Nano-sized light mill drives micro-sized disk (w/ Video)

July 5, 2010 by Lynn Yarris
Nano-sized light mill drives micro-sized disk (w/ Video)
This STM image shows a gammadion gold light mill nanomotor embedded in a silica microdisk. Inset is a magnified top view of the light mill.

While those wonderful light sabers in the Star Wars films remain the figment of George Lucas' fertile imagination, light mills - rotary motors driven by light - that can power objects thousands of times greater in size are now fact. Researchers with the Lawrence Berkeley National Laboratory and the University of California Berkeley have created the first nano-sized light mill motor whose rotational speed and direction can be controlled by tuning the frequency of the incident light waves. It may not help conquer the Dark Side, but this new light mill does open the door to a broad range of valuable applications, including a new generation of nanoelectromechanical systems (NEMS), nanoscale solar light harvesters, and bots that can perform in vivo manipulations of DNA and other biological molecules.

“We have demonstrated a plasmonic motor only 100 nanometers in size that when illuminated with linearly polarized light can generate a torque sufficient to drive a micrometre-sized silica disk 4,000 times larger in volume,” says Xiang Zhang, a principal investigator with Berkeley Lab’s Materials Sciences Division and director of UC Berkeley’s Nano-scale Science and Engineering Center (SINAM), who led this research. “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.”

The success of this new light mill stems from the fact that the force exerted on matter by light can be enhanced in a metallic when the frequencies of the incident are resonant with the metal’s plasmons - surface waves that roll through a metal’s conduction . Zhang and his colleagues fashioned a gammadion-shaped light mill type of nanomotor out of gold that was structurally designed to maximize the interactions between light and matter. The metamaterial-style structure also induced orbital angular momentum on the light that in turn imposed a torque on the  nanomotor.

“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 video will load shortly
Filmed through water, a silica microdisk embedded with a gold, gammadion-shaped light mill nanomotor rotates in one direction under illumination from laser light at 810 nanometers wavelength. When the wavelength is switched to 1,715 nanometers, the rotational direction is reversed. Torque is produced when the laser light frequencies resonate with the frequencie of the metal’s plasmons. (Movie courtesy of Zhang group)

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 motors is very high. As a bonus, the rotational direction is controllable, a counterintuitive fact based on what we learn from wind mills.”

The directional change, Liu explains, is made possible by the support of the four-armed gammadion structure for two major resonance modes - a wavelength of 810 nanometers, and a wavelength of 1,700 nanometers. When illuminated with a linearly polarized Gaussian beam of laser light at the shorter wavelength, the plasmonic motor rotated counterclockwise at a rate of 0.3 Hertz. When illuminated with a similar laser beam but at the larger wavelength, the nanomotor rotated at the same rate of speed but in a clockwise direction.

Optical forces induced on a light mill motor by an illumination wavelength of 810 nanometers (left) and 1,700 nanometers. At 810 nm wavelength, light impacts the outer side of the arms, inducing a counterclockwise torque on the motor. At 1,700 nm, light passes through the gaps and impacts the elbow of the motor, providing a clockwise torque. (Image courtesy of Zhang group)

“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 says. “For example, a silica disk embedded with four plasmonic nanomotors attains the same rotation speed with only half of the power applied as a disk embedded with a single motor.”

The nanoscale size of this new light mill makes it ideal for powering NEMS, where the premium is on size rather than efficiency. Generating relatively powerful torque in a nanosized light mill also has numerous potential biological applications, including the controlled unwinding and rewinding of the DNA double helix. When these light mill motors are structurally optimized for efficiency, they could be useful for harvesting solar energy in nanoscopic systems.

“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,” Liu says.

Explore further: Transformation optics make a U-turn for the better

More information:
For more information about the research of Xiang Zhang visit
For more information about the Berkeley Nano-scale Science and Engineering Center visit

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5 / 5 (3) Jul 05, 2010
I'll say it first. It looks like a swastika. Now nobody has to say anything more about it.

100nm motor? That is amazing. The shear size, or lack thereof could easily enable "nanobot" type medical devices. There are so many applications, but my lack of experience in this subject limits my imagination. Any other great applications I'm missing?
not rated yet Jul 05, 2010
Lovely! :-)
not rated yet Jul 05, 2010
I think it is more a proof of concept. I can't see how you can shine a laser on a motor that swims in a blood stream. Maybe the laser has to be mounted on the same chip inside the robot to power the motor. But then really small. Can we do that? (Can we fix it? ....)
5 / 5 (2) Jul 05, 2010
Richard Feynman would have loved it. Remember his prize for making a tiny motor, won in a few months to his surprise. This would take the cake however!
5 / 5 (1) Jul 05, 2010
Awesome. One of the more reliable nanoscale mechanisms.
not rated yet Jul 05, 2010
Quote from article:
"Optical forces induced on a light mill motor by an illumination wavelength of 810 nanometers (left) and 1,700 nanometers. At 810 nm wavelength, light impacts the outer side of the arms, inducing a counterclockwise torque on the motor. At 1,700 nm, light passes through the gaps and impacts the elbow of the motor, providing a clockwise torque." (Image courtesy of Zhang group)

I find it a little odd that the longer wavelength light enters the gap, one would think it would be the shorter wavelength. I wonder what the explanation is for that?
4.5 / 5 (2) Jul 05, 2010
@Question: I don't claim to understand it but I think the answer has to do with the forces created by resonance of certain parts of the gammadion, and not directly by light squeezing through and forcing certain parts of gammalion. See this _free_ suplemental information for the paper: http://www.nature...8-s1.pdf
5 / 5 (1) Jul 07, 2010
I do need to comment on the swastikaness of it... It is important to note that the Nazi swastika's arms turned the opposite direction. What we have here is the ancient svastik symbol, which has stood for good luck, good fortune, and has represented the Sun at different points in history...
An analysis of the 5,000 year history of the svastika can be found here:

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