Trapping and guiding microscopic particles with light-induced forces

Abstract

Contact-free trapping and manipulation of light absorbing micrometer and nanometer-scale particles in air and in vacuum utilises radiation pressure, which results from momentum transfer from photons, and a pressure-dependent thermal force, caused by momentum transfer from gas molecules to the confined particles. Both forces are linearly proportional to the illuminating laser intensity, and both push the particles towards regions of lower intensity. While the radiation pressure of light was predicted and described more than a century ago, the theory of thermal forces, the so called photophoretic force, is still under development. It depends on a number of poorly described factors, such as the temperature gradient across the illuminated particle and thermal creep of heated gas along the particle surface due to temperature and pressure gradients.

In this thesis I use doughnut-shaped structured laser beams to levitate and guide light-absorbing micron-size particles aiming to uncover the optically induced forces in air at variable pressure ranging from 10-2000 millibar. First, I designed and built a counter-propagating optical pipeline to uncover the influence of polarisation on the particle movement. Second, I designed and constructed a vertically directed diverging vortex beam trap, a `funnel' trap, to conduct a quantitative evaluation of the photophoretic force and trapping stiffness by levitating graphite particles and carbon-coated glass shells of calibrated sizes in a carefully characterised vortex beam. Third, from the measured size of the particles and the position of the particle in the beam on the one hand, and the known density of the particles and the intensity distribution of the funnel trap on the other hand, I characterised the optically induced thermal forces in the axial and transverse directions. Fourth, I compared the contribution of thermal force to the light-pressure force and their dependence on atmospheric pressure. Based on the results of my experiments I determined the parameter space for guiding particles with hollow-core vortex and Bessel beams, taking into account the particle speed, size, and offset from the laser axis, all linked to the optical beam properties such as beam divergence, optical polarisation and power. The results of this thesis are used in the development of a touch-free optical system for pin-point delivery of macromolecules to the X-ray focal spot at the Free Electron Laser facility at the DESY (Deutsches Elektronen-Synchrotron) synchrotron in Hamburg, Germany, for coherent diffractive imaging experiments on nanometer-scale morphology. I conclude with a discussion of avenues for future work in contact-free manipulating of particles with structured laser beams to enhance significantly the efficiency of nanometer-scale morphology of proteins and biomolecules.