Tiny Microdrones Propelled by Light-Driven Nanomotors

Even though it emits a directed stream of light particles, a hand-held laser pointer creates no discernible recoil forces when it is "fired." This is due to its massive mass in comparison to the little recoil impulses that light particles create when they exit the laser pointer.

Optical recoil forces, on the other hand, have long been known to have a big impact on microscopic particles. Because of light pressure, comet tails, for example, point away from the Sun. Light sails have also been proposed as a means of propulsion for light spacecraft, most recently in association with the "starshot" concept, which involves sending a fleet of small spacecraft to Alpha Centauri.

Ordinary quadcopter drones as models

Würzburg physicists led by Professor Bert Hecht (Chair of Experimental Physics 5, Nano-Optics Group) have now demonstrated for the first time that light can not only efficiently propel micrometer-sized objects in an aqueous environment, but also precisely control them on a surface with all three degrees of freedom in the journal Nature Nanotechnology (two translational plus one rotational).

They were inspired by quadcopter drones, which have four independent rotors that offer complete control of movement. Such control capabilities open up entirely new possibilities for the typically challenging handling of nano- and micro-objects, such as nanostructure building, nanometre-level surface research, and reproductive medicine.

Polymer discs with up to four light-driven nanomotors

The Würzburg microdrones are made out of a transparent polymer disc with a diameter of 2.5 micrometers. This disc contains up to four independently addressable gold nanomotors.

“These motors are based on optical antennas developed in Würzburg, – that is, tiny metallic structures with dimensions less than the wavelength of light,” says Xiaofei Wu, a postdoc in the Hecht research group.
 “These antennas were specifically optimized for receiving circularly polarised light. This allows the motors to receive the light regardless of the orientation of the drone, which is crucial for applicability. In a further step, the received light energy is then emitted by the motor in a specific direction to generate optical recoil force, which depends on the sense of rotation of the polarisation (clockwise or counterclockwise) and on either of two different wavelengths of light.”

The researchers were able to control their microdrones efficiently and precisely only because of this concept. Extreme accelerations are possible due to the drones' tiny bulk.
The microdrones' development proved difficult. It all began in 2016 with a VW Foundation research grant targeted to high-risk ventures.

Precise fabrication based on single-crystal gold

The microdrones' function is dependent on the nanomotors' incredibly precise construction. The ability to cut nanostructures from monocrystalline gold using accelerated Helium ions has proven to be a game changer. The drone body is then created using electron beam lithography in the following processes. Finally, the drones must be separated from the substrate and dissolved.

In future tests, a feedback loop will be used to automatically rectify environmental influences on the microdrones, allowing them to be controlled more accurately. Furthermore, the study team is working to complete the control choices so that the drones' height above the surface can be managed as well. Another goal is to include functional tools into the microdrones.
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