From: rev@enh.nist.gov
Newsgroups: sci.physics
Subject: RE: photon 'detectors' - how reliable?
Date: 21 Jan 93 23:14:12 GMT
Organization: NIST

>In a previous article, merritt@macro.bu.edu (Sean Merritt) wrote:
>>In article <wwadge.727584610@csr> wwadge@csr.UVic.CA (Bill  Wadge) writes:
>>>   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?
>>>
>>>   Seems unlikely, but then I'm not a physicist ...
>> 
>>What your asking about is the "quantum efficiency" of the detector.
>>If this is a device that uses a method of converting photon
>>flux into an electrical current(photo-electric device) if we
>>ignore noise:

[description of detector responsivity deleted]

>>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
>>than this. If you consider quantum noise or S/N it just lowers
>>the qe more. Although it is much more mathematically complicated.
>>

Depending on the photon energy this is not always the case.  Detectors that 
have quantum efficiencies greater than 1 are available, but you need to work in 
the extreme ultraviolet or soft x-ray region.  These are not photoemmitters, 
but semicondutor devices where the incident radiation creates electron-hole 
pairs in a diode.  For example, silcon has an electron-hole pair energy of 
about 3.63eV (I think).  If a photon with 7.26eV is absorbed, you can get two 
electron-hole pairs, and the quantum efficiency is 2, or 200% depending on how 
you want to put it.

>>Usually  the qe is specified interms of wavelength, and a function
>>called "responsivity" R = R (lambda).
>> 
>> 
>>qe(lambda) = [R(lambda)*h*c]/(e*lambda)
>> 
>>the units of R are [area/power] power is in watts or lumens.
>>somettimes itis given in [volume/power].
>>

Well, we just leave it as QE(lamda) in units of electron/photon.

But returning to the original question, even though you can get more than one 
electron out of your detector for every photon going in, you don't actually, 
detect every photon and no others.  As was mentioned there is noise, usually 
from thermal excitation of electrons.  Also there are loss mechanism that come 
into play so not every photon gets detected.  There are reflections off the 
front surface (and any other boundaries inside) of the detector, there are dead 
regions in the detector (SiO2 absorbs photons but does not result in any signal 
in a photodiode), and there is always the possibilty that the photon will just 
go through the detector without interacting with it at all.

To my mind, the "perfect" detector can't be made, but it is possible to 
calibrate detectors by knowing their quantum efficiency and that allows you to 
do science as if you did see every photon coming your way, even when you might 
only "really" see one in every ten.

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