Direct radiation pressure measurements for lightsail membranes

Abstract

Ultrathin lightsails propelled by laser radiation pressure to relativistic speeds are currently the most promising route for flyby-based exoplanet exploration. However, there has been a notable lack of experimental characterization of key parameters essential for lightsail propulsion. Here we present a platform for optomechanical characterization and model development of laboratory-based lightsail prototypes. We propose an approach for simultaneous measurement of optical forces and powers based on the multiphysics dynamics induced by the excitation laser beam. By modelling the lightsail with a 50-nm-thick microscopic silicon nitride membrane suspended by compliant springs, we quantify force from off-resonantly driven displacement and power from heating-induced mechanical mode softening. With this approach, we calibrate the measured forces to the driving powers by operating the device as a micromechanical bolometer. We report radiation pressure forces of 70 fN using a collimated beam of 110 W cm⁻² and noise-robust common-path interferometry. Moreover, we quantify the effects of incidence angle and spot size on the optical force and explain the non-intuitive trend by edge scattering. As lightsails will also experience lateral forces, we demonstrate measurement of in-plane motion via grating interferometry. Our results provide a framework for comprehensive lightsail characterization and optomechanical manipulation of macroscopic objects by radiation pressure forces.