Imaging Science Ph.D. Defense: Lucy Ying-Ju Chu
Ph.D. Dissertation Defense
Measurement of Radiation Pressure on Diffractive Films
Lucy Ying-Ju Chu
Imaging Science Ph.D. Candidate
Chester F. Carlson Center for Imaging Science, RIT
Advisor: Dr. Grover Swartzlander
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Abstract:
One of the few ways to reach distant stars is by radiation pressure, in which photon momentum is harnessed from free sunlight or extraordinarily powerful laser systems. Large but low mass light-driven sails reflect photons and transfer momentum to the sailcraft, providing large velocity from continuous acceleration. Over the past decade, demonstrative reflective light sail missions were enabled by cost-efficient small satellites and the emerging private space economy. The maneuver of these metal-coated polyimide films is mechanically cumbersome because the sail must be rapidly tilted towards and away from the sun line during navigation. Modern diffractive films such as high-efficiency single-order gratings, liquid crystal cycloidal diffractive wave-plates, and meta-material gratings may provide enhanced control schemes with radiation pressure tangential to the sail surface. The potential to replace motorized control components with all-optical components also offers a reduction in mass and the risk of mission failure. Before spending considerable resources sending a rocket to deploy a solar sail, it must be verified that the sail will behave as expected in a lab on Earth. This is challenging since Earth’s gravity, electro-static forces, thermal effects, and environment vibrations exceed the relatively weak effects of radiation pressure. In this dissertation, we designed and constructed an opto-mechanical torsional pendulum in a vacuum environment that measures radiation pressure on diffraction films with sub-nano-Newton precision. With the system, we observed a large component of force parallel to the surface of a diffraction grating owing to “grating momentum”. Furthermore, we proposed, designed, and validated Diffractive Beam-Rider structures that enable spatially varying forces to pull and align the sailcraft to the beam. We parametrically “cooled” the turbulence on the Beam-Rider, which demonstrates its potential for implementation on a laser sail. This experimental stability verification was performed on a centimeter-sized bi-grating and a diffractive axicon with one and two-dimensional restoring force, respectively.
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