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



thanks -- this gives me further ideas for demonstrations. I already
have artificial optical activity using Cu wire.

bc, however, still awaits answer(s).

Brian Whatcott wrote:

I was very encouraged to spot this URL:

<http://www.enzim.hu/~szia/cddemo/edemo14.htm>

By Andr·s Szil·gyi (szia (at) enzim (dot) hu)

This has some exceptionally worthy graphical depictions
of the various effects.


Elswhere, [to answer my own trirefringence suggestion, in part]
I see that pleochroism is described as a 3 color effect.

The allotrope of carbon to which I referred earlier lost a letter:
it is better spelled 'ceraphite' by the way.

Regards

Brian W


At 12:56 AM 4/14/2005, you wrote:

I'd never read or heard of the third definition. [Thanks Bryan] I, of
necessity, differentiated between the first two. (WRT polarization, not
frequency)

I repeat, w/ a variation, my question about producing circular light
using a dichroic plate. Can it be done w/ the incident light exactly at
45 deg. wrt the optical axis of the xtal? [reason next paragraph]
Intuitively by using other than 45 deg. one compensates for the
dichroism. My source claimed the need to use other than 45 deg. was
because the microwave optical axis is not quite the same as determined
visually. If my supposition is correct, it merits a note to the
journal. Being deficient in maths, I'd find several xtals w/ differing
dichroic ratios and measure the incident angles to achieve circular
light. (Another sci. fair exercise?)

I found my technique was deficient. W/ more care and lotsa time, I
achieved an e of ~ 0.3 instead of the 0.6 below. To do this I had to
use an angle of ~ 64 deg. (It is critical, and my goniometer is very
crude! *)

BTW the xtal is very non uniform. I first tried using a "point"
detector. Its position was very critical and I couldn't adjust the
correct thickness of the plate to achieve reasonable circularity. A
small horn was better, and w/ a larger one I was reasonably successful.
I presume this is because it "averaged" the xtal.

* Crude also in the sense that the detector axis is not collinear w/ the
microwaves pencil. I think this caused error by using differing parts
of the xtal. That's why rochon prisms are superior to most polarizers.

I've thought some more about the relationship of dichroism and
birefringence. According to the classical theory refringence is caused
by absorbing oscillators. Birefringence, by two with differing
frequency at some angle. I'll bet one could make a polaroid 1/4 wave
plate plate at the blue end where there is extinction failure. Of
course Bryan is correct in the vis. as there is complete extinction (A >
3) w/ one polarization. OTOH not. At the thickness necessary for a
vis. 1/4 wave plate, there may not be nearly total absorption. This may
be a good science fair exercise.

bc

p.s. there wasn't any response 'till BW's (at least in my inbox.


Brian Whatcott wrote:


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!




Brian Whatcott Altus OK Eureka!

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