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Plasmonic Motor Powered Mechanically and Directly by Light, Developed by UC Berkeley

From left to right: Ming Liu, Xiang Zhang and Thomas Zentgraf

All photovoltaic solar cells have basically the same working principle: a photon hits the photosensitive material, kicks an electron off his state in the valence band and send it into the conduction band. Still, light has some other properties that could enrich what solar cells could yield.

Researchers from the University of California Berkeley, led by Xiang Zhang, a principal investigator with the university’s Materials Sciences Division and director of UC Berkeley’s Nano-scale Science and Engineering Center (SINAM), demonstrated for the first time practically how light can actually move material nanometric particles of gold by using surface plasmons.

Surface plasmons are surface waves that roll through a metal’s conduction electrons. The light-driven motor designed by Zhang and his team can harvest the light’s energy to spin a gammadion-shaped gold particle. Not that they’d have something with former German political structures, but that structures literally maximizes the interaction between light and matter, which is interesting.

“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 […] 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,” says Zhang.

The silica disk-embedded gammadion nanomotor

The frequencies of the incident light resonate with the surface plasmons and thus enhance the force exerted on the gold gammadion, whose shape is used to give the light an 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,” 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.”

Ming Liu, a PhD student in Zhang’s team, says the power density of the plasmonic motor they conceived is very high. They even got to drive a micrometer-sized silica disk that is some 4,000 times larger than the gammadion. He also states that “the typical motors had to be at least micrometres or even millimeters in size in order to generate a sufficient amount of torque.”

“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,” Liu says.

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.

These light-harvesting properties of the light-driven gold gammadion can be used in powering nanoelectromechanical (NEMS) devices, manipulating the DNA’s double helix and even use them in solar panels, which can eventually turn the energy into electrons, but the main purpose would be mechanical work.

“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 laser power applied as a disk embedded with a single motor […] 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.”

In an e-mail interview given exclusively to The Green Optimistic, Liu answered a few questions not covered in the initial press release from EurekAlert!. I asked him three questions whose answers he concluded in one, straight explanation. The questions were:

1. What is the currently achieved efficiency of the nano-device?
2. If used in solar cells, can you theoretically determine if the efficiency of this technology, applied in harvesting the energy of light, will ever compare to current photovoltaic solar cells’?
3. How does the system behave in high temperatures, unavoidable in the case of sunlight conversion?

Liu answered:

I think the efficiency of the light mill would never comparable with photovoltaic solar cells, because photons carry mainly energy, but not momentum. They are actually massless. The advantage of a light mill is that it neglects all the steps between transforming energy from light to work, for instance chemical fuel, electricity, or water in reservoirs. It cuts the intermediates and changes the energy form directly from light to work.The gold lightmill works well as long as it is not melted, which means a few hundred degrees.

Even though it may not prove very efficient to use in solar cells, this kind of plasmonic motors could serve very well to help them, by covering another layer besides the photovoltaic and the thermal one, with the electrons that are left out of conversion after the photovoltaics had done their job.

Being the first time such a mechanism has been demonstrated practically (it already exists in theory since 1936), it’s normal not to have high expectations regarding its efficacy. Further research is needed to perfect it and, maybe in a few years, we’ll have yet another type of solar cells or who knows what other materials that can transfer mechanical energy directly, without any middleman.

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