Photonic crystal lightsail with nonlinear reflectivity for increased stability.

Recent research has studied the feasibility of using laser radiation pressure to propel lightweight spacecraft, such as sails, at relativistic speeds. One major challenge is the effect of laser beam distortion on sail stability. We propose and investigate the use of lightsails based on Kerr nonlinear photonic crystals as a passive method for increasing sail stability. The key concept is to flatten the dependence of reflected intensity on incident intensity at the laser wavelength, using a specially designed, guided-resonance mode of the nonlinear photonic crystal. We use coupled-mode theory to analyze the resonance characteristics that yield the flattest curve. We then design a silicon nitride photonic crystal that supports a resonance with the desired properties. We show that our design simultaneously provides both high stability and high thrust on the sail, unlike designs based on linear materials.