Torsion pendulum design for metasurface-based diffraction light sail optical force measurement

The exchange of momentum between photons and objects is what optical force is all about. The research of optical forces can be traced back to the astronomer Kepler's observations of comet tails. The understanding of the optical force has been enhanced by the Maxwell's classical electromagnetic field theory and Einstein's light quantum model. The continuous development of lasers has led to a new stage in optical technology, such as optical tweezers, laser cooling, and solar sails. Light sails are considered one of the ways to achieve interstellar travel, with the advantages of not needing to carry propellant and carrying a high payload. In recent years, the continuous development of metasurface technology has enabled researchers to apply diffraction optical force technology based on sub-wavelength structures to light sails to solve the problems of traditional solar sails, such as difficult attitude control and limited acceleration, thereby improving the feasibility of light sail driving technology. Spatial positions have an impact on the polarization state of the vector optical. When combined with spatially multiplexed metamaterial surfaces, the degrees of freedom for attitude manipulation of the diffractive light sail can be increased, leading to greater maximum acceleration, self-stabilizing thrust, and attitude controllability. In space, even subtle mechanical effects can cause the flight trajectory of the diffraction light sail to deviate from the preset orbit. Experimentally verification is necessary for the mechanical effects between the diffraction light sail and the vector optical field. Therefore, accurately measuring the multi-axis optical force generated by the diffraction light sail under vector light fields is crucial. There are few experiments that directly measure optical force in the world, and the measurement accuracy is generally not high. These experiments usually use torsion pendulums, piezoelectric crystals, flat capacitors, and other measurement tools. Based on the weak force measurement technology of torsion pendulums, this paper proposes the design of optical force torsion pendulums for regular-shaped and irregular-shaped diffraction light sails, and performs error analysis on the two torsion pendulums. For regular-shaped light sails, measurement accuracy can be improved by strictly controlling the size of the torsion pendulum and the relative position errors of each component. The force measurement has a relative error of 0.55‰. In order to meet the optical force measurement of irregular-shaped light sails and reduce the requirements for the processing accuracy and assembly accuracy of each component of the torsion pendulum, this article proposes a design scheme for an optical torsion pendulum suitable for complex objects. The fundamental element is the mid torsion pendulum horizontal rod and two standard balls. Measurement of the torsion pendulum’s motion with and without the ball allows for accurate k-value estimation of the system. Different fixtures designed to match the main body of the torsion pendulum cam be used to measure the light force on different samples. Complex shapes can be accommodated by this design. The optical power measurement needs to be tested on the sample. The optical torsion pendulum for irregular-shaped light sails is theoretically capable of achieving a relative error of 0.38% through theoretical error analysis. The design of this article enhances the accuracy of optical force measurement and the efficiency and flexibility of experiments, paving the way for applications like laser-driven light sail propulsion and space debris remediation.