By creating a distinct nanoscale patterning on the surface of an object, researchers at the California Institute of Technology (Caltech) have designed a way to levitate and propel objects using only light. The patterning interacts with light in such a way that the object can right itself when perturbed, creating a restoring torque to keep it in the light beam. Rather than requiring highly focused laser beams, the researchers designed the patterning of the object as a way to “encode” stability into the object. Self-stabilizing optical manipulation of objects — which can be millimeter, centimeter, and even meter in scale — is achieved by controlling the anisotropy of light scattering along the object’s surface. According to the researchers, the light source can be millions of miles away. Conceptual illustration of a nano-patterned object reorienting itself to remain in a beam of light. Courtesy of the Atwater laboratory. In a scalable design that features silicon resonators on silica substrate, the researchers identified nanophotonic structures that can self-stabilize when rotated and/or translated relative to the optical axis. By engineering the scattered phase, the researchers achieved nanoscale control of scattering across a large area, creating restoring behavior in the object, without the need to focus incident light or excessively constrain the shape, size, or material composition of the object. Although still theoretical, the work could lead to platforms for manipulating macroscopic objects, with applications ranging from contactless wafer-scale fabrication and assembly, to trajectory control for ultralight spacecraft and even laser-propelled light sails for space exploration. “We have come up with a method that could levitate macroscopic objects,” said professor Harry Atwater. “There is an audaciously interesting application to use this technique as a means for propulsion of a new generation of spacecraft. We’re a long way from actually doing that, but we are in the process of testing out the principles.” In theory, this spacecraft could be patterned with nanoscale structures and accelerated by Earth-based laser light. Without needing to carry fuel, the spacecraft could reach very high, even relativistic, speeds and possibly travel to other stars. Atwater also envisions that the technology could be used on Earth to enable rapid manufacturing of ever-smaller objects, like circuit boards. The research was published in Nature Photonics (https://doi.org/10.1038/s41566-019-0373-y).