hobbs should be aware of.
-- Bill Sloman, Nijmegen