Using just light to propel micrometer-sized drones and exercising precision control: For the first time, physicists from the University of Würzburg have achieved this. Their microdrones are a fraction of the size of red blood cells.
Even though it creates a directed stream of light particles, a hand-held laser pointer produces no visible recoil forces when it is “fired.” This is due to its high bulk in comparison to the modest recoil impulses that light particles create when they leave the laser pointer.
Optical recoil forces, on the other hand, have long been known to have a significant impact on small particles. Comet tails, for example, point away from the Sun in part due to light pressure. The use of light sails to propel light spacecraft has also been proposed, most notably in connection with the “star shot” project, which will send a fleet of small spacecraft to Alpha Centauri.
Models are simple quadcopter drones.
Models are simple quadcopter drones. 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 common 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 characterization, and reproductive medicine.
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Polymer discs with up to four nanomotors powered by light
The Würzburg microdrones are made out of a transparent polymer disc with a diameter of 2.5 micrometers. This disc has up to four gold nanomotors that may be controlled separately.
“These motors are based on optical antennas created in Würzburg,” says Xiaofei Wu, a postdoc in the Hecht research group. “Optical antennas are tiny metallic structures with dimensions shorter than the wavelength of light.” “These antennas have been designed specifically to receive circularly polarised light.”
This ensures that the motors receive light independent of the drone’s orientation, which is critical for the application. The received light energy is then radiated in a specific direction by the motor to generate optical recoil force, which is dependent on the sense of rotation of the polarisation (clockwise or counterclockwise) and one of two wavelengths of light.”
The researchers were only able to control their microdrones efficiently and precisely because of this concept. Extreme accelerations are possible because of the drones’ tiny bulk.
Microdrones took a long time to develop. It all began in 2016 with a research grant from the Volkswagen Foundation, which was allocated to high-risk projects.
The microdrones’ ability to function depends 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 immersed in water.
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, of course, to equip the microdrones with useful equipment.
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