Extinction cross section measurements for a single optically trapped particle

Michael I. Cotterell; Thomas C. Preston; Bernard J. Mason; Andrew J. Orr-Ewing; Jonathan P. Reid

doi:10.1117/12.2189174

25 August 2015 Extinction cross section measurements for a single optically trapped particle

Michael I. Cotterell, Thomas C. Preston, Bernard J. Mason, Andrew J. Orr-Ewing, Jonathan P. Reid

Proceedings Volume 9548, Optical Trapping and Optical Micromanipulation XII; 95480C (2015) https://doi.org/10.1117/12.2189174
Event: SPIE Nanoscience + Engineering, 2015, San Diego, California, United States

Abstract

Bessel beam (BB) optical traps have become widely used to confine single and multiple aerosol particles across a broad range of sizes, from a few microns to < 200 nm in radius. The radiation pressure force exerted by the core of a single, zeroth-order BB incident on a particle can be balanced by a counter-propagating gas flow, allowing a single particle to be trapped indefinitely. The pseudo non-diffracting nature of BBs enables particles to be confined over macroscopic distances along the BB core propagation length; the position of the particle along this length can be finely controlled by variation of the BB laser power. This latter property is exploited to optimize the particle position at the center of the TEM₀₀ mode of a high finesse optical cavity, allowing cavity ring-down spectroscopy (CRDS) to be performed on single aerosol particles and their optical extinction cross section, σ_ext, measured. Further, the variation in the light from the illuminating BB elastically scattered by the particle is recorded as a function of scattering angle. Such intensity distributions are fitted to Lorenz-Mie theory to determine the particle radius. The trends in σ_ext with particle radius are modelled using cavity standing wave Mie simulations and a particle’s varying refractive index with changing relative humidity is determined. We demonstrate σ_ext measurements on individual sub-micrometer aerosol particles and determine the lowest limit in particle size that can be probed by this technique. The BB-CRDS method will play a key role in reducing the uncertainty associated with atmospheric aerosol radiative forcing, which remains among the largest uncertainties in climate modelling.

Conference Presentation

Citation Download Citation

Michael I. Cotterell, Thomas C. Preston, Bernard J. Mason, Andrew J. Orr-Ewing, and Jonathan P. Reid "Extinction cross section measurements for a single optically trapped particle", Proc. SPIE 9548, Optical Trapping and Optical Micromanipulation XII, 95480C (25 August 2015); https://doi.org/10.1117/12.2189174

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