[Phys-L] Re: dichroism, birefringency, and circularly polarized EM rad. question

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

At 05:05 PM 4/8/2005, Bernard Cleyet wrote:
> I'm a bit embarrassed to ask this question as elliptical polarization
> was part of my disst.  But it was ~ 35 years ago.
>
>
> How can one obtain circularly polarized light from a birefringent xtal
> that is also rather (T ratio > 2) dichroic (WRT polarization plane) from
> plane polarized light. I didn't find the answer from a perusal of Hecht.
>
>
> The best I've obtained w/ my xtal so far is e * ~ 0.6  (X band
> radiation) could it be just a rather "poor" (non uniform **) xtal
>
> * e = sqrt[1 - b^2/a^2]
>
>
> Peripheral question.  Am I correct in thinking a dichroic (WRT to
> polarization) xtal is always birefringent?
>
> bc


bc's note attracted no qualified public responses.
I see my opportunity to offer herewith, an
unqualified response.

Dichroism is an unfortunate term. It has three disparate meanings.

1) the intuitive one: a material that processes two different light
  frequencies [colors] differently.     The usual process is to transmit
one frequency and reflect another.    This is readily understood as a
  band-pass, high-pass, or low-pass filter. A current commercial
offering is a thin dielectric multi-coating on the surface of
  a "dichroic" glass sheet: iridescent colors are to be had  on a glass sheet
  of 16 sq ins area: $100 to $150  (!)

2) A material that processes a light beam of two different
linear polarizations differently. Early scientific examinations of the
  property used materials that had steeply frequency [color]  dependent
  properties along with the polarization anisotropy, which led to this
unfortunate labeling.

3) A material that can distinguish circular polarizations of
opposite handedness


bc's questions:
Q - How to obtain circularly polarized  beams from a plane-polarized beam?

A - The stock response is to mention a wave plate - so that a 1/4 wave delay
provides the desired rotation.

Q - Is a dichroic (type 2) crystal always birefringent?

A - If the crystal in question can act like a Polaroid sheet, then the answer
is obviously 'no'

Peripheral pondering of my own.....
if birefringent materials can be uniaxial or biaxial,
due to their anisotropic dielectric qualities [described in terms of a
'rank 2 tensor' aka electric susceptibility tensor] the 3X3 matrix involved
would seem to permit triaxially anisotropic materials -
  i.e trirefringent dielectrics - but the literature I casually scanned
  does not seem to mention this flavor of optically active crystal.
The decorative quartz minerals onyx, chalcedony, agate,  and eleven others,
the white carbon allotrope called craphite, and calcite are merely bi-

One could meander on with Kerr and Pockels, but one won't.

However, on the topic of fast optical shutters a la Kerr which rely on
  electrically induced rotation of the polarization plane to allow 100nS
shutters,    I see that New Scientist is carrying news of a rather faster
  'shutter' or should it be called rather an optical sampling oscilloscope?

This little beauty can visualize a light pulse consisting of two cycles,
in the visible.   I will scan a picture when I have a moment, for your
  amusement.


Brian Whatcott    Altus OK    Eureka!
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