5
$\begingroup$

In general, if you shine some EM radiation through an arbitrary polarizer which works for visible light, it's not guaranteed to polarize your EM radiation. E.g. you can take polarizing sunglasses and shine unpolarized X rays or radio waves through it, and this radiation most likely go through almost unaffected.

But I've never heard of any term which would refer to polarizer's rejection spectrum – i.e. how much of the "wrongly" polarized radiation of given wavelength it blocks. Is there any such term? What is it?

$\endgroup$
2
$\begingroup$

The quantity I was interested in is called polarization extinction ratio. See the usage of this term e.g. in the description of this wire grid polarizer, like in the following graph taken from that page's Graphs section:

Example of the usage of the term

$\endgroup$
  • $\begingroup$ Indeed. As with all questions of this nature, the best approach is to go to a supplier and ask for the specs of the product. $\endgroup$ – Emilio Pisanty Nov 8 '18 at 12:37
  • $\begingroup$ @EmilioPisanty but not necessarily trust those specs without a good, independent background and understanding of the subject. Supplier specs are sometimes generated by, or at least edited by sales & marketing. It shouldn't happen, but it does. $\endgroup$ – uhoh Nov 8 '18 at 15:18
  • $\begingroup$ @uhoh I'll kindly thank you very much not to put words in my mouth, particularly if you then want to quote the CoC at others. $\endgroup$ – Emilio Pisanty Nov 8 '18 at 15:19
  • 1
    $\begingroup$ @uhoh the question was indeed purely about terminology. Veracity of vendor-supplied specs is quite off topic in this thread. $\endgroup$ – Ruslan Nov 8 '18 at 15:24
  • $\begingroup$ @Ruslan I agree. I found examples of the term you first mentioned in peer-reviewed literature, rather than a vendor-supplied source. I only mentioned vendors in the answer in a cautionary tale about why you should not trust terminology used by a vendor. I'm happy to see that you found "extinction ratio" is the term that you were looking for! $\endgroup$ – uhoh Nov 8 '18 at 15:29
0
$\begingroup$

Is there any such term?

Yes indeed there is "anything like a polarizer rejection spectrum"!

A search in google scholar turns up papers, though they tend to be paywalled. Here's just one example from 2018: All-fiber circular polarization filter realized by using helical long-period fiber gratings "...we propose and demonstrate a novel method for the fabrication of a HLPG with a flat-top band-rejection spectrum, which is...""

Skip to the end for the direct answer, here is some background leading up to it. I feel it is pretty essential to read for people working with polarizers.

In optics, it is dangerous to trust sales terminology and spec-writing at face value; for example you can go to your favorite optics catalog and see "non-polarizing beam splitters" that polarize by 10 or 20% or more. They just mean that they aren't the polarizing beam splitters found on the next page. So I will address the physics of the question and leave the sales aspect to the sales reps.

"non-polarizing" beam splitter

Source: Thorlabs

The reason the other two answers don't really explain the term or deny that it could have meaning is that they are thinking of the most common kind of every-day polarizer, based on absorption. Traditionally they were made from stretched organic chains (plastic or gel) laced with iodine atoms that provided the electrons that interacted with the incident light in a directional way; light polarized parallel to the iodine-laced polymers was absorbed, and never seen again.

Absorptive Polarizer Source: hyperphysics

However, there are also other ways to use prism combinations as polarizers.

There are several very different kinds of polarizers based on pairs of prisms. You can read more about them in Wikipedia or this nice tutorial purewavepolarizers. No matter which kind, they have two outputs, one for each polarization. Generally these polarizers have very low absorption in their useful range, so whatever light is rejected from one exit direction is present in the other direction.

enter image description here Source: hyperphysics

There are also Wire Grid Polarizers made from long lines of metal (wire, or nanofabricated grids) with a sub-wavelength period that also efficiently reflect one polarization while transmitting the other. While these can be a bit lossy since the wires are sub-wavelength wires or nano-fabricated lines of metal, the transmitted and reflected light is often less than the incident light, so the spectrum of light from one output would be less representative of the rejected spectrum from the other.

