Optical spanners and improved optical tweezers

Abstract

This thesis describes the experimental and theoretical work that investigated the transfer of orbital angular momentum from light to matter. This was achieved by combining two established areas of laser physics which were "optical tweezers" and Laguerre-Gaussian laser modes. The optical tweezers are essentially a tightly focussed laser beam from a high numerical aperture microscope objective lens, which traps particles in three dimensions just below the beam focus. By incorporating a Laguerre- Gaussian laser mode into the tweezers system, the trapping efficiency was doubled. These improved optical tweezers have been successfully demonstrated both theoretically and experimentally. In addition to the spin angular momentum which is associated with the polarisation state, the Laguerre-Gaussian laser modes also possess orbital angular momentum. The "optical spanners" utilised this property by transferring orbital angular momentum from the laser beam to the trapped particle, causing it to rotate whilst being held in the optical trap. This effect was theoretically modelled and experimentally observed. Using the optical spanners, the spin angular momentum of the laser was used to directly cancel the orbital angular momentum in the beam, which was observed as a cessation in rotation of the trapped particle. This demonstrated the mechanical equivalence of the spin and orbital components of angular momentum in a light beam, and gave experimental evidence for the well defined nature of the orbital angular momentum present in Laguerre-Gaussian laser modes.