The Nintendo 3DS notwithstanding, 3D technology still comes under the classification of cumbersome. The viewing angle is usually poor, or you need to wear special glasses. Neither of these options are particularly attractive. An alternative is to use holograms, but this comes with its own set of problems. For instance, the holograms on credit cards are visible from a wide range of angles... but, well, they aren't called rainbow holograms for nothing. Holograms with better color reproduction have a limited viewing angle. All in all, it is rather fraught.

Riding to the rescue are engineers bearing surface plasmon polaritons. By combing some fairly standard holography techniques along with some surface plasmon polariton tricks, the researchers have created holograms that have both a wide viewing angle and good color reproduction.

So, how did they do it? First, a short primer on surface plasmon polaritons, entities that are part light and part electric charge. When light is incident on a metallic surface, it begins to move charges back and forth. Normally, this will result in the light being reflected from the surface. But charge motion can also become collective and begin to transit along the surface of the metal. When this happens, the light is not reflected—it's absorbed into the metal as a surface plasmon polariton.

To excite a surface plasmon polariton, we must match the speed of light along the surface to the speed of the surface plasmon polariton. This requires that we hit the surface at some angle that is not perpendicular to the surface, and because light in vacuum is just so damned fast, we have to slow it down. We do this by putting the metallic surface on some glass. This slows the light down by about 30 percent, allowing us to choose an angle of incidence and wavelength of light that will generate a surface plasmon polariton.

Having gotten a surface plasmon polariton into the metal, what do we do with it? We get it to radiate as light again. To do this, you pattern the metallic surface with a series of lines. These lines disrupt the movement of the surface plasmon polariton, causing it to emit light. However, the emission is in the form of a bunch of stripes that all add together to make a single bright spot at a single angle. By altering the lines, you could also create a pattern that doesn't allow the surface plasmon polariton to radiate at all, or allow red colors to radiate vertically, while green colors radiate at 45 degrees.

How does this relate to holograms? A hologram is recorded by mixing light that has bounced off an object with light that is undisturbed. The resulting fringe pattern records the amplitude and phase of the light associated with the object. These two bits of information are what gives a hologram its depth, in contrast to a normal photograph, which records the intensity of scattered light.

If this fringe pattern is recorded with different colors, the combined fringe pattern also records the colors that make up the object as well. The other thing to note is that a fringe pattern is just a bunch of lines that cross each other at various angles—just the sort of thing used to make a surface plasmon polariton radiate.

The researchers took advantage of this by recording their hologram on a metallic surface layered on a piece of glass. The cool thing is that if you illuminate the back of the glass plate with red, green, and blue lasers at the right angles, you end up with red, green, and blue surface plasmon polaritons traveling through the metal. These then hit the fringe pattern of the hologram—which, remember, records not just amplitude and phase, but color as well. The fringe pattern causes red colors to only be viewable in directions from which red was scattered in the original pattern. Likewise for green and blue, allowing for good color reproduction over the range of angles.

As with many things, this isn't all pixies and fairy dust. The angular range over which the hologram can be viewed with color accuracy is basically the same angular range over which the original hologram was illuminated, so to get a wide range of viewing angles, a number of holograms have to be overlapped. This makes the combination of wide viewing angle and accurate white balance very hard to achieve (but achievable all the same).

Furthermore, I suspect that the size of the image is rather small. While you might envision this as a display for something like a phone or a portable gaming system, it probably won't scale to a television display. On the technical development side, the researchers have shown static images, but display technology needs to be dynamic. They will need to develop a way to write the fringe patterns on the metallic substrate and erase them again in just a few milliseconds in order for this to progress to a workable 3D system. This last may well be a real barrier to any products and will require some innovative thinking.

Science, 2011, DOI: 0.1126/science.1201045

Listing image by Science/AAAS