  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.
Therefore, a model platform for optomechanical characterization of lightsail
prototypes made from realistic materials is needed. We propose an approach for
simultaneous measurement of optical forces and driving powers, which
capitalizes on the multiphysics dynamics induced by the driving laser beam. By
modelling the lightsail with a 50-nm thick silicon nitride membrane suspended
by compliant micromechanical springs, we quantify force from off-resonantly
driven displacement and power from heating-induced mechanical mode softening.
This approach allows us to calibrate the measured forces to the driving powers
by operating the device as a mechanical bolometer. We report radiation pressure
forces of 80 fN using a collimated pump beam of 100 W/cm2 and noise-robust
common-path interferometry. As lightsails will inevitably experience non-normal
forces, we quantify the effects of incidence angle and spot size on the optical
force and explain the nonintuitive trend by edge scattering. Our results
provide a framework for comprehensive lightsail characterization and laboratory
optomechanical manipulation of macroscopic objects by radiation pressure
forces.
