Optomechanical Characterization of Laser-Driven Nanostructured Lightsails

We report direct optomechanical measurements of a metagrating-based microscopic lightsail by characterizing its translational and rotational dynamics in response to laser radiation pressure. Laser-driven lightsails – ultrathin, meter-scale membranes propelled to extreme speeds by high-intensity radiation pressure – represent a new generation of light-fueled space probes for interstellar exploration [1]. Successful lightsail development will rely on the integration of nanophotonic designs for dynamical stability [2-4] and the ability to monitor their influence on a lightsail’s motion within a sensitive and scalable characterization platform.

Here, we design and fabricate high-stress silicon nitride lightsails of up to 80 µm in size suspended by compliant springs and patterned with metagratings to induce in-plane optical forces and torques for self-stabilizing dynamics. Unpatterned lightsails as characterized in our previous work [5] will inevitably experience perturbations during acceleration, causing them to veer away from the laser beam. On the other hand, a nanostructured lightsail can ride the laser beam without external guidance when patterned with photonic designs for generation of restoring forces and torques. To characterize radiation-pressure induced torsional motion and observe optical torques, we map the nm-scale out-of-plane displacement across a tethered lightsail’s surface in response to off-centered collimated illumination using noise-robust common-path interferometry. We access light-induced in-plane motion by proposing and implementing single-beam grating interferometry, where lateral translation of our metagrating-based lightsail is encoded in a phase shift of the interference of two reflected diffraction orders from the sample. Upon incident high-power laser intensity at 1064 nm, a wavelength at which high-stress silicon nitride exhibits ultralow absorption necessary for thermally stable lightsail dynamics, we are able to measure lateral displacement and observe in-plane optical forces of a lightsail prototype designed for enhanced in-plane mechanical susceptibility. Our results provide a framework for comprehensive lightsail characterization, paving the way toward full dynamical verification and propulsion of self-stabilizing lightsails made of ultrathin, dielectric materials for long-range manipulation and interstellar travel.

[1] H. A. Atwater et al., Nature Materials (2018)
[2] O. Ilic, H. A. Atwater, Nature Photonics (2019)
[3] R. Gao, M. D. Kelzenberg, Y. Kim, O. Ilic, H. A. Atwater, ACS Photonics (2022)
[4] R. Gao*, M. D. Kelzenberg*, H. A. Atwater, Nature Communications (2024)
[5] L. Michaeli*, R. Gao*, M. D. Kelzenberg, C. U. Hail, A. Merkt, J. E. Sader, H. A. Atwater, arXiv (2024)

*These authors contributed equally.