# Rotary Photon Drag Enhanced by a Slow-Light Medium. Right? Right.

Remember that scene in the Jody Foster movie called Contact when they got all of those drawings of “the machine?”  There was a part of the movie where Ellie realized that the images were encoded somehow, and the key to encoding them was by looking at them in three dimensions.  Remember that minute little detail?

I read an article on this just the other day, and after I read the entire article in the journal Science, I really want to share the gist of this thing with you all.  It totally reminds me of this for some reason.  I was explaining this all to a friend on Skype, and I got tired of typing, and then the researcher slice of my brain started going ape-sh**.  Pardon me.

First, read the abstract of the article written by Sonja Franke-Arnold (School of Physics and Astronomy (SUPA), University of Glasgow, Scotland), Graham Gibson and Robert W. Boyd (Department of Physics, University of Ottawa, Ottawa, Canada), and Miles J. Padgett (The Institute of Optics and Department of Physics and Astronomy, University of Rochester, Rochester, NY):

Transmission through a spinning window slightly rotates the polarization of the light, typically by a microradian. It has been predicted that the same mechanism should also rotate an image. Because this rotary photon drag has a contribution that is inversely proportional to the group velocity, the image rotation is expected to increase in a slow-light medium. Using a ruby window under conditions for coherent population oscillations, we induced an effective group index of about 1 million. The resulting rotation angle was large enough to be observed by the eye. This result shows that rotary photon drag applies to images as well as polarization. The possibility of switching between different rotation states may offer new opportunities for controlled image coding.

Ok, got it?  Yeah, read it a few times, but generally the concept of the experiment is pretty simple, and the results are very interesting!  What these folks were doing was shining a shaped, collimated beam of light through a spinning ruby disk rotating at a given speed – in this case a maximum of 30 cycles per second.  The ruby disk causes a bit of “drag” on the photons travelling through it, causing the light to refract and exhibit some interesting behavior.  Check out this little video, from the paper and from the journal Science:

to view the .MOV file, click here

Ruby has a heavy Index of Refraction, which means the light is slowed down (refracted) at a rate of X when it leaves the air and enters the ruby itself.  If you imagine the 1.0 value of the Index of Refraction as how light travels through regular ol’ air (and not taking into account humidity, pollution, or any of that schtuff), anything greater than 1.0 is refracting.  Diamond has an Index of Refraction of about 2.42, and Ruby’s Index of Refraction is about 1.77.  Ruby refracts less than diamond.  Make sense if you didn’t already get it?

Here’s the weird thing – Ruby is not what we consider isotropic – meaning that no matter what the incidence angle is and no matter what the orientation of the crystal is, the light travels through the crystalline matrix equally as it travels through the medium.  Glass, sodium chloride crystals, and a lot of polymers exhibit this kind of “perfect” structure.  Sodium chloride is basically a cubic structure, relatively perfectly bonded in a cube matrix.  Ruby, on the other hand, is an anisotropic crystalline structure, meaning that there are more than one axes that are different within the structure of the crystal matrix.

Here’s a good image of the difference between an isotropic and anisotropic crystal structure, optically, from Olympus America’s Microscopy Resource Center.  Figure A is a sodium chloride crystal, which is isotropic.  Figure B is a calcite crystal, which has calcium ions and carbonate ions in it.  Calcite is anisotropic.  Check it:

Ok – now if you think of a crystal structure with light shining through its matrix, and the light is going to pass through two different planes of refraction, essentially – what do you expect to happen to one beam of light as it enters the anisotropic crystal structure and slows down?

Who said it’s going to split into two beams?  (DJ Lemma, pout your hand down, I know you already know the answer!)  You’d be correct – the incident beam splits into two beams, each sort-of along that individual crystal plane.  Take a look at this image of a calcium carbonate crystal, and how it is creating a double image:

This phenomenon is called birefringence.  Deep breath – bi-re-frin-gence.  Ruby, the gem used in the experiment, is also an anisotropic crystal, and it exhibits traits of birefringence.

So, imagine taking that birefringent crystal disk, spinning it at a relatively high rate (30 Hz), and shining a very specific wavelength of light (ie, a laser), that is in a certain shape through the ruby disk as it spins.  A bunch of stuff was discovered with this experiment, all related to the image.  The generalities of the experiment, as I paraphrase, is that the group shone a very bright laser with a square-ish shape through the ruby disk, noted the position that the laser had ont he other side of the ruby disk after it was on the other side of the disk.  When you shine a shaped laser beam at 532 nanometers (green) through a spinning ruby disk (which is a very slow-light medium, slowing the light down to just a few tens of meters per second) spinning at a rate between not spinning and 30 rotations per second, the image refracts from about a third of a degree to about ten degrees as the ruby disk increases from slow revolutions per second to thirty revs per second.

What a crazy experiment, huh?  I needed a good dose of photonics and optics in my Thursday!

The ramifications of this experiment have to do with encoding images with extra data – if you can imagine an image that has more information in it depending on which way the image is spinning, that is some trippy Minority Report shizzyhizzle.  “Oh, you’ve stolen my image!  But since you don’t know which wavelength to use and at which speed to spin the image, you’ll never decode my super secret plans of world domination!!!

Yeah, I have a vivid imagination.

HUGE number of thanks:
the journal Science
PhysOrg
PhysicsWorld
the Index of Refraction of Ruby and Sapphire (actually a very cool fact doc, check it out!)