Fractals for Sharper Vision
A “superlens” that focuses visible or UV light beyond the diffraction limit could be used to image biomolecules or lithographically carve nanocircuits. Researchers have attempted to build such lenses using metamaterials, but demonstrated schemes work only at a single frequency and introduce significant optical losses. Now, Geoffroy Lerosey at the Langevin Institute in Paris, France, and colleagues have demonstrated with microwaves that a superlens made of a fractal material can overcome such limitations—a strategy that might work with visible light.
A superlens images an illuminated object by picking up the tiny wavelets of radiation that scatter from its subwavelength features. Such “near-field” wavelets generally decay within a few nanometers of the object’s surface. But a lens made of a metamaterial whose refractive index is negative can amplify the wavelets, making them detectable. However, negative refraction is a resonant effect, so it only works at a precise frequency set by the geometry of the metamaterial.
The new design from Lerosey’s group uses a planar lens made of a wire folded into a so-called Hilbert curve. Such a curve has fractal properties—it looks the same on different spatial scales. Because of this self-similarity, the lens can couple to many electromagnetic modes at different frequencies and spatial scales, some of which are much smaller than the wavelength of the illuminating radiation. In experiments, the team showed that their planar lens could focus a beam containing a spectrum of microwaves (1–4 GHz) onto a spot 15 times smaller than the wavelength. The authors argue that the scheme could be extended to visible light by using thin metallic films made of disordered nanoclusters, whose microscopic structures are known to have fractal properties.
This research is published as a Rapid Communication in Physical Review B.
–Matteo Rini
Matteo Rini is the Deputy Editor of Physics.