Monday, April 18, 2005

Slow Light

Phew. It's finally finished.

I gave this talk for my advisor about "slow light". Because the experiment we recently finished involved slowing light down, and he wanted me to learn about how that worked, and then prove to him I understood it. (Of course, light slows down when it travels through any medium, including air, water, glass, etc. It's why lenses work. But we slowed it down a lot more.) When and if that data ever gets published, I'll add a couple of slides to my talk showing that this theory actually describes reality. I'd also like to clean up my references, and make sure the solution I got for the differential equation I used actually matches the one I found on-line. (But I know it will.)

And, for the benefit of those who haven't studied differential equations, I'd like to write a layman's-terms version which introduces the ideas of absorption and energy levels and electromagnetic waves, and answer everyone's favorite question, "What's it good for?"

But I'm going to put off doing those things for a while, 'cause I'm tired of working on it, to tell you the truth. Anyway I think what I've got, as it stands, could be useful to some people... Undergrads maybe, or people like me, starting out in optics research. So I'm going to make it available on the internet, and anyone who isn't interested in physics, but is curious about what I do at work all day (hi, Mom) can take a look too.

Ken worked with me on every step of this, coming up with answers to my questions, asking me questions that forced me to clarify my ideas, teaching me what he knew and learning with me. So really this is a joint effort (though any errors, of course, are my own). Every scientist should be so lucky.

Here it is, as a PDF:
An Introduction to Slow Light

(n.b. They took away our webpages! I'll upload this somewhere else, when I get around to it.)


Andy said...

Hey Mary~

I would appreciate the layman's version... this topic, like quantum computing, has fascinated me every since I read about it. Say hey at some point -- I have a bunch of things to talk with you about.


Ashi said...

*rubs his chin, perpliexed* I know more about this than the average chemist and I certainly can't follow enough of that slideshow:P But I get the impression that you're not actually slowing down the light and that the lagging phase shift is a result of different timescales between adsorption and fluorescence for different frequencies of light in that medium?...


Anonymous said...

I'm still double checking your equations. Not.


Mary said...

I guess it's fair to say that we're "not actually slowing down the light." After all, my first sentence was "Light travels at c. Period."

But we're slowing it down in the only sense anyone ever can -- by sort of interfering it with a time-delayed version of itself, so that the various peaks and pulses of the signal come out later than they would if they medium weren't there.

Yes, if you want to think of it that way, you can say that the "timescale between absorption and fluorescence" is the reason for that time-delayed wave. But I prefer to think about it in terms of intereferences.

Ashi said...

I guess it's not actually absorbance and flourescence from the way you put it. Is it as a result of the light interacting with the time-delayed light (which is a result of the medium), or is it directly as a result from interference from the medium?

Sorry if I am asking stupid questions, I do find it interesting

Mary said...

The medium produces a wave which is usually a copy (but not a perfect copy) of the wave it absorbed. That's the key point.

Details: Technically this is called "stimulated emission" rather than fluorescense. Fluorescense is more random. I think in the real world both are always going on at the same time, but I mostly ignore the fluorescence for purposes of this talk. It's sort of like friction -- it dissipates energy, keeps all these resonances from blowing up to infinity -- appears in the harmonic oscillator models as "damping."

But if by "fluorescense" you just mean "light emitted by the atoms"... Well, the time delay between the incident wave and the copy-wave emitted by the medium is *equivalent* to the time delay between the absorption of a photon and its re-emission. Either picture, captured photons or interfering waves, is valid. Both predict the correct results. As a matter of personal taste, I prefer the wave picture.