Smaller than blood cells and yet precisely controllable: Researchers have developed tiny drones that can be propelled and steered by the recoil force of light. The function of the propellers of flight drones is taken over by light motors made of ingeniously configured gold particles. They act as nano-antennas that respond to laser light of different wavelengths and polarizations by emitting it in specific directions. The recoil forces generated in this way then set the micro-drones into directed movements or rotations, depending on the type of light excitation. In this way, they can be maneuvered dexterously through a liquid medium, the researchers report.
If we hold a laser pointer or a flashlight in our hand, we don’t notice the effect – but in principle it is there: Directed light can exert force similar to an air flow. However, the radiation pressure of the photon stream can only have a significant influence on small and light structures. This can be seen in comets, for example: sunlight can clearly deflect their tails made of fine particles. This power of light has also been the focus of technology for some time. For example, researchers are working on ways to power light spacecraft using light sails. However, the technology that the researchers led by Bert Hecht from the University of Würzburg are now presenting is not based on capturing the luminous flux, but on using the recoil forces that light particles cause when they leave an object.
Polymer discs with light engines
They used this effect with the aim of making drones so small that they could one day perform missions in the micro-world. As the scientists explain, their design was based on the basic principle of locomotion of ordinary flying drones. In the so-called quadrocopters, four independent propellers ensure directed air currents, which enable control over the movements. In the microscopic versions developed by the Würzburg scientists, light engines take on this role by controllably emitting light in specific directions when illuminated by laser light with different characteristics.
The micro drones consist of a transparent polymer disc with a diameter of 2.5 microns. In order to make the respective orientation easily recognizable, the disc has a lateral notch. The scientists have embedded up to four light engines in these structures, which can be controlled individually. They consist of rod-shaped gold particles in a specific arrangement. The configuration determines the type of radiation to which they respond. “These motors are based on optical antennas developed in Würzburg – i.e. on tiny metallic structures with dimensions in the range of light wavelengths. They were also specially optimized for receiving circularly polarized light,” says first author Xiaofei Wu.
Guided by emitted light
As the researchers explain, the incident light is emitted laterally in specific directions by the respective motors. “This happens depending on the direction of rotation of the polarization – clockwise or counterclockwise – as well as for two different light wavelengths,” says Wu. By irradiating them with four different types of radiation, the motors can be selectively activated with laser light. The recoil forces generated are sufficient to move and rotate the tiny drones in a directed manner. The experiments showed that they can be controlled quickly and precisely via a base in a liquid medium.
As the University of Würzburg finally reports, the team is now working on further optimizing their sophisticated micro-drones. Above all, they want to expand the previously two-dimensional control options to also control the height of the drones above the surface. They are also working on attaching tools to the micro-drones. All of this should serve the overarching goal: Precisely controllable mini-probes could one day assemble nanostructures, analyze fine surface features or be sent on medical missions, according to the vision.
Source: University of Würzburg, specialist article: Nature Nanotechnology, doi: 10.1038/s41565-022-01099-z