"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," says Zhang. "The imposed torque results solely from the gammadion structure's symmetry and interaction with all incident light, including light which 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 pre-determined, the illuminating source can be a simple linearly polarized plane-wave or Gaussian beam."
The results of this research are reported in the journal Nature Nanotechnology in a paper titled, " Light-driven nanoscale plasmonic motors." Co-authoring the paper with Zhang were Ming Liu, Thomas Zentgraf, Yongmin Liu and Guy Bartal.
It has long been known that the photons in a beam of light carry both linear and angular momentum that can be transferred to a material object. Optical tweezers and traps, for example, are based on the direct transfer of linear momentum. In 1936, Princeton physicist Richard Beth demonstrated that angular momentum in either its spin or orbital form - when altered by the scattering or absorption of light can produce a mechanical torque on an object. Previous attempts to harness this transfer of angular momentum for a rotary motor have been hampered by the weakness of the interaction between photons and matter.
"The typical motors had to be at least micrometres or even millimeters in size in order to generate a sufficient amount of torque," says lead author Ming Liu, a PhD student in Zhang's group. "We've shown that in a nanostructure like our gammadion gold light mill, torque is greatly enhanced by the coupling of the incident light to plasmonic waves. The power density of our m
|Contact: Lynn Yarris|
DOE/Lawrence Berkeley National Laboratory