Smaller than blood cells, yet precisely controlled: Scientists have developed tiny drones that can be propelled and controlled by the force of light recoil. The function of the drone’s propellers is taken over by light engines made of artfully configured gold particles. They act as nano-antennas that respond to laser light of different wavelengths and polarities by emitting it in specific directions. The recoil forces generated in this way set the microdrons in directional movements or rotations, depending on the type of light excitation. This way, they can be skillfully maneuvered in a liquid medium, the researchers say.
If we hold a laser pointer or a flashlight in our hand, we do not notice the effect – but in fact it is: directed light can exert a force similar to the flow of air. However, the radiation pressure of the photon flux can have a significant effect only on small and light structures. This can be seen, for example, in comets: sunlight can clearly deflect their tails, which are made of tiny particles. This power of light has also been of interest to technology for some time. For example, scientists are working on ways to power light spacecraft with light sails. However, the technology currently being presented by scientists led by Bert Hecht of the University of Würzburg is not based on capturing the luminous flux, but on using the recoil forces that cause light particles to leave the object.
Polymer disks with lightweight motors
They used this effect to make the drones so small that one day they would be able to perform missions in the microworld. As the scientists explain, their design was based on the basic principle of the movement of ordinary flying drones. In the so-called In microscopic versions developed by the scientists in Würzburg, light motors perform this role by emitting light in a controlled manner in specific directions when illuminated by laser light with different properties.
Microdrons consist of a transparent polymer disk with a diameter of 2.5 microns. The disc has a side notch to make it easier to recognize the correct orientation. Scientists placed up to four light motors in these structures, which can be individually controlled. They consist of rod-shaped gold particles in a specific arrangement. The configuration determines the type of radiation to which they respond. “These engines are based on optical antennas developed in Würzburg – that is, tiny metal structures with dimensions in the wavelength range of light. They have also been specially optimized for the reception of circularly polarized light, ”says first author Xiaofei Wu.
Guided by the emitted light
The scientists explain that incident light is emitted laterally in certain directions by appropriate motors. “This happens depending on the direction of the polarization rotation – clockwise or counterclockwise – and for two different wavelengths of light,” 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 enough to move and rotate the tiny drones in a targeted manner. Experiments have shown that they can be quickly and precisely controlled with a base in a liquid medium.
As finally reported by the University of Würzburg, the team is currently working on further optimizing its sophisticated microdrons. First of all, they want to expand their existing two-dimensional control options to also control the height of the drones above the surface. They are also working on attaching tools to the microdrons. All of this should serve an overriding purpose: precisely controlled minisens may one day assemble nanostructures, analyze fine surface features, or be sent on medical missions as per vision.
Source: University of Würzburg, specialist article: Nature Nanotechnology, doi: 10.1038 / s41565-022-01099-z