Measurement of radiation pressure and tailored momentum transfer through switchable photonic devices

Abstract

Light carries momentum, which can be transferred to an object upon reflection or absorption. The resulting force per unit area from light, so called radiation pressure, is rather weak but can have macroscopic consequences. For example, sunlight imparts momentum on dust particles causing a comet's tail to be directed away from the sun. In a microscopic world, micro/nano-mechanical transducers have become sensitive enough that radiation pressure can influence them greatly. However, photothermal effects often accompany and overwhelm the radiation pressure, complicating its measurement. In this thesis, we first show a quantitative measurement of the radiation force on an uncoated silicon nitride microcantilever in an ambient condition. We identify and separate the radiation pressure and photothermal effects through an analysis of the cantilever's frequency response. Further, we demonstrate the first measurement of a wavelength-dependent radiation pressure due to optical interference in a silicon microcantilever. We utilize an in-situ optical transmission measurement at the excitation position to determine the local optical properties.

Another interesting application of radiation pressure is a solar sail. Solar sails use solar radiation pressure for propulsion and offer an opportunity for propellant-free space travel, enabling long-term and long-distance missions that are impossible with traditional methods. Although solar sail propulsion alleviates the need to carry chemical fuel, attitude control and steering are still performed using traditional methods involving reaction wheels and propellant ejection. In the second part of the thesis, we present a steerable solar sail concept based on a polymer dispersed liquid crystal (PDLC) device that switches between transparent and scattering states, enabling attitude control without mechanically moving parts or chemical propellant. Devices are fabricated and characterized (transmission, reflection, absorption and scattering) over the visible and near infrared range of the solar spectrum (400 nm - 1100 nm) and are found to outperform previous designs by more than a factor of four in terms of over-all weighted momentum switchablility between on and off states. Devices require no power in the diffusely reflective state and dissipate less than 0.5 mW/cm^2 while in the on state, showing great potential as a low-power switching mechanism for solar sail attitude control.