Wire Grid Polarizer

Source: Quora NOTE: The diagram is misleading, with non-normal incidence on this oriented surface, you have the possibility that the plane of incidence does not contain the direction of the lines, and so the simple concepts of s- and p-polarization don't apply. Take this, and all drawings of wire grid polarizers with a grain of salt!

There is a nice summary of the "family of polarizers" in this tutorial: https://pe2bz.philpem.me.uk/Lights/-%20Laser/Info-902-LaserCourse/c06-10/mod06_10.htm

Answer:

Optics is such a broad field and widely written about. The phenomenon of linear polarization was noted directly about two centuries ago, effects due to polarization were noted even earlier.

A rejection spectrum can be either

  1. Simply the incident spectrum minus the transmitted spectrum, literally the light that was rejected, or not passed out one particular output of a polarizer (most polarizers have two outputs, only the familliar absorptive polarizer has only one)

  2. The output spectrum from the "other output", the output not of interest, for a prism or wire-grid polarizer. This is a less precise measure, but if you don't have the facilities to easily subtract two spectra (remember this is hundreds of years old) then measuring the other output is a good indicator and lower limit to what was rejected from the output of interest.

$\endgroup$
  • 1
    $\begingroup$ This is nowhere close to an answer to the question as posed. $\endgroup$ – Emilio Pisanty Nov 8 '18 at 12:39
  • $\begingroup$ @EmilioPisanty I disagree. To the question "Is there anything like a 'polarizer rejection spectrum'?" I've replied "yes" and explained what it can be. I've also illustrated why the other two "no there isn't" answers are not necessarily correct. I think your "...is nowhere close to.." is one of those down-vote strange attractors that isn't particularly helpful. Can you write something more constructive? Do you disagree that the answer would be the reflected or redirected component from a wire-grid or prism polarizer? If so, in what way? If the last sentence isn't clear, I can rewrite it. $\endgroup$ – uhoh Nov 8 '18 at 13:56
  • $\begingroup$ The question is a specific request for terminology, which this answer does not provide. Instead it is a barely-coherent rambling about how polarizers work, together with screenshots of text and other irrelevant material. You're welcome to disagree but I'm keeping that downvote and my initial characterization; if it acts as a "down-vote attractor" then that's a natural consequence of the answer being bad. I'm uninterested in additional discussion absent fixes to the answer. $\endgroup$ – Emilio Pisanty Nov 8 '18 at 14:03
  • $\begingroup$ I thought that manner of speaking to others had mostly dissipated here. Okay I will take a few minutes and eliminate the text that appears within the images. I don't recall images which also happened to contain text were off-limits, I think you are stretching that point thin. btw existence of text does not cause the answer to not be an answer. $\endgroup$ – uhoh Nov 8 '18 at 14:08
  • $\begingroup$ @EmilioPisanty done. $\endgroup$ – uhoh Nov 8 '18 at 14:47
-2
$\begingroup$

There wouldn't actually be a rejection spectrum because photons either make it through or they don't. The ones that do not are absorbed. The ones that do are rotated parallel with the slit as they go through. You could imagine a rejection spectrum to be the opposite of what actually happens. There would be photons of all polarization's contributing but more from the ones originally polarized perpendicular than the ones parallel.

$\endgroup$
  • 1
    $\begingroup$ First, they don't have to be absorbed: they can be simply reflected. Second, there would be a rejection spectrum – a function which would describe which part of all the photons polarized in the unwanted direction happen to come through, depending on the wavelength. I can't imagine why such a function could not exist. $\endgroup$ – Ruslan May 8 '18 at 17:02
  • $\begingroup$ Reflection is a photon being absorbed and then a new one being emitted. As for your second comment I kinda agree but as I said there wouldn't actually be a spectrum. Just a calculation which would obviously be the opposite of what we see. $\endgroup$ – Bill Alsept May 8 '18 at 17:08
  • 1
    $\begingroup$ @BillAlsept "Reflection is a photon being absorbed and then a new one being emitted." In your opinion. Also a spectrum is just a function so there would be but that's just semantics. $\endgroup$ – MJC May 10 '18 at 9:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.