From: aephraim@physics3 (Aephraim M. Steinberg)
Newsgroups: sci.physics
Subject: Re: photon 'detectors' - how reliable?
Date: 21 Jan 1993 19:24:39 GMT
Organization: /etc/organization
NNTP-Posting-Host: physics3.berkeley.edu

In article <MERRITT.93Jan21103409@macro.bu.edu> merritt@macro.bu.edu (Sean Merritt) writes:
>In article <wwadge.727584610@csr> wwadge@csr.UVic.CA (Bill  Wadge) writes:
>
>>
>>    .. towards a detector which records every photon ..
>>
>>   Is this possible? Can one really build a device so sensitive that it will
>>   detect 100% reliably every photon that enters it, and never
>>   go off by accident?
>
>First, Bill if you are really interested in this stuff, get yourself an
>Electro-Optics Handbook, put out by RCA(Solid State division, Electro
>Optics and Devices division. This and your last question are fully
>answered there.
>
> . . . . . . . 
>
>the parameters here are i and Popt. It is easy to see that you
>can't hope to get 100% qe, infact real devices seldom do better
>than 25%. I think the best photomultipliers are something less


Sorry, in my earlier post I forgot to answer the "by accident" question.
These detectors DO periodically go off by accident, primarily due to
thermal fluctuations (this is why the devices are run cold, typically
around -25C) and impurities in the semiconductors.  The resulting
pulses are called "dark counts"-- by the way, your eye has much the same
problem (it too acts as a single-photon detector in sufficiently low light,
albeit of quite low efficiency.  I don't think anyone knows for certain
how much of the efficiency problem is due to the intrinsic "quantum
efficiency" of the cells in your retina and how much is due to geometric
effects).  Typical APD's have dark count rates of 500 or 1000 counts per
second, but can register 100s of thousands of real counts per second 
before they start to saturate (and drop in efficiency).  The saturation
is usually due to the speed of the electronics, and can therefore be
pushed to higher count rates by building faster circuitry.

The SPCM's I mentioned are "super-low" impurity-level devices, so their
dark count rates are as low as 50 counts per second.  (They too begin
to saturate around 100,000 cps, but they currently have comparitively
slow electronics, which should change soon.  The APD operates in a 
"metastable" state like a supercooled liquid-- it is held above the 
diode's breakdown voltage, but cold enough that it takes about 20ms for 
the breakdown to occur.  A single photon is usually sufficient to trigger
an avalanche of electron-hole pair-production, which yields a macroscopically
detectable current.  After the avalanche, the device needs to be "reset,"
or "quenched," and the speed with which this is done determines the
saturation level.  The time of detection, however, is accurate to
something on the order of 100ps.)

Also, a lot of the losses in many of these detectors have to do with 
reflections off the devices.  The effective efficiency can be improved
by using the same trick that makes cats' eyes appear to glow: a mirror
which gives the photons a "second chance" to be detected.


-- 
Aephraim M. Steinberg                    | "WHY must I treat the measuring
UCB Physics                              | device classically??  What will
aephraim@physics.berkeley.edu            | happen to me if I don't??"
                                         |       -- Eugene Wigner
Newsgroups: sci.physics