Theuwissen takes imaging to the limits

Professor Albert Theuwissen (EEMCS) was elected Electronic Imaging Scientist of 2011 last January. What are the latest trends in his field of research?

Standard detectors become ever more sensitive. (Photo: Hans Stakelbeek)

Sure, the Fuji Gold Medal Award in 2008 was prestigious, but to be selected ‘primus inter pares’ by one’s colleagues makes this award even more interesting, says Professor Albert Theuwissen, who couldn’t attend the ceremony in San Francisco due to a PhD defence in Delft.
Last year Theuwissen co-authored the book ‘Single-photon Imaging’ (Springer Verlag, 2011), which is about the holy grail of imaging: imaging with the absolute minimum of light - one photon.

Until recently, single photon imaging could only be done with vacuum tubes called photo multipliers. High voltages between a series of cathodes turn a single electron, kicked free by a photon, into an avalanche of electrons and hence into a detectable current peak.

The first solid-state devices that applied the same trick where called EM-CCDs and EM-CMOS, where ‘EM’ stands for electron multiplication. They first hit the market some ten years ago, but they’re still very expensive. The applications are mostly restricted to medical research, says Theuwissen, for example for the detection of fluorescent molecules.

Another solid-state single-photon detector is the SPAD (single photon avalanche diode), which acts as an all-or nothing detector. One photon is enough to trigger a pulse of millions of electrons, after which the device needs to be reset.
Its response is very non-linear, as the surface is very small and needs a relatively high voltage to function. These issues make SPADs difficult to work with.
However, what they can do very precisely is timing the exact moment at which the (first) photon hits. This quality makes SPADs suitable to measure distances by determining the photon’s exact time-of-flight.

This kind of imaging, which is the expertise of Professor Edoardo Charbon (EEMCS), can acquire 3D images of a scene in millimetre resolution in real time.
Theuwissen adds that ongoing research keeps reducing noise levels in CMOS detectors, so that eventually also these ‘standard’ imaging chips will approach single photon sensitivity.