Trapping

Abstract: Summary form only given. We demonstrate that a low-index dielectric particle can be stably trapped in three-dimensions using a stationary, focused Gaussian beam containing an optical vortex. This work is, to our knowledge, the first demonstration of three-dimensional trapping of a spherical low-index particle using a single, stationary beam. Vortex traps allow the trapping of low- and high-index particles with less risk of damage and better isolation.

Abstract: Metallic objects reflect light and have generally been considered poor candidates for optical traps, particularly with optical tweezers, which rely on a gradient force to provide trapping. We demonstrate that stable trapping can occur with optical tweezers when they are used with small metallic Rayleigh particles. In this size regime, the scattering pictures for metals and dielectrics are similar, and the larger polarizability of metals implies that trapping forces are greater. The latter fact makes the use of metal particles attractive for certain biological applications. Comparison of trapping forces for latex and gold spheres demonstrates that the gradient force is the major determinant of trapping strength and that competing effects, such as scattering or radiometric forces, are relatively minor.

Abstract: The complex hydrogen trapping characteristics of iron-titanium-carbon alloys, contain-ing both reversible and irreversible traps have been fully analyzed. The key to this quantitative analysis is a complete identification of the type and number of each operating trap. The trapping parameters were obtained from an analysis of the relevant hydrogen permeation transients. Titanium substitutional atoms have been shown to be reversible, low occupancy traps with an interaction energy with hydrogen,E (Ti-H), of 0.27 eV. Typi-cal rate constants for these alloys are; a hydrogen capture rate constant of approximately 10-24 cm3/atom .s a release rate constant of approximately 10-3 s-1 and a trapping rate of the order of 1015 atoms, H/cm3 .s. TiC particles are irreversible traps with a large oc-cupancy and an interaction energy, .E(TiC-H), of 0.98 eV. The irreversible trapping parameters are calculated from the first permeation transient, where mixed trapping oc-curs. The trapping kinetics are about an order of magnitude faster than when only rever-sible trapping exists. The role of trapping on the effective diffusivity of hydrogen is dis-cussed as is, briefly, its role in affecting hydrogen-induced damage. Finally, guidelines are given to permit the trapping behavior of more general alloys to be analyzed.