Manipulating light at the nanoscale is the big topic in optics these days. Nanoscopic materials are exciting because they're smaller than many of the wavelengths of light we typically work with. If light is squeezed into volumes that are much smaller than its wavelength, then the intensity of the light becomes very large. And, just like smacking a sleeping lion on the nose, lots of exciting things happen in a very short time when the light is bright enough.

One of the ways to generate very bright, but very localized spots of light is through the use of localized surface plasmon resonances. Unfortunately, with a few exceptions, the bright prospects of surface plasmon resonances have remained just that: prospects. In a fever of excited calculating, physicists have now discovered why the fields associated with surface plasmon resonances aren't always as bright as expected—cue disappointment. But, the best thing? These new findings will generate all sorts of new and exciting ideas.

Putting surface plasmons in a cage

Surface plasmon polaritons are moving waves that combine electron motion in a metal with the electric field of a light wave. Localized surface plasmon resonances differ in one respect: the plasmon doesn't travel anywhere when it is a resonance. If we excite a surface plasmon in a tiny gold bead, then the plasmon reflects off the sides and meets itself on the way back. The result is that it interferes with itself—everyone does—and doesn't move anywhere. Or, in more simple terms, because the plasmon has a component made up of electrons, it is confined by the boundary of the metal, so it rattles around inside its metal cage, unable to go anywhere

Typically, these gold beads are just 50nm wide, while the wavelength of light used to excite the plasmon might be as long as 700nm. But, as a plasmon, the wave is confined to the bead, meaning that the wavelength has been compressed, and the energy associated with the wave is also compressed, so, anything in the vicinity of the bead will experience an electric field that is much more intense.

This by itself is nice, but things get more interesting when we start adding more beads to the mix. When two beads are close to each other, the electrons in one bead tend to drag the electrons in the other around. At the right moments in time, the sides of the beads closest to each other are oppositely charged, and the field in the gap between the two is huge. If we stick something in the gap, it is going to react to all that light.

The quantum key to the jail

We have recently covered an example of what happens to electrons when intense electromagnetic fields are applied. In a more extended version of that work, a team of researchers from France, Spain, and the US, used calculations to determine the quantum behavior of the electrons as the field in the gap between two gold beads got stronger. They then used these results to determine what the light does as a result, creating a reasonably complete picture of the physics of a localized surface plasmon resonance.

The researchers found that as the field gets stronger—either by turning up the laser power or by moving the beads closer to each other—electrons begin to jump from one bead to the other. This reduces the difference in charge between the two beads and reduces the field between the two. The upshot is that as you attempt to turn up the field, the electrons act to turn the field down.

What does this do to the light? Now, if all were well and the electrons stayed confined to their respective beads, the field in the gap would exactly replicate the light field that was applied, but be a lot brighter. As the electrons tunnel from bead to bead, they change the field in the gap, so that it doesn't track the applied light field. The result is that the beads emit light colors that were not present in the original light field.

Think of it like this: you have a single nanoparticle of gold and shine a red laser on it. It glows red... a much brighter red than expected, but still red. Now, you move a second gold bead closer, and as the bead gets closer, the beads start to glow blue. The closer they get, the more blue they become, until they just barely touch, whereupon the blue disappears entirely.

In some respects this is a disappointing finding. Why? Because lots of people have been banking on the idea that you can stick molecules in the gap and do things to them. The idea being that sensors become more sensitive when the light field intensity is turned up. Or, if you require sufficient intensity to start a reaction, this gap might be a place to get things started without an expensive high power laser. What this tells us is that the intensity will top out, and the gain from placing something in the gap will not be as great as you might hope for.

There is, however, a big upside. All this tunneling behavior results in the light fields behaving in a nonlinear fashion. This tells us that it has the potential to be a great tool for various types of spectroscopy. In a more—or perhaps not—practical direction, optical computing requires the light to behave nonlinearly. Which, ordinarily, requires very intense light fields. Designing devices that enhance that nonlinearity is the only way to make optical computers feasible. Unfortunately, in the configurations investigated here, the fields are still too large, but, at least there may be hope.

Nano Letters, 2012, DOI: 10.1021/nl300269c