Clean motor oil is almost transparent in bulk, but tiny oil droplets on the surface of water can appear to be any color of the rainbow. Depending on their size, the droplets preferentially scatter certain colors of light toward your eyes, and others away from them. The ability of tiny particles of dielectric materials like oil or glass to redirect light would be useful for technologies like solar cells and optical sensors, if we could design and fabricate them reliably.

Scattering from large transparent objects has been well understood for hundreds of years. We usually think of them as redirecting “rays” of light. A lens, for example, redirects parallel rays to a point at its focus. Just like an oil droplet, a lens treats each wavelength differently, focusing each color at a slightly different point. Smaller objects are somewhat more complicated. As the size of the scattering object decreases below the wavelength of light (in the range of hundreds of nanometers), it makes less sense to think about light as being composed of rays. Instead, we can think in the “wave” picture, in which incoming electromagnetic waves excite electrons in the material. The electrons in turn produce electric and magnetic waves moving in all directions, which add to the incident waves to produce a scattering pattern.

If we want to use dielectric nanoparticles to redirect light in new technologies, they need to be simple enough that we can fabricate them reliably, but complex enough that they can be adjusted to give the right response. In this paper, van de Groep and Polman suggest that cylindrical nanoparticles are a good candidate, since they have exactly two adjustable parameters: radius and height. By adjusting these parameters independently, the electric and magnetic response of the particle to an incident field can be changed. By using numerical simulations, they show that the particles can be “tuned” to respond to desired wavelengths, and some amount of control can be exerted over the scattering pattern. A third adjustable parameter can be introduced as well. For most applications, the nanoparticles need to sit on top of a substrate. By choosing the index of refraction of the substrate, the scattering pattern can be changed substantially.

The physics of scattering from small particles is dictated by Maxwell’s equations, and is understood rigorously. But implementing the solution can be quite complicated for all but the simplest objects, like spheres in a homogeneous medium. Cylinders on substrates are at the edge of simple application, where numerical simulations become the most viable avenue to progress. The authors believe that these results will provide rules to aid in the design of high-performance integrated optical circuitry, solar cells, or other technologies in which light must be manipulated.

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