Abstract
A method is described to measure the refractive index dispersion with wavelength of optically trapped solid particles in air. Knowledge of the refraction properties of solid particles is critical for the study of aerosol; both in the laboratory and in the atmosphere for climate studies. Single micron-sized polystyrene beads were optically trapped in air using a vertically aligned counter-propagating configuration of focussed laser beams. Each bead was illuminated using white light from a broadband light emitting diode (LED) and elastic scattering within the bead was collected onto a spectrograph. The resulting Mie spectra were analysed to accurately determine polystyrene bead radii to ±0.4 nm and values of the refractive index to ±0.0005 over a wavelength range of 480-700 nm. We demonstrate that optical trapping combined with elastic scattering can be used to both accurately size polystyrene beads suspended in air and determine their wavelength dependent refractive index. The refractive index dispersions are in close agreement with reported values for polystyrene beads in aqueous dispersion. Our results also demonstrate a variation in the refractive index of polystyrene, from bead to bead, in a commercial sample. The measured variation highlights that care must be taken when using polystyrene beads as a calibration aerosol.
Highlights
Atmospheric aerosol is known to impact on many areas of life, such as air pollution, human health, and modern climate change.[1,2] The climate of the Earth is directly affected by aerosols scattering and absorbing solar radiation and indirectly affected by aerosols acting as cloud condensation nuclei
We demonstrate that optical trapping combined with elastic scattering can be used to both accurately size polystyrene beads suspended in air and determine their wavelength dependent refractive index
Building on the initial work of Ward et al.[10] and Moore et al.[11] this study demonstrates that light scattering from a white light emitting diode (LED) and application of Mie theory can be used to accurately size polystyrene beads suspended in air and determine the refractive index dispersion over a broadband wavelength range
Summary
Atmospheric aerosol is known to impact on many areas of life, such as air pollution, human health, and modern climate change.[1,2] The climate of the Earth is directly affected by aerosols scattering and absorbing solar radiation and indirectly affected by aerosols acting as cloud condensation nuclei. The study presented here demonstrates the technique of optical trapping of polystyrene beads in air combined with white-light scattering[10,11,12,13] and the application of Mie theory,[14,15,16] to determine the radius and wavelength-dependent values of refractive index. Bateman et al.[5] used transmission photospectrometer measurements to determine the size and refractive index dispersion of a suspension of polystyrene beads in water as preliminary work towards the characterisation of biological hydrosols. Building on the initial work of Ward et al.[10] and Moore et al.[11] this study demonstrates that light scattering from a white LED and application of Mie theory can be used to accurately size polystyrene beads suspended in air and determine the refractive index dispersion over a broadband wavelength range
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