Fiber based optical trapping of aerosols

Continuous-feed optical sorting of aerosol particles

We consider the problem of sorting, by size, spherical particles of order 100 nm radius. The scheme we analyze consists of a heterogeneous stream of spherical particles flowing at an oblique angle across an optical Gaussian mode standing wave. Sorting is achieved by the combined spatial and size dependencies of the optical force. Particles of all sizes enter the flow at a point, but exit at different locations depending on size. Exiting particles may be detected optically or separated for further processing. The scheme has the advantages of accommodating a high throughput, producing a continuous stream of continuously dispersed particles, and exhibiting excellent size resolution. We performed detailed Monte Carlo simulations of particle trajectories through the optical field under the influence of convective air flow. We also developed a method for deriving effective velocities and diffusion constants from the Fokker-Planck equation that can generate equivalent results much more quickly. With an optical wavelength of 1064 nm, polystyrene particles with radii in the neighborhood of 275 nm, for which the optical force vanishes, may be sorted with a resolution below 1 nm.

Optical binding in white light

We experimentally demonstrate, for the first time, binding of aerosols of various sizes and shapes in white light. The optomechancial interaction between particles is long range and is in the underdamped regime. Incoherency allows mitigation of interference fringes to enable monotonically changing the distance between particles from 60 μm to contact, constituting a parametrically controlled testbed for transition studies at new scales.

Aerosol droplet optical trap loading using surface acoustic wave nebulization

We demonstrate the use of surface acoustic wave nebulization (SAWN) to load optical traps. We show that the droplets sizes produced can be tuned by altering the RF frequency applied to the devices, which leads to more control over the sizes of trapped particles. Typically the size distribution of the liquid aerosols delivered using SAWN is smaller than via a standard commercial nebulization device. The ability to trap a range of liquids or small solid particles, not readily accessible using other ultrasonic devices, is also demonstrated both in optical tweezers and dual beam fiber traps.

Towards an integrated optical single aerosol particle lab

We present a manipulation and characterization system for single airborne particles which is integrated onto a microscope slide. Trapped particles are manipulated by means of radiation pressure and characterized by cavity enhanced Raman spectroscopy. Optical fibers are used to deliver the trapping laser light as well as to collect the Raman scattered light, allowing for a flexible usage of the device. The system features a sample chamber which is separated from an aerosol-flooded injection chamber by means of a light guiding glass-capillary. The coupling of this device with an aerosol optical tweezers setup to selectively load its trapping sites is demonstrated. Finally, a route towards chip-integrated handling and processing of multiple particles is shown and the first results are presented.

Longitudinal optical trapping and sizing of aerosol droplets

We present evidence that aerosol droplets, approximately 1-2microm in diameter, can be optically bound over a 4mm distance within a volume formed by the overlap of the central cores and rings of two counterpropagating Bessel beams. The sizes of the individual polydisperse aerosol particles can be estimated from the angular variation of the elastic light scattering. Scattered light from the two orthogonally polarized trapping beams and from a Gaussian probe beam of different wavelength can be used to provide independent estimations of size. The coalescence of two droplets was observed and characterized.

Selective trapping of multiple particles by volume speckle field

We suggest a new approach for selective trapping of light absorbing particles in gases by multiple optical bottle-beam-like traps created by volume speckle field. We demonstrate stable simultaneous confinement of a few thousand micro-particles in air with a single lowpower laser beam. The size distribution of trapped particles exhibits a narrow peak near the average size of an optical speckle. Thus, the speckleformed traps act as a sieve with the holes selecting particles of a similar size.

Optical guiding of absorbing nanoclusters in air

We suggest a novel approach in all-optical trapping employing a photophoretic force for manipulation of absorbing particles in open air. We demonstrate experimentally the robust three-dimensional guiding, over the distances of a few millimeters, of agglomerates of carbon nanoparticles with the size spanned from 100 nm to 10 microm, as well as their acceleration up to velocities of 1 cm/sec. We achieve stable positioning and guiding of particles as well as simultaneous trapping of a large number of particles in a dual-beam optical trap created by two counter-propagating and co-rotating optical vortex beams.

Fiber cross-sectional imaging by manually controlled low coherence light sources

We couple a variable-coherence light beam into a multimode optical fiber and observe the fiber cross-sectional images. The variation in the fiber imaging is explored as we change the degree of optical coherence of the incident light. Low coherence light is shown to be capable of improving the quality of the fiber images. Various mode patterns of a multimode optical fiber are also shown numerically and experimentally to elucidate the fiber coupling characteristics.