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Q & A: circular dichroism

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Most recent answer: 02/20/2008
Q:
My question relate to materials which possess the property of circular dichroism. Iím not thinking of the type of filters which combine a linear polarizer with a ľ wave retarder plate, since despite their output they are actually selecting for plane-polarized waves. Iím interested in the sorts of substances (for example certain biological molecules) that preferentially absorb circularly-polarized waves of one handedness over the other. So here goes! 1) Do these substances absorb any plane-polarized waves that pass through them, and if so does the degree of absorption vary with respect to the orientation of the plane about the axis of propagation? 2) The only circularly dichroic substances that I have read about are the afore-mentioned biological molecules, suspended in solution. I understand that with these substances the dichroic effect is very weak Ė in the order of one part in a hundred-thousand (I donít know how accurate that is). Are there any solid circularly-dichroic materials? And if so, is the effect any more pronounced?
- Matthew Hoey (age 40)
Brisbane, Australia
A:
Let me start with the part where I know the answer offhand. Later we'll update when we find more complete information.
1) Yes, all these substance absorb linearly-polarized light. Every linearly-polarized wave can be described as a sum of two equal-magnitude waves with opposite circular polarization. So even if you have a 100% circular dichroic (CD) absorber, it'll still absorb that part of the linearly polarized wave with one of the circular orientations. Of course what's left is no longer purely linearly polarized.
2) Yes, there are solid dichroic materials. Lots of crystals (especially ones of biological materials, but not just those) show optical rotatory dispersion (ORD) in the visible- they rotate plane waves. Sugar is an example. I think it was Pasteur who noticed that synthetic sugar crystals had ORD, some of one sign and some of the other. Now there's a deep mathematical relation (Kramers-Kronig) that says that ORD always must be accompanied by CD at some frequency. If we find some data on what solids have particularly high CD we'll update this,

Mike W.

(published on 02/20/2008)

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