Susan E. Skelton

Trapping volume control in optical tweezers using cylindrical vector beams.

We present the result of an investigation into the optical trapping of spherical microparticles using laser beams with a spatially inhomogeneous polarization direction [cylindrical vector beams (CVBs)]. We perform three-dimensional tracking of the Brownian fluctuations in the position of a trapped particle and extract the trap spring constants. We characterize the trap geometry by the aspect ratio of spring constants in the directions transverse and parallel to the beam propagation direction and evaluate this figure of merit as a function of polarization angle. We show that the additional degree of freedom present in CVBs allows us to control the optical trap strength and geometry by adjusting only the polarization of the trapping beam. Experimental results are compared with a theoretical model of optical trapping using CVBs derived from electromagnetic scattering theory in the T-matrix framework.

Trapping and deformation of microbubbles in a dual-beam fibre-optic trap

We present results of numerical calculations to evaluate the performance of a dual-beam fibre-optic trap for low refractive index particles such as ultrasound contrast agent microbubbles. Using a geometrical optics approach, we determine the range of parameters of microbubble size and beam dimensions over which the optical trap is stable and evaluate the trapping forces and spring constants. Additionally, we calculate the optically induced stress profile over the surface of the microbubble and evaluate the resulting deformation of the microbubble using elastic membrane theory. Our results suggest that such an experiment could be a useful tool for quantifying the mechanical properties (elastic modulus) of the shell material of an ultrasound contrast agent microbubble.

Optically bound particle structures in evanescent wave traps

We present a study of the manipulation of microparticles and the formation of optically bound structures of particles in evanescent wave traps. Two trapping geometries are considered: the first is a surface trap where the evanescent field above a glass prism is formed by the interference of a number of laser beams incident on the prism-water interface; the second uses the evanescent field surrounding a biconical tapered optical fiber that has been stretched to produce a waist of submicron diameter. In the surface trap we observe optical binding of microparticles in to one-dimensional chain structures. In the tapered optical fiber trap we demonstrate both particle transport for long distances along the fiber, and the formation of stable arrays of particles. In both experiments we use video microscopy to track the particle locations and make quantitative measures of the particle dynamics. The experimental studies of particle structures are complemented by light scattering calculations based on Mie theory to infer how the geometries of the observed particle structures are controlled by the underlying incident and scattered optical fields.

Optical trapping and optical binding using cylindrical vector beams

We report on the use of cylindrical vector beams for optical manipulation of micron and sub-micron sized particles using the methods of a single-beam gradient force trap (optical tweezers) and an evanescent-feld surface trap (optical binding). We have demonstrated a stable interferometric method for the synthesis of cylindrical vector beams (CVBs), and present measurements demonstrating polarization-controlled focal volume shaping using CVBs in an optical tweezers. Furthermore we show how appropriate combinations of CVBs corresponding to superpositions of optical fbre modes can be used for controlled trapping and traffcking of micro- and nanoparticles along a tapered optical fbre.

Shaping of the trapping volume in optical tweezers using cylindrical vector beams

We present the result of an investigation into the optical trapping of micropaticles using laser beams with a spatially inhomogeneous polarization (cylindrical vector beams). We perform three-dimensional tracking of the Brownian fluctuations in position of a trapped particle and extract the trap spring constants. We characterize the trap geometry by the aspect ratio of spring constants in the directions transverse and parallel to the beam propagation direction and evaluate this figure of merit as a function of polarization angle. We show that the additional degree of freedom present in cylindrical vector beams (CVBs) allows us to control the optical trap strength and geometry by adjusting the polarization of the trapping beam only. Experimental results are compared with a theoretical model of optical trapping using CVBs derived from electromagnetic scattering theory in the T-matrix framework.

Optical squeezing of microbubbles: ray optics and Mie scattering calculations

We consider the trapping of low refractive index objects, such as ultrasound contrast agent microbubbles, in a dual-beam fibre-optic trap. We confirm numerically that such a configuration results in stable trapping and we present the calculated trapping forces and spring constants. Furthermore we calculate the photonic stress profile over the surface of the trapped microbubble using both ray optics and Mie scattering approaches, and compare the results. We then find the optical stress-induced deformation of the microbubble for both the ray optics and Mie scattering stress profiles using linear elastic membrane theory. We suggest that this method could be a useful tool for quantifying the mechanical properties of the shell material of an ultrasound contrast agent microbubble.

Micro- and Nanoparticle Optical Trapping Using Cylindrical Vector Beams

We report on the optical trapping of a number of micro- and nanoparticles using beams with a non-uniform state of polarization and show how geometry of the trap can be shaped by the polarization state.