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Extinction cross section measurements for a single optically trapped particle

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Original languageEnglish
Title of host publicationProceedings of SPIE - The International Society for Optical Engineering
Publisher or commissioning bodySociety of Photo-Optical Instrumentation Engineers (SPIE)
ISBN (Print)9781628417142, 9781628417142
DatePublished - 2015
EventOptical Trapping and Optical Micromanipulation XII - San Diego, United States
Duration: 9 Aug 201512 Aug 2015


ConferenceOptical Trapping and Optical Micromanipulation XII
CountryUnited States
CitySan Diego


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, I 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 I ext with particle radius are modelled using cavity standing wave Mie simulations and a particleâ™s varying refractive index with changing relative humidity is determined. We demonstrate I 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.

    Research areas

  • aerosol, Bessel beam optical trap, cavity ring-down spectroscopy, extinction cross section, light scattering, Lorenz-Mie theory, radiative forcing, refractive index


Optical Trapping and Optical Micromanipulation XII

Duration9 Aug 201512 Aug 2015
CitySan Diego
CountryUnited States
SponsorsThe Society of Photo-Optical Instrumentation Engineers (SPIE) (External organisation)

Event: Conference



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