Physicists at the University of Würzburg have used light to propel micrometer-size objects in an aqueous environment and control them precisely on a surface with all three degrees of freedom — two translational degrees and a rotational degree. The scientists said that the demonstration of this level of control over nano- and micro-objects opens possibilities to precisely control the assembly of nanostructures, such as for the analysis of surfaces with nanometer precision. Additional applications involve reproductive medicine, the transport and release of cargos, and sensing of nano- and mesoscale objects. “When photons interact with matter, forces and torques occur due to the transfer of linear and angular momentum, respectively. The resulting accelerations are small for macroscopic objects but become substantial for microscopic objects with small masses and moments of inertia, rendering photon recoil very attractive to propel micro- and nano-objects,” the researchers said in a published paper. For example, a hand-held laser pointer produces no noticeable recoil forces when it is “fired” even though it emits a directed stream of light particles. This is because its mass is very large compared to the very small recoil impulses that the light particles cause when leaving the laser pointer. Still, optical recoil forces can have a very large effect on correspondingly small particles; the tails of comets point away from the sun partly due to light pressure. Propulsion of light spacecraft via light sails has also been discussed repeatedly, most recently in connection with the Breakthrough Starshot project, in which a fleet of miniature spacecraft will be sent to Alpha Centauri. The size comparison between quadrocopter and microdrone. University of Würzburg physicists used quadrocopters as inspiration to develop microdrones in an aqueous environment and control them precisely on a surface with all three degrees of freedom. The work supports applications for sensing at the nanoscale, as well as reproductive medicine. Courtesy of Xiaofei Wu/Universität Würzburg. The Würzburg team drew inspiration from ordinary quadrocopter drones, in which four independent rotors allow complete control of the movements. Though it is known that nano- and micro-objects are useful to the assembly of nanostructures, it is usually extremely difficult to handle these objects. The scientists called the tiny disc-shaped objects that they developed microdrones. The microdrones consist of a transparent polymer disc measuring 2.5 μm in diameter, and they are significantly smaller than red blood cells. Up to four independently addressable nanomotors made of gold are embedded in the disc. The team’s nanomotors were chiral plasmonic nanoantennas, which resonantly scattered the circular polarization components of the driving light into well-defined directions. The scientists used two overlapping unfocused light fields at wavelengths of 830 and 980 nm as light sources. “These motors are based on optical antennas developed in Würzburg — that is, tiny metallic structures with dimensions less than the wavelength of light,” said Xiaofei Wu, a postdoctoral researcher in the group of professor Bert Hecht, chair of Experimental Physics 5, Nano-Optics Group at Würzburg, who led the team that made the demonstration. “These antennas were specifically optimized for receiving circularly polarized 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 polarization (clockwise or counterclockwise) and on either of two different wavelengths of light.” Due to the very small mass of the drones, extreme accelerations can be achieved, the researchers said. Artistic representation of a microdrone with two active light-driven nanomotors between red blood cells. Courtesy of Thorsten Feichtner/Universität Würzburg. Hecht and his team began work to develop these microdrones in 2016. They aimed to develop nanodrones to be both controlled and powered with polarized laser beams. The extremely precise fabrication of the nanomotors is crucial for the function of the microdrones. The researchers cited the use of accelerated helium ions as a means to cut nanostructures from monocrystalline gold as critical to the work. In further steps of the research, the drone body is produced using electron beam lithography. Finally, the drones must be detached from the substrate and brought into solution. In further experiments, a feedback loop is being implemented to automatically correct external influences on the microdrones to control them more precisely. Furthermore, the research team strives to complete the control options so that the height of the drones above the surface can also be controlled. Another goal is to attach functional tools to the microdrones. The research was published in Nature Nanotechnology (www.doi.org/10.1038/s41565-022-01099-z).