diffractive polarizers

[P3D Peter Abrahams <telscope@xxxxxxxxxx>]

I was very intrigued by the note from sci.optics, quoted by Greg Erker
<erker@xxxxxxxxxxxxxx>

>A diffraction grating functions as a polarizer, if the grating
>periodicity is smaller than the wavelentgh. If you think in terms of
>lines per mm, that's more than 2500 lines per mm in the visible
>region. Then the light does not "see" the grating and passes
>through/reflects without diffraction. Only the state of polarization
>is changed.

I couldn't find anything substantial on this subject on their web site
either, though it was inspiring to find such a concentration of talent in a
climate that makes Canada's look temperate.
There are a few paragraphs on diffractive polarizers in v2 of the OSA's
Handbook of Optics, p3.27 and 3.35. There is a whole family of these
optical elements, though I'm sure none of them are found outside high-tech
facilities.  Unfortunately, they indicate that these polars will not solve
our problems:
"Diffraction-type polarizers include diffraction gratings....that have
properties similar to those of dichroic polarizers except that they
transmit one component of polarization and reflect the other when the
wavelength of the radiation is much longer than the grating.....None of
these polarizers has as high a degree of polarization as prism polarizers."
Later, it is described how 80 percent of the light that is polarized
parallel to the grooves, and 5 percent of the perpendicular rays, will be
reflected off a very specialized grating, but only "in the zero order at a
50 degree angle of incidence" and for wavelengths of a very limited range.
The 80 percent sounds great, but 5 percent sounds high enough to give a
ghost image.  
Apparently, this field got some or all of its impetus because anomalies in
gratings can cause polarization, which reduce the efficiency of the grating. 
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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