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Q & A: Faraday effect materials

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Most recent answer: 05/18/2011
Q:
Do you know where I can get a crystal of TGG, a rod of Terbium doped Borosilicate, or a rod of some other type of glass that has a high Verdet constant? I am a senior in high school and want to build a Faraday rotator as a project. My budget is between $100 and $200. All the places I have asked have sent me quotes for like $4000, which is insane. I want to learn about the physics behind the phenomenon as well.
- Dylan (age 17)
NY, USA
A:
That's a sophisticated project. The quick answer is no, we don't know where to get a great price on these things.

Maybe a little idea connected with the physics might help. The Faraday effect is the rotation of the polarization plane of polarized light caused by a magnetic field in some materials. It's distinct from the optical rotation exhibited by some materials in the absence of a field. The Faraday effect does not reverse sign when the light goes the opposite direction through the material, unlike standard optical rotations. This is ultimately because magnetic fields break time-reversal symmetry.

So of what use is that blather to you? Since the Faraday rotation adds up as light goes back and forth through a material, you might be able to work with a thin slice of one of your favorite materials (like TGG), put mirrors on it, and have the light make multiple passes back and forth to amplify the effect. (This obviously wouldn't work for non-magnetic rotation.) The light would enter the slab at a slight angle just outside the mirrors, bounce back and forth a few times between the mirrors, then escape on the other side of the mirrors. This mirror geometry geometry would make it hard to use the rotator except on a skinny beam of light, probably obtained from a laser.

The best way to make the mirrors would probably be by depositing metal films (maybe aluminum) on part of each side of the slab, if you have access to an evaporator. Otherwise you could just use good mirrors and maybe use a drop of optical oil between the Faraday slab and the mirrors to reduce reflections.

So maybe, if you contact the TGG supplier, explain that you're a high-school student on a very limited budget, describe the project, perhaps they'll give you a break on a few-mm thick slice of TGG or other material.

Good luck!

Mike W.

(published on 12/18/2010)

Follow-Up #1: Faraday effect

Q:
Ok, I finally found a suitable glass rod, I think: Do you know if this will work with no mirrors, and with two 1" x 1" x 1/2" neodymium magnets that each can pull 25 pounds? If this isn't strong enough, do I need to wind a massive electromagnet that plugs straight into 120V AC? This glass rod is called SF-59, and has a Verdet constant for 650 nm light of 23 rad/Tm. I can get my hands on a 532nm or 405nm laser to use. Can you explain the calculations and the concept? I am currently taking AP Physics C as my first physics course. Thank you very much!
- Dylan (age 17)
NY, USA
A:
Dylan-

I've removed the commercial link from your question, in part because I believe that the boss of that company is an old friend and we don't want any conflict of interest here. Let me make a crude estimate. Your magnets probably give a surface field of several thousand Gauss, say 0.2 Tesla. I don't know if the field is parallel to the large surface, which would be most convenient.  Say that is maintained for 5 cm total. With that Verdet constant, you get about 0.01 T-m *23 rad/T-m. Maybe about 0.2 rad. That's a modest but easily measurable rotation. (My guess is it should be easy to find a cheap red laser to use, if you've got a polarizer sheet for it.)

Hooking up a big electromagnet to an ac source requires major safety precautions, both to avoid shocks and to make sure it doesn't overheat.  Also, if you us ac the Faraday rotation, which depends on the sign of the field, will also be alternating. If you have a nice photodetector, amplifier, and scope, ac rotation can be even easier to detect than dc. However, if you're just looking by eye, dc is probably easier to work with.

My guess: If you've got a couple of nice crossed polarizers, you should be able to see the Faraday effect with that $100 rod and your magnets. Getting quantitative about it will be harder. The guys at that company can probably  give some more detailed advice.

Mike W.

(published on 12/21/2010)

Follow-Up #2: Faraday effect magnets

Q:
Are two of these magnets suitable? http://www.kjmagnetics.com/proddetail.asp?prod=BX0X08-N52&cat=168 I am figuring that I can use them like this by placing them flat on a steel plate with a wooden spacer to keep them apart. I could then attach the glass rod directly on top of the magnets.
- Dylan (age 17)
NY, USA
A:
Those are nice powerful magnets. Notice that the field points at right angles to the big surface, if I understand the specs right.  You want to make sure that you can arrange for the fields from the two to add up and also to be approximately parallel to the direction of light transmission through the special rod. I'm not sure I understand what arrangement you're suggesting, but that point about the orientation of the field with respect to the direction of propagation is crucial. Fortunately the specs show a nice picture of the magnetic field lines, which will help your design.

Mike W.

(published on 01/23/2011)

Follow-Up #3: completed Faraday rotator

Q:
Thank you so much for your help! here is the finished product: http://www.instructables.com/id/Faraday-Rotation-EPIC/
- Dylan (age 18)
NY
A:
Very cool. Those instructions you wrote look really clear.

Mike W.

p.s.- In the links you provide, you might include us. The reason I'm being pushy about that is, after looking over some other science Q&A sites, we think that people would get more reliable info here. Therefore we're trying to advertise some.

(published on 05/18/2011)

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