20 February 2010

Gold finger: photonics galore

Chris Lee of Ars Technica provides an insightful analysis of a significant new capability in photonics. What impresses me most about this is how well the stage is set for further development and then industrial implementation through recombination with rapidly emerging industrial technologies. I've clipped a summary below and indicated one such synergy, but you should read the whole thing.

Making an optical switch by drawing lines in gold

...Normally, metals make for horrible nonlinear optics—in part because metals fail at transparency—but, they do have one advantage: lots of free electrons. If you shine light on the metallic surface in just the right way, then the electrons start to respond to the light field by oscillating in sympathy. This oscillation moves along the surface of the metal as something called a surface plasmon polariton—which is jargon for electrons that set up an oscillation that maintains a spatial orientation on the surface of the metal.

These plasmons travel at a much slower speed than light, have a much shorter wavelength, and are confined to the metal's surface. As a result, the electric fields associated with the charge oscillation are quite intense—intense enough, in fact, to drive nonlinear interactions. As a result, metals provide light with quite a good medium for things like four-wave mixing, provided you can get the light into the metal. Plasmons are made for this job because they are essentially light waves traveling along the surface of a metal.
[The researchers] ruled lines on the metal that were about 100nm in width...

(...a barn-door for nanoimprint lithography --S.J.)

and separated by around 300nm (center to center). These lines act to slow down the waves, with the delay depending on how close the plasmon wavelength is to the separation of the lines. This also controls the direction of the emission. Light only exits the structure at the point where the emission from each individual line is in phase with the rest, which depends on the spacing of the lines. But, not every color can find a spacing for which this occurs. In this case, the second of the two emitted waves can't find an emission angle that works for it at all, so the device emits only a single wavelength of light.
So, the end result is that, by ruling lines on the gold surface, you can choose which of the two colors you want generated and which direction it's emitted in. As an added bonus, the lines provide sharp points on the surface, which accumulate charge, resulting in very high electric fields (think of a lightening rod on a building). As a result, the four-wave mixing process becomes more efficient.
What's next? That's hard to say. I know that there are some ideas about how these nonlinear optical processes can be made more efficient, and maybe even useful, using plasmonic surfaces. So we may see some plasmonic optical switching devices. The big selling point in plasmonics is usually sensing, though, so things may go in that direction.

One thing that intrigues about this approach is how it leverages and manages those surface plasmon waves. Note the point Lee mentions about the plasmon waves' shorter-than-light wavelengths, necessitated by their comparatively slow velocity. Their short wavelengths would seem to be a useful property for probing and sensing phenomena and physics on the nanoscale, perhaps providing a new tool to bridge the region between light-based microscopies and electron microscopies, which are limited by the electron's Compton wavelength (on the order of 10^-12 m) in the same way that optics are diffraction-limited by photons' wavelengths. In particular, materials with even slower surface plasmon velocities but still generous free electron populations (which I'd imagine equates to "conductivity," though I'm rusty) would have shorter plasmon wavelengths still. Depending on how far down the process can be driven, things could get really interesting, and really weird.

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