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Ultrathin lightsails, propelled to relativistic speeds by laser radiation pressure, are being actively explored as a new generation of interstellar spacecraft probes, spearheaded by the Breakthrough Starshot Initiative. Realizing laser-driven lightsails necessitates precise characterization of the optical forces on a material platform capable of exhibiting mechanical, beam-riding, and thermal stability.

We performed sensitive force measurements that rely on three key components: a noise-robust common-path interferometer with picometer resolution, rational design of the tethered lightsail for enhanced mechanical susceptibility, and an off-resonant driving scheme for quasi-static, linear dynamics. For a laser power density of 200 W/cm 2 at 514 nm, we measure displacements of~ 10 pm, resulting from optical forces of~ 30 fN. Contrary to optical trapping of microscopic objects, motion is induced by a collimated laser beam …