OK, first of all, what’s a light mill? It’s a simple rotary motor consisting of four flat vanes mounted to a central axis, which spins when subjected to light. Light mills have been around since 1873, mostly just as novelty items, and have pretty much always been at least a few inches tall. Less than a week ago, however, scientists at California’s Lawrence Berkeley National Laboratory announced in a research paper that they had created a light mill just 100 nanometers in size. Unlike its bigger brothers, this tiny device might actually have some very practical applications.

The nano light mill rotates when exposed to laser light, and the speed and direction of the rotation can be changed by manipulating the frequency of that light. Most intriguingly, the mill can generate enough torque to drive a micrometer-sized silica disk, 4,000 times larger in volume than the mill itself. This means that the nano mill could conceivably be put to use, in things such as nanoelectromechanical systems (NEMS), nanoscale solar light harvesters, and nanobots that could perform actions such as unwinding and rewinding DNA double helixes.

The nano mill was constructed out of a gold-based metamaterial, which is the key to why it works. Metals contain plasmons, which are surface waves that roll through its conduction electrons. The amount of force exerted by light on a metallic nanostructure can be enhanced when the frequency of the light waves resonates with those plasmons. The metamaterial was designed to maximize this effect. Previous attempts at such motors had to be much larger in order to generate any torque, because they did not exploit the interaction between photons and plasmons.

While a single mill attached to a silica disk was indeed able to turn it, the Berkeley Lab researchers found that the torque could be greatly increased by attaching multiple mills. A disk with four mills, for instance, required just half the laser power to achieve the same rotational speed as a disk with only one. "By designing multiple motors to work at different resonance frequencies and in a single direction, we could acquire torque from the broad range of wavelengths available in sunlight," said lead author Ming Liu.

The research paper was published in the journal Nature Nanotechnology.

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