STFC, Central Laser Facility, Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire, OX11 0FA, U.K..
Phys Chem Chem Phys. 2013 Dec 21;15(47):20735-41. doi: 10.1039/c3cp53498g. Epub 2013 Nov 6.
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.
描述了一种测量空气中被光阱捕获的固体颗粒的折射率色散与波长关系的方法。了解固体颗粒的折射特性对于气溶胶的研究至关重要，无论是在实验室还是在大气中进行气候研究。使用垂直对准的聚焦激光束的反向传播配置，将微米级的聚苯乙烯珠粒在空气中用光阱捕获。使用宽带发光二极管（LED）的白光照射每个珠粒，并将珠粒内的弹性散射收集到光谱仪上。分析得到的 Mie 光谱，以准确确定聚苯乙烯珠粒的半径为±0.4nm，折射率值在 480-700nm 的波长范围内为±0.0005。我们证明，光学捕获结合弹性散射可以既准确地测量空气中悬浮的聚苯乙烯珠粒的大小，又可以确定它们的波长相关折射率。折射率色散与在水相分散体中报道的聚苯乙烯珠粒的值非常吻合。我们的结果还表明，在商业样品中，聚苯乙烯珠粒之间的折射率存在变化。测量到的变化突出表明，在使用聚苯乙烯珠粒作为校准气溶胶时必须小心。
