Lightsail Spacecraft and Radiation Pressure Optomechanics

The rich history of employing light to manipulate matter beginning with Ashkin's optical tweezers has enabled trapping and manipulation of simple subwavelength objects using optical gradient forces from a focused laser beam. However, controlling and directing objects whose size is larger than the wavelength requires design of optical force gradients within object itself. Then using a nearly collimated beam with a waist size matching the object dimensions, optical forces can made to exert precisely control with planar films and nanostructured devices. This approach can be scaled from microscopic to macroscopic levels, opening the path for light-matter interactions to control objects ranging from millimeters to meters in size, such as radiation pressure propelled lightsails.

Ultrathin lightsails, propelled to relativistic velocities by laser radiation pressure, are being actively explored to enable a new generation of interstellar spacecraft probes, spearheaded by the Starshot Initiative. While the achievable speeds of conventional spacecraft technology are limited by ejection of chemical reaction mass, light momentum serves as an alternative, external propellant enabling ultralight spacecraft to reach significantly higher velocities, allowing the reduction of travel time by up to three orders of magnitude for interstellar missions. In contrast to solar sails, which rely on radiation pressure from the broadband spectrum of sunlight and its limited irradiance for propulsion, laser-driven lightsails could be accelerated to extreme velocities if propelled by an earth-based kilometer-sized laser array with a power density of ~ MW/cm2 and ultralight weight of a few grams. We are exploring the fundamental optomechanics of radiation pressure force interaction with nanoscale structures, and applications to development of laser-driven lightsail spacecraft.