Passively Stabilized Dynamics of Flexible Metagrating-Based Laser-Propelled Lightsails

We combine flexible flat membrane modeling with spin-stabilization and nanophotonic design motives based on anisotropic optical scattering to report passive structural and dynamical stabilization of laser-propelled lightsails. Near-term interstellar exploration of exoplanets as envisioned by the Breakthrough Starshot initiative relies on propulsion of ultrathin membrane-like probes, or <i>lightsails</i>, to relativistic speeds via a directed laser source [1]. Successful deployment will depend on the ability to preserve the overall structural integrity of the lightsail while ensuring stable beam-riding behavior [2]. To date, studies on passive stabilization of laser-driven, photonically engineered lightsails assumed rigid-body dynamics [3-10]. In reality, lightsails with extreme area-to-mass ratios will be flexible and thus capable of developing shape deformations that could not only result in structural instability, but also compromise beam-riding ability.<br/><br/>We shed light on the flexible-body dynamics of flat silicon nitride membranes by using finite-element-based time-domain simulations in the linear elastic regime. Silicon nitride is a promising material candidate due to its low absorption in the near-infrared propulsion band and its wafer-scalable fabrication. Spin-stabilization is utilized to prevent shape distortion and collapse of the lightsail without needing to introduce additional mass or material. To passively stabilize such spinning flat membranes, we present a nanophotonic design based on asymmetrically diffracting optical metagratings. With the design search accounting for gyroscopic effects, we highlight how varying the spinning frequency and width of the Gaussian beam can disrupt or enable stable dynamics. Specifically, for a laser beam with a peak intensity of 1 GW/m² at 1064 nm and a Gaussian beam profile of <i>w</i> = 0.3 m, our simulations show how a meter-sized flexible flat membrane with a mass of 0.87 g, spinning at 120 Hz and patterned with our metagrating design will self-stabilize for a minimum duration of 5 seconds even if being initially displaced by <i>x</i> = <i>y</i> = 0.05 m and tilted by 2° (pitch and roll). Assuming constant, but realistic material properties, we also confirm that the sails develop strain and exhibit peak temperatures that are sufficiently below silicon nitride’s maximum tensile strain limit and crystallization temperature, respectively.<br/><br/>Our results pave the way towards more realistic simulations of lightsail deployment and propulsion from the microscopic to the macroscopic scale. Moreover, our open-source simulation tool can be readily expanded to include other types of optical interaction or experimental temperature-dependent material properties, while allowing to develop strategies for damping of the lightsail’s lateral oscillations and payload integration.<br/><br/>[1] Harry A. Atwater <i>et al.</i>, Nature Materials (2018)<br/>[2] Ognjen Ilic and Harry A. Atwater, Nature Photonics (2019)<br/>[3] Elena Popova <i>et al.</i>, Mathematical Methods in the Applied Sciences (2017)<br/>[4] Zachary Manchester and Abraham Loeb, The Astrophysical Journal Letters (2017)<br/>[5] Prateek R. Srivastava <i>et al.</i>, Optics Letters (2019)<br/>[6] Joel Siegel <i>et al.</i>, ACS Photonics (2019)<br/>[7] Ying-Ju L. Chu <i>et al.</i>, Physical Review Letters (2019)<br/>[8] Mohammad M. Salary and Hossein Mosallaei, Laser & Photonics Reviews (2020)<br/>[9] Niels Gieseler <i>et al.</i>, Optics Express (2021)<br/>[10] Avinash Kumar <i>et al.</i>, Physical Review Applied (2021)