Abstract

Small spherical particles (from 5 to 30 µm) were charged and suspended in an electric quadrupole trap which was mounted in a chamber connected to a vacuum pump. The particles were coated with a liquid having a relatively low vapor pressure at normal atmosphere (such as glycerine). When the chamber was evacuated, light from a He-Ne laser scattered at right angles from the coated particle was monitored as the liquid coating shrank due to evaporation. Sharp variations in the scattered light intensity as a function of changing particle size are termed Mie resonances or partial wave resonances occurring when the wavelength and particle radius match a spherical cavity-type mode. For the large layer thickness, this resonance spectrum matