Optical Pulling, Optical Deflection, and Optical Trapping of Single Airborne Particles using Negative Photophoresis

Abstract

Optical manipulation is the use of light to control the motion of objects. This thesis focuses on the development of optical transportation technology via optical pulling with application to the characterization of single airborne particles. Optical pulling, optical deflection, and optical trapping of single airborne particles using negative photophoresis force were demonstrated for the first time. Optical pulling and deflection were demonstrated for multiple absorbing particles. Raman scattering spectra of optically pulled particles were collected, thus demonstrating a method for particle identification. Using photography, digital video images were collected and analyzed for optically pulled and deflected particles. Optically pulled particles moved at a constant velocity for a given laser beam power. The velocity of optically pulled particles increased nonlinearly as laser power increased, approaching a saturation limitation. This saturation limitation was found to change with the atmospheric pressure. The efficiency for optical deflection was found to increase as the beam power ratio, deflection to pulling, increased approaching the limit of 100%. Using a fast photodiode, scattering light signals were collected from optically pulled and deflected particles, which were found to be periodic, suggesting a constant rotational frequency in the motion. This rotational frequency was found to depend on laser beam power. From the video images and scattering light data, theoretical models for optical pulling and optical deflection were derived. Optical manipulation and characterization of single airborne particles is of significance for the analysis related to air pollution, human health, and global climate change.