Precise measurement of optical force and torque via "microdrones" — Enabling accurate manipulation and control of nanoparticles and nanomachines

Optical pressure, which is the minuscule pushing force generated when light illuminates matter, and optical torque, which is the rotational force it exerts, are essential elements in nanoparticle manipulation and nanomachine technologies. However, due to a property called the "diffraction limit," which prevents light from being focused into an area smaller than its wavelength, it has traditionally been difficult to precisely control and measure the position and orientation of nanoparticles.

A research team including Professor Yoshito Tanaka at the Research Institute for Electronic Science at Hokkaido University, has developed a "microdrone" capable of three-dimensionally and precisely measuring the minute mechanical forces generated by illuminated nanostructures. Using this drone, the team evaluated the mechanical responses of a V-shaped gold nanostructure under light irradiation in detail. They successfully observed lateral optical torque, namely, rotation induced around an axis within a plane perpendicular to the optical axis, for the first time, and demonstrated that this phenomenon is driven by the "twisting of light."

The research team embedded the target nanostructure at the center of a cross-shaped drone body and trapped the vehicle three-dimensionally using four focused laser beams, effectively eliminating the disruptive effects of the thermal motion of surrounding molecules. By precisely tracking the vehicle's movement upon light irradiation, they established a technology to track a total of six degrees of freedom, encompassing both the position and posture of the nanostructure, with high precision. Furthermore, by matching the refractive index of the drone's material to that of the surrounding liquid, they rendered the vehicle optically transparent, allowing them to accurately extract only the optical force and torque acting on the nanostructure.

These results enable the precise control of optical nanoparticle manipulation and optical nanomachines. As a technology that visualizes previously "invisible forces," ranging from biomolecules to quantum mechanical forces, it is expected to find applications across a wide spectrum of scientific fields.

(Article: Masanori Nakajo)