Optical application and measurement of torque on microparticles of isotropic nonabsorbing material

We show how it is possible to controllably rotate or align microscopic particles of isotropic nonabsorbing material in a TEM00 Gaussian beam trap, with simultaneous measurement of the applied torque using purely optical means. This is a simple and general method of rotation, requiring only that the particle is elongated along one direction. Thus, this method can be used to rotate or align a wide range of naturally occurring particles. The ability to measure the applied torque enables the use of this method as a quantitative tool--the rotational equivalent of optical tweezers based force measurement. As well as being of particular value for the rotation of biological specimens, this method is also suitable for the development of optically-driven micromachines.Comment: 8 pages, 6 figure

Globally-Linked Vortex Clusters in Trapped Wave Fields

We put forward the existence of a rich variety of fully stationary vortex structures, termed H-clusters, made of an increasing number of vortices nested in paraxial wave fields confined by trapping potentials. However, we show that the constituent vortices are globally linked, rather than products of independent vortices. Also, they always feature a monopolar global wave front and exist in nonlinear systems, such as Bose-Einstein condensates. Clusters with multipolar global wave fronts are non-stationary or at best flipping.Comment: 4 pages, 5 PostScript figure

Optically levitated stylus for scanning force microscopy

A single-beam optical gradient trap could potentially be used to hold a stylus for scanning force microscopy. An optically trapped particle can be modeled as a harmonic oscillator and can thus be characterised by its spring constant. This paper determines the spring constants for particles trapped in both Gaussian and donut beams, using a novel technique of measurement of the backscattered light. A portion of the backscattered laser light from the trapped stylus is collected by a photodiode. The effective spring constant of the optically trapped 'cantilever' can then be determined by fitting the experimentally determined power spectrum to a theoretical spectrum. The measured spring constants are up to several orders of magnitude smaller than available scanning force microscope cantilevers, indicating that an optically trapped stylus may be suitable for scanning of soft samples. With appropriate choice of wavelength, the laser light will not damage the sample

Vortex-beam-induced lensing of off-resonance probe beam in Rb vapor

The nonlinear optical effects in an atomic gas with a LG donut beam are investigated. The LG donut beam was produced with a Ti:sapphire laser using a computer-generated hologram. The nonlinear medium was Rb atomic vapor in a cylindrical Pyrex cell 10 cm long and 2.5 cm in diameter, heated to approximately 120 °C. The power of the donut beam at the cell window was 450 mW. By tuning the laser frequency close to the Rb atomic D line at 780.1 nm, resonant enhancement of the nonlinearity are obtained

Angular momentum transfer from a circularly polarized Gaussian laser beam

The trapping of absorbing particles with the use of a Gaussian laser beam has been proposed. The force from an absorbing particle from radiation pressure is dependent on the angle in which light impinges. Modeling shows that two-dimensional trapping can be achieved if the particle is in the region of the focused beam before the waist, and the curvature of the wave front is such that the resulting force can be resolved into a radially inward force and a force in the direction of beam propagation. A Gaussian beam can also be used and move these absorbing particles providing an opportunity to the study the effect of the polarization of the beam on the trapped particle. Trapped CuO particles on the order of a few micron in a Gaussian beam is presented

Optically driven oscillations of ellipsoidal particles. Part I: Experimental observations

We report experimental observations of the mechanical effects of light on ellipsoidal micrometresized dielectric particles, in water as the continuous medium. The particles, made of polystyrene, have shapes varying between near disk-like (aspect ratio k = 0.2) to very elongated needle-like (k = 8). Rather than the very tightly focused beam geometry of optical tweezers, we use a moderately focused laser beam to manipulate particles individually by optical levitation. The geometry allows us varying the longitudinal position of the particle, and to capture images perpendicular to the beam axis. Experiments show that moderate-k particles are radially trapped with their long axis lying parallel to the beam. Conversely, elongated (k > 3) or flattened (k < 0.3) ellipsoids never come to rest, and permanently “dance” around the beam, through coupled translation-rotation motions....Méthodes avancées pour la caractérisation optique de systèmes particulaires complexesEuropean ITN COMPLOID