Yvonne Carts-Powell

News of the quantum weird

In Uncategorized on September 5, 2008 at 10:23 am

I’m writing a magazine story right now for Optics & Photonics News, and the deeper I delve into the topic, the weirder it gets. And by “it” I mean reality on very small scales.

It can make the group velocity go faster than the speed of light

So there’s these things called surface plasmon-polaritons. When you have a surface of, say, a metal, in air or vacuum or another dielectric, and light hits the surface… weird things can happen.

For example, suppose there’s a hole the size of your fist in a metal plate. Light shines on the metal plate and gets blocked, while some of the light goes through the hole and off to the other side. The amount of light that gets through is related to the size of the hole: if you have a big hole, you have lots of light; small hole and only a little light gets through — all very Newtonian, very commonsensical. Now suppose you shrink the hole, down to the size of a pinhole: then only a tiny bit of light gets through. (Shrink it some more, to roughly the size of the wavelength, and you start seeing effects from diffraction and quantum effects: you can no longer treat light like a continuous stream but have to take into account it’s either particle or wave nature. And one of these days, someone is going to come up with a better analogy for light than that.)

But that’s a conversational tangent. What I meant to talk about was how, if you have a pinhole in a metal plate, only a tiny bit of light gets through. Except… more light gets through the plate than one can account for based on the size of the hole. The surface around the hole grabs some of the light and funnels it through the hole!

What’s happening is that the photons (ie, electromagnetic waves) hit the surface, and some of them turn into surface plasmon-polaritons. Part of the energy from the photon is inside the metal, where it moves as charge-density waves and part of it floats above the surface in the dielectric. The wave disperses and disappears pretty quickly, but on short scales, it can have effects — like traveling along the surface and through a pinhole before turning back into a photon that comes free of the surface and zooms off into the dielectric again, continuing with it’s photon-y business.

This is weird. But it’s not the part that’s bending my brain today.

Some clever folks realized that a very thin metal line would be more interesting than a big metal sheet, for playing with plasmons. Among other things, “plasmonics” provide a new way of manipulating light. (There’s a PDF-format term paper about plasmonics by Luis Prill Sempere at http://www.ph.utexas.edu/~niugroup/nano/Sempere.Luis.pdf . It’s a little dated, but not too badly.) One really interesting idea is to not use a continuous metal line, but create a lot of tiny (nanoscale) spheres of metal placed pretty closely together. This provides an even better way to control the wave.

This is where it gets odd: So these guys at University of Pennsylvania wondered what would happen if they didn’t use spheres? What if they squashed them a bit so they were more ovalish (“oblate spheriods” is their term.) They fired up the computers and ran simulations… and came up with some pretty darn weird behavior. Remember how I said that the wave disperses quickly? With their setup, the rate of dispersion depends on how deformed the spheres are. And that’s not all: if you design your line of nanodots just right, you can tune the characteristics of the surface-plasmon-polariton, including the speed of the group velocity. It can make the group velocity go faster than the speed of light in the dielectric.

This isn’t claiming that Einstein was wrong, or that the speed of light isn’t the upper limit on how fast matter and information can move. Group velocity is not the speed of a thing — it’s the speed of a change. And changes can move faster than light — we’ve seen that experimentally.

Still… it’s very odd.  I’m slowly wrapping my brain around this and feeling the need to quote Dave Barry: “I am not making this up!”

And I’m trying to figure out how I even begin to explain this in less than 400 words that the publication allows me. Welcome to the glamorous life of a science writer eh?


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