20 July 2010—For decades, scientists have known that photons possess enough linear and angular momentum to turn a pinwheel-shaped dielectric, so long as the pinwheel is the right size and light enough. Exactly how this works remains unclear, and so far these motors, called light mills, have been too weak to be useful outside the laboratory.
But a team of researchers at the University of California, Berkeley, has now developed a light-driven nanosize motor that addresses the limitations of earlier light mills. The new motor, made of gold, generates comparable torque, but it is much smaller. At 100 nanometers across (one-tenth the size of other motors) it would make possible things like unwinding DNA in living cells and nanoscale harvesting of solar energy, the scientists say. The work, which was funded by grants from the National Science Foundation and the U.S. Department of Energy, was reported 4 July in Nature Nanotechnology.
Despite the light mill's diminutive size, it can rotate a silica disk 4000 times its volume. Its power is derived largely from what's known as the plasmonic effect, says Yongmin Liu, a postdoctoral researcher at UC Berkeley who worked with mechanical engineering professor Xiang Zhang on the motor. "The plasmonic effect can significantly enhance light-matter interaction at the nanoscale," Liu says.
The energy of photons falling onto the mill is transferred to "plasmons," electron density waves that are confined to the surface of the metal. These plasmonic oscillations increase the apparent surface area of the mill. So even though it's tiny, the new motor can draw upon the same amount of energy as a motor many times its actual size.
Plasmons in the mill have characteristic "resonance modes" that are determined by the type of metal and the geometry of the motor itself. When the frequency of the incident light matches these particular modes, the maximum energy from the incoming photons is transferred to the mill, resulting in the strongest torque.
The researchers found that they could control the direction of rotation of the motor by varying the wavelength of the light they used. For example, when the team illuminated the experimental plasmonic motor with light at 810 nm and 1700 nm—its two resonance frequencies—it rotated at the same speed but in opposite directions.
Lih Lin, a professor of electrical engineering at the University of Washington and an expert on nanoscale photonic devices, is impressed with the research. "Very high optical intensities are often required to generate sufficient torque, which limits the scope of potential applications," says Lin. The experimental motor requires only milliwatts of optical power, "making this a versatile tool for potential applications in nano-optomechanical transducers, energy conversion, and biological manipulations," she says. The UC Berkeley team could improve upon the device, Lin says, by developing a method for controlling the motor's position once it comes to a halt.
Ming Liu, a doctoral researcher on the UC Berkeley team, agrees that work remains. "It is like we know how to send more gasoline into a tiny motor of a car," he says, but "there is still a long way to go to increase the mileage of the motor."