ALMOST PERFECT [+]Enlarge Credit: ACS Photonics

Highly reflective mirrors help telescopes capture images of faint, distant galaxies and enable lasers to produce intense beams of light. Researchers have come up with a new way to make such mirrors, by starting with self-assembled nanoparticles. These near-perfect reflectors demonstrate the promise of metamaterials, a relatively new class of materials designed to interact with light in novel ways (ACS Photonics 2015, DOI: 10.1021/acsphotonics.5b00148).

The common type of mirror, the kind found over bathroom sinks, is a piece of glass backed by a smooth aluminum film and reflects about 90% of visible light—good enough for checking your outfit. But high-performance applications, such as telescope lenses or the reflective cavities in lasers, need more efficient reflection—even a very small absorption of light by the mirror will degrade performance. So these instruments use Bragg reflectors—stacks of thin films that refract light in such a way as to reflect almost 100% of incident light at the wavelengths of interest. But they are costly to produce.

Last year, Jason Valentine, a mechanical engineer at Vanderbilt University, and colleagues demonstrated a third kind of mirror—a metamaterial—that combined the simplicity of metal films with the near-perfection of Bragg reflectors (Appl. Phys. Lett. 2014, DOI: 10.1063/1.4873521). Metamaterials are engineered to have properties, typically derived from nanoscale patterning, that do not occur in the bulk material. They made their reflector by patterning a silicon wafer’s surface with an array of silicon cylinders a few hundred nanometers in diameter. Each of the cylinders acted like a tiny resonator for particular light frequencies—analogous to the way certain sound frequencies will make a tuning fork hum. By adjusting the size of the cylinders, Valentine could control how well they reflected light of a given frequency. This mirror reflected more than 99% of light at the peak wavelength.

Although the work showed that metamaterial mirrors could be effective, the method for making them was far from practical: The researchers painstakingly created the arrays using electron-beam lithography, which is difficult to scale up. So Valentine and his team adopted a simpler method to make bigger, near-perfect reflectors.

They started with off-the-shelf polystyrene beads 820 nm across and dropped them into a film of water. Driven by electrostatic forces, the beads self-assembled into a monolayer on the water’s surface with a repeated hexagonal pattern. Valentine’s group then drained the water, lowering the bead layer onto a submerged silicon wafer, and used a plasma etching process to shrink the beads to 560 nm. Finally, they used the bead layer as a lithographic mask to pattern the underlying silicon. The resulting 2-cm2 arrays were covered in silicon cylinders, each 335 nm tall and 480 nm across the top.

The arrays reflect 99.7% of incident infrared light at 1,530 nm. Valentine is now working on making larger area reflectors by patterning with silicon nanospheres.