Direct Measurement of Radiation Pressure Forces on Membrane Lightsails

We report direct measurement of radiation pressure forces exerted on a 100-nm-thick silicon nitride lightsail membrane. Our sensitive measurements 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. 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 [1,2]. Realizing laser-driven lightsails necessitates precise characterization of the optical forces on a material platform capable of exhibiting mechanical, beam-riding, and thermal stability. For a laser power density of 200 W/cm² 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 filling substantial part of the lightsail, mimicking the initial acceleration stage of interstellar lightsails. Furthermore, to predict the tilt-dependent dynamics of subwavelength thick lightsails, we characterize the non-intuitive trend of the optical force versus incidence angle in the range of ±20° for TE and TM polarization. Our study represents a critical milestone in realizing an experimental testbed for lightsail characterization, thus advancing the development of laser-driven spacecraft, and opening the door for manipulation of macroscopic objects through optical forces.