A newly developed light mill could lead to a whole new crop of nanoscale devices, including nanoscale solar light harvesters, nanoelectromechanical systems, and nanobots that could manipulate DNA and other biological molecules in vivo. Researchers with the US Department of Energy’s Lawrence Berkeley National Laboratory and the University of California, Berkeley, have created the first nano-size light mill motor whose rotational speed and direction can be controlled by tuning the frequency of the incident light waves. “The light mills are as small as 100 nm in radius. [They] can be even smaller, and the smallest I ever made was actually 50 nm in radius,” said lead author Ming Liu, a PhD student. “It is only limited by the fabrication technique we have here.” The plasmonic motor may be small, but the 100-nm motor generates a torque great enough to power a micrometer-size silica disk with a volume 4000 times larger when illuminated with linearly polarized light. “In addition to easily being able to control the rotational speed and direction of this motor, we can create coherent arrays of such motors, which results in greater torque and faster rotation of the microdisk,” said Xiang Zhang, a principal investigator with Berkeley Lab’s Materials Sciences Division and director of UC Berkeley’s Nano-scale Science and Engineering Center, who with Liu led this effort. Ming Liu, left, Xiang Zhang, center, and Thomas Zentgraf have created the first nano-size light mill motor whose rotational speed and direction can be controlled by tuning the frequency of the incident light waves. This new concept opens the door to a broad range of valuable applications in energy and biology as well as in nanoelectromechanical systems. Courtesy of Roy Kaltschmidt and Lynn Yarris, Berkeley Lab. It’s not news that photons carry momentum that can be transferred to a material object. Photonic tools such as optical traps and tweezers are based on the direct transfer of linear momentum. And it isn’t only linear momentum that can be transferred to an object. Researchers have found that the transfer of angular momentum can produce a mechanical torque on an object when affected by the absorption or scattering of light. Until now, using that torque to power a rotary motor has been a challenge, thanks to the weak interaction between photons and matter. “The typical motors had to be at least micrometers or even millimeters in size in order to generate a sufficient amount of torque,” Liu said. “We are the first group, as far as we know, to understand, measure and to utilize the giant plasmonic forces generated by such carefully designed structures.” The new light mill works because metallic structures can enhance the force that light exerts on matter when the incident light waves resonate with the metal’s plasmons. Zhang’s team devised the light mill from gold with a structural design intended to maximize light-matter interactions. The metamaterial-type structure also produces a torque on the nanomotor through induced orbital angular momentum. “The planar gammadion gold structures can be viewed as a combination of four small LC circuits for which the resonant frequencies are determined by the geometry and dielectric properties of the metal,” Zhang explained. “The imposed torque results solely from the gammadion structure’s symmetry and interaction with all incident light, including light that doesn’t carry angular momentum. “Essentially, we use design to encode angular momentum in the structure itself. Since the angular momentum of the light need not be predetermined, the illuminating source can be a simple linearly polarized plane wave or Gaussian beam.” Coupling incident light to plasmonic waves enhances the torque, according to Liu. “The power density of our motors is very high,” he said. “As a bonus, the rotational direction is controllable, a counterintuitive fact based on what we learn from windmills.” The four-armed structure supports two major resonance modes: wavelengths of 810 and 1700 nm; this enables the directional change. The plasmonic motor rotated counterclockwise at 0.3 Hz when exposed to a linearly polarized Gaussian beam at 810 nm; a similar beam at 1700 nm caused it to rotate clockwise but at the same speed. “When multiple motors are integrated into one silica microdisk, the torques applied on the disk from the individual motors accumulate, and the overall torque is increased,” Liu said. “For example, a silica disk embedded with four plasmonic nanomotors attains the same rotation speed with only half of the laser power applied as a disk embedded with a single motor.” The team’s research is reported in the journal Nature Nanotechnology. Thomas Zentgraf, Yongmin Liu and Guy Bartal co-authored the paper with Zhang and Ming Liu. Light mills, Liu said, enable energy to be transferred directly from light to mechanical works at the nanometer scale, without worrying about the intermediates. “The immediate next step,” he added, “is to optimize the light mill structure for a higher overall energy efficiency. We are also cooperating with biologists to connect the light mill to DNA molecules to study the winding and unwinding properties of DNA.”