Nanoscale Light Mill Spins a Motor with a Beam of Light

The Nanoscale Light Mill Motor

Ignore the Reich-i-ness of the motor's shape and you'll notice that at a shorter 810-nanometer wavelength the light strikes the outside of the motors arms, turning the motor counterclockwise. A larger 1,700-nanometer wave passes through to strike the elbows, turning the motor the opposite direction.Image courtesy of Zhang group

Whether wielded by Egyptian sun gods, Luke Skywalker, or your run of the mill solar-thermal power plant, light has the potential to do big things. Thanks to a breakthrough by UC Berkeley and the DOE's Lawrence Berkeley National Lab, we can now make light do very small things as well. Researchers there have created the first nano-sized light mill motor that can be manipulated in both speed and direction by tuning the frequency of the light waves that serve as its power source.

The plasmonic motor is only 100 nanometers in size, but given the right kind of power in the form of a linearly polarized light beam, it can produce enough torque to turn a silica disk 4,000 times larger than the motor itself. This type of light-driven motor isn't new, but the kind of power derived from a motor this small is unheard of up to this point. Previous attempts at plasmonic motors required the devices to be at least many micrometers in size, and even then they produced far less torque per unit volume.

The breakthrough is in the gammadion gold structures that make up the motor. The gammadion structure's symmetry coupled with the way it interacts with all incident light – not just light coming in from a particular angle – a simple linear polarized beam can coax a great deal of torque from the motor. That increased interaction between light and gammadion structures spells more power from less motor.

The motor is also sensitive to wavelength, which can be used to change the direction of the motor. As you can see (sort of) in the video below, a shorter wavelength of 810 nanometers gets the motor humming in a counterclockwise direction. A similar beam at 1,700 nanometers gets it cranking in the opposite direction.

Such tiny, controllable light-powered motors could have myriad biological applications, not least of which is manipulating DNA in vivo, using the motor to unwind and rewind a double helix. They could also lead to improved nanoelectromechanical systems and better solar harvesting devices.