B Photon detection methods -- do they always involve displacing an electron?

[jeremyfiennes]

TL;DR Summary

Involves dispacing an electron?

Am I right in thinking that all photon detection methods depend on a photon displacing an electron, that then displaces other electrons to give a detectable electric current pulse?

 

[phyzguy]

No. In a CCD detector, each incoming photon only generates a single electron. The electrons generated are stored and then the total charge stored in a given pixel is clocked out and read. Also radio antennas don't involve detecting individual photons.

 

[jeremyfiennes]

Thanks. So even in a CCD detector, the intial event is still a photon generating a free electron?

 

[phyzguy]

Thanks. So even in a CCD detector, the intial event is still a photon generating a free electron?

Yes. It actually generates a hole-electron pair, but usually only one of the two is counted. I tried to think of examples where the initial event was something other than generating a free electron, but I couldn't think of any. But that doesn't mean there aren't any.

 

[jeremyfiennes]

Thanks. What is interesting me is an operational definition of a 'photon-for-us' as "The light energy that displaces a bound electron". Creating an electron-hole pair being essentially the same (?). If you do think of any other ways, please advise as that would invalidate this definition.

 

[PeterDonis]

Am I right in thinking that all photon detection methods depend on a photon displacing an electron

It depends on what you mean by "displacing".

In a CCD detector, as @phyzguy noted, an incoming photon creates a hole-electron pair. That means an electron is "displaced" by being moved from a bound state (bound to a specific atom) to a free state.

In a radio antenna, however (which @phyzguy also referred to), incoming photons (better viewed as an incoming EM field) "displace" electrons that are already in a free state (more precisely, in the conduction band of a metal, not bound to any specific atom), causing them to wiggle around and create a current.

These are two different processes, even though both could be described using the word "displace".

 

[f95toli]

Summary:: Involves dispacing an electron?

Am I right in thinking that all photon detection methods depend on a photon displacing an electron, that then displaces other electrons to give a detectable electric current pulse?

No, at least for microwave photons you can "trap" them in a resonator/cavity and then detect their presence using the dispersive shift of a qubit.
Whether or not you consider"trapping" to involve electrons is up to you;formally it is still an excitation of EM field but of course you also have also induced currents in the walls of the cavity.

 

[jeremyfiennes]

Thanks again. I'm mainly interested in the single-photon case, which doesn't apply to radio antennae. I agree that here a field (wave) approach is better.
A better single-photon operational definition would then be "The light energy sufficient to cause an electron to move from a bound to an unbound (free) state". Which with an applied potential can be amplified (multiplied) to a detectable level .
This then means that a detector material with an electron binding energy E can only detect photons with a frequency f=E/h or greater. Is that correct?

 

[tech99]

As far as I know, a photon can be created by accelerating any charge. So for instance a proton would do it. And the converse must surely apply - a photon would set a proton into motion, or at least apply a force to it.

 

[jeremyfiennes]

Ok. But doesn't photon detection normally use electron displacement?

 

[DaveE]

Ok. But doesn't photon detection normally use electron displacement?

yes

 

[DaveE]

I suppose you could detect photons with many other particles: Positrons, protons, muons, even neutrinos or other photons. But you probably wouldn't want to, you know, unless you're showing off, or were trying to get tenure 70 years ago.

 

[f95toli]

A better single-photon operational definition would then be "The light energy sufficient to cause an electron to move from a bound to an unbound (free) state". Which with an applied potential can be amplified (multiplied) to a detectable level .

I don't see how this would be useful. It is an "operational definition" which is only applicable in some specific cases.
Also, in most situations the electrons would not become "free"; typically the electron would -for detectors that operate this way- still stay within the material meaning that when it is absorbed it is kicked into some higher energy state; it would not become "free".

Note also that in superconducting single photon detectors (which are very widely used) the effect of the photon to break more or one Cooper pairs thereby generating lots of quasiparticles and phonons; , in this situation it is not very useful to think of single electrons moving between states.

 

[jeremyfiennes]

Thanks. My basic question is: when a photon detector gives a "click", what actually happens inside it? My idea was that in some kind of way an electron is liberated. And that this then liberates other electrons, that end up producing a current pulse sufficient to give the "click". It is this idea that I am wanting to have either confirmed or refuted.

 

[f95toli]

There is no single answer to this since there are many different detectors. The way the superconducting detectors I referred to above (see e.g. https://singlequantum.com/) is quite different from the ways SPADs work (see e.g. http://www.micro-photon-devices.com/Products/SPAD-by-Technology) .
That said, I don't know of any efficient detector that directly use the photoelectric effect which seems to be what you are describing.
Even in SPADs I don't believe you can think of "liberated" electrons, you have photo-generated carriers but these are not "free".

