Nanophotonic Lightsail Optomechanics for Long-Range Optical Manipulation and Interstellar Exploration

Author: Gao, Ramon

Year: 2026

Degree: Dissertation (Ph.D.)

Advisor: Atwater, Harry Albert

Committee Members: Roukes, Michael Lee; Sader, John E.; Faraon, Andrei; Vahala, Kerry J.; Atwater, Harry Albert

Option: Applied Physics

Abstract

Laser propulsion, where light serves as fuel based on the principle of photon momentum transfer, might one day allow us to explore other planetary systems. Directing a high-intensity laser beam onto an ultrathin reflective space probe could enable acceleration to relativistic velocities needed for interstellar missions to our neighboring exoplanets within a human lifetime. These laser-driven lightsails, envisioned to be dielectric membranes of square meters in area while weighing at most a few grams, must be prone to shape deformations, withstand laser-induced heating, and harness photon momentum for efficient propulsion and self-correcting dynamics.

In this thesis, we employ optomechanical systems and leverage nanophotonic design principles to address these unprecedented challenges. Importantly, we establish a platform for experimental characterization and model development of laboratory-based lightsail prototypes. We identify silicon nitride membranes as a promising material platform due to its ultralow absorption and fabricate microscopic lightsails based on spring-supported compliant resonators. By using off-resonant collimated laser excitation and noise-robust common-path interferometry while simultaneously operating our device as a micromechanical bolometer, we quantify the propulsive optical force and associated laser power in the linear regime. As we vary the angle of incidence and increase the laser spot size to overfill the lightsail, we reveal noticeable effects of edge scattering on the radiation pressure force --- an observation that represents one of many considerations and lessons to be learned for lightsail development. When driving these tethered lightsails with resonant radiation pressure to micron-scale displacements, nonlinear dynamic behavior emerges due to their geometric nonlinearity. We explore how transitions between bistable mechanical states in the Duffing regime can be controlled with optical forces through noise-mediated sidebands and intermodal coupling.

Next, we introduce anisotropically scattering metagratings patterned into silicon nitride membranes as a way to engineer the transfer of light momentum and to generate restoring optical forces and torques for passive stabilization. We characterize the intensity and angular distribution of laser light diffracted from the suspended metagratings versus incidence angle, from which we infer the forces and torques and thus the engineered self-stabilization mechanism. Then, we integrate these metagratings into compliant mechanical resonators optimized for enhanced susceptibility to optical in-plane forces and torques, and describe a route for parallelizing and scaling up fabrication of nanostructured lightsail prototypes. To measure in-plane motion, we propose and implement grating interferometry, whereby interference of two diffracted orders from the metagrating produces fringes that shift as a function of lateral displacement. This allows us to characterize the noise-excited in-plane mode, nanometer-scale translations of and optical in-plane forces on our tethered nanostructured lightsail. Together with our measurements of torsional motion in response to off-centered laser illumination and detected via common-path interferometry, our set of experiments showcase the capabilities of our characterization platform to monitor all motional and rotational degrees of freedom relevant for lightsail dynamics, which paves the way for direct measurement of their associated forces and torques.

Finally, we set our sights on the future where our tethered nanostructured lightsails could be brought to the macroscopic domain and released for levitation and propulsion experiments. In particular, we explore the open and critical question of structural deformations by developing a time-domain flight simulator for flexible membranes based on a mass-spring model to study their multiphysics acceleration dynamics. We find that a combination of spin-stabilization and metagrating topology redesigned to account for gyroscopic effects could enable shape stability and beam-riding stability of flat flexible lightsail membranes that are meters in size and subwavelength in thickness.

Our numerical and experimental results mark the very beginning of a long-term effort to turn laser-driven lightsails into reality. From a broader perspective, our works seek to advance a complementary avenue in optical manipulation, where complex dynamics and nontrivial optical forces can be achieved by sculpting the material rather than shaping the laser beam. This alternative approach opens the door for controlling the motion of larger objects over longer distances with laser light, which could transform the fields of robotics, manufacturing, spacecraft technology, and interstellar exploration.