For the first time, University of Würzburg physicists have been successful in propelling micrometer-sized drones using just light and exerting clear-cut control. These microdrones are considerably smaller than red blood cells.
A hand-held laser pointer creates no obvious recoil forces when it is "fired" — even though it releases a directed flow of light particles. The reason for this is its extremely large mass compared to the minute recoil impulses that the light particles create when departing from the laser pointer.
Nevertheless, it has long been evident that optical recoil forces can truly have an extremely large impact on correspondingly small particles. For instance, the tail of a comet points away from the Sun partially because of light pressure.
The propulsion of light spacecraft using light sails has also been deliberated frequently, most recently regarding the "star shot" project, wherein a convoy of small spacecrafts is to be launched to Alpha Centauri.
Ordinary Quadcopter Drones as Models
Würzburg physicists guided by Professor Bert Hecht (Chair of Experimental Physics 5, Nano-Optics Group) have currently illustrated for the first time that it is possible to not only competently propel micrometer-sized objects in an aqueous surrounding with light but also regulate them on a surface with all three degrees of freedom (two translational and one rotational). Details of their efforts have been presented in the journal Nature Nanotechnology.
The researchers were inspired by basic quadcopter drones, where four separate rotors enable full regulation of the movements. Such control practicability provides totally new options for the generally very tough handling of nano- and micro-objects, for instance, for the examination of surfaces with nanometer accuracy, for the assembly of nanostructures or in the domain of reproductive medicine.
Polymer Disks with up to Four Light-Driven Nanomotors
The Würzburg microdrones comprise a transparent polymer disk measuring 2.5 μm in diameter. Up to four individually addressable nanomotors composed of gold are inserted into this disk.
These motors are based on optical antennas developed in Würzburg,— that is, tiny metallic structures with dimensions less than the wavelength of light. These antennas were specifically optimised 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.
Xiaofei Wu, Postdoctoral Researcher, Hecht Research Group, University of Würzburg
“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," Xiaofei Wu added.
It was only by using this idea that the scientists could regulate their microdrones resourcefully and precisely. Owing to the tiny mass of the drones, extreme speeds can be accomplished.
The creation of the microdrones was tough. It commenced in 2016 with a research grant provided by the VW Foundation committed to risky projects.
Precise Fabrication Based on Single-Crystal Gold
The exceptionally precise creation of the nanomotors is vital for the working of microdrones. The utilization of accelerated Helium ions as a way to cut nanostructures from monocrystalline gold has been a game-changing factor. In additional steps, the drone’s body is created using electron beam lithography. As a final step, the drones have to be separated from the substrate and transported into a solution.
In additional experiments, a feedback loop is being applied to automatically rectify outside influences on the microdrones to more accurately regulate them. Moreover, the researchers aim to finish the control options so that the drone’s height above the surface can also be regulated. In addition, another aim is to fasten functional tools to the microdrones.
Wu, X., et al. (2022) Light-driven microdrones. Nature Nanotechnology. doi.org/doi.org/10.1038/s41565-022-01099-z.