 

[jeremyfiennes]

Thanks. I looked at the refs, but still couldn't suss out exactly how they work, what physical process initiates the detection procedure. Would "An incident photon creates an electron-hole pair, that then ..." be correct

 

[Klystron]

For the sake of completeness, at the molecular level the dark adapted human eye can detect individual photons via modification of proteins in the photoreceptors.

 

[jeremyfiennes]

Interesting. But how does that then result in an electrical impulse being sent to the visual cortex of the brain, which is the basis of all perception?

 

[Klystron]

For the sake of completeness, at the molecular level the dark adapted human eye can detect individual photons via modification of proteins in the photoreceptors.

Interesting. But how does that then result in an electrical impulse being sent to the visual cortex of the brain, which is the basis of all perception?

Also an interesting question; though outside my areas of expertise.

Some of our "Other Science" scientists (@BillTre) appear expert in biological vision and light detection systems including bioluminescence. Consider framing a specific question and choosing an appropriate forum to improve your knowledge of biological and chemical light detection methods.

I know a little about light detection and color generation in certain freshwater fish as a hobbyist. I understand that cephalopod eyes may be close analogues to primate vision; a simpler nervous system along with high intelligence in many cephalopod species permitting detailed study.

 

[hutchphd]

My idea was that in some kind of way an electron is liberated. And that this then liberates other electrons, that end up producing a current pulse sufficient to give the "click"

This seems to me true in the granddaddy of them all: the photomultiplier tube with cascading dynodes. There are now lower noise solutions but the PMT can count individual quanta.

 

[BillTre]

Interesting. But how does that then result in an electrical impulse being sent to the visual cortex of the brain, which is the basis of all perception?

It occurs under special conditions.
Being visually dark adapted makes the whole visual system more sensitive to low light levels.

There are several amplification steps involved in getting a signal indicative of absorbing a single photon to the brain:
Some amplification occurs in the receptor cell, some occurs as the signal passes though the retina, and in other neural areas before it reached optical brain areas.

wiki description of phototransduction in a rod receptor (low light level, B&W discrimination) cell using rhodopsin:

Unlike most sensory receptor cells, photoreceptors actually become hyperpolarized when stimulated; and conversely are depolarized when not stimulated. This means that glutamate is released continuously when the cell is unstimulated, and stimulus causes release to stop. In the dark, cells have a relatively high concentration of cyclic guanosine 3'-5' monophosphate (cGMP), which opens cGMP-gated ion channels. These channels are nonspecific, allowing movement of both sodium and calcium ions when open. The movement of these positively charged ions into the cell (driven by their respective electrochemical gradient) depolarizes the membrane, and leads to the release of the neurotransmitter glutamate.

When light hits a photoreceptive pigment within the photoreceptor cell, the pigment changes shape. The pigment, called iodopsin or rhodopsin, consists of large proteins called opsin (situated in the plasma membrane), attached to a covalently bound prosthetic group: an organic molecule called retinal (a derivative of vitamin A). The retinal exists in the 11-cis-retinal form when in the dark, and stimulation by light causes its structure to change to all-trans-retinal. This structural change causes opsin (a G protein-coupled receptor) to activate its G protein transducin, which leads to the activation of cGMP phosphodiesterase, which breaks cGMP down into 5'-GMP. Reduction in cGMP allows the ion channels to close, preventing the influx of positive ions, hyperpolarizing the cell, and stopping the release of neurotransmitters.[16] The entire process by which light initiates a sensory response is called visual phototransduction.

There is a lot of potential for amplification in those steps.

Changes in photoreceptor neurotransmitter release can evoke responses in the retinal cells (bipolar cells, amacrine cells, horizontal cells).
These signals eventually get to the ganglion cells (the output cells of the retina) and travel down their axons to their first contacts in the brain.
The signal has to pass through other cells before the signal gets to optic cortex.

Neuronal mechanisms can amplify signs as they are propagated through these cell populations.

 

[jeremyfiennes]

Thanks. Very interesting and informative. The "signal" is however an electric potential. And this is always due to electrons being in some way 'displaced' from their initial states?

 

[BillTre]

The "signal" is however an electric potential. And this is always due to electrons being in some way 'displaced' from their initial states?

The charges moving in electrophysiological phenomena are almost exclusively ions (atoms which have either gained or lost one or more electrons, thus having a charge).
Once the electrons are lost, the atom (your electron displacement) is a charged ion which can move around (mostly through membranes from the inside to the outside of the cell (or visa-versa).

This is somewhat reminiscent of how old Ben Franklin (unaware of electrons) had to decide to how explain current flow in some way and chose the (usually) wrong way, moving of positive charges not negative charges).
A better example of biological electron movement is the ETC (Electron Transport Chain) which powers much of life.

 

[jeremyfiennes]

Thanks. The depth of this is now way beyond my original query, but thanks and I have learned a lot of new things.