Hundreds of electrical transistors can fit in a single square micron on a chip. But the footprint of photonic devices is far from commensurate with that density. Whereas the atom sets the ultimate scaling limit to Moore’s law for electronic components,1 the minimum size of photonic ones is usually constrained by the diffraction limit of the light waves they handle. For devices that manipulate signals in the IR—the most common wavelength range in telecommunications—that limit is on the scale of microns.
Fortunately, researchers can circumvent the diffraction limit by directing light at the interface between a metal and a dielectric. Doing so induces an interaction between the photons and mobile electrons at the metal’s surface. The interaction excites surface plasmons, charge-density oscillations that propagate along the interface at the same frequency as the light and can readily be channeled into nanometer-scale circuit elements. For the past couple of decades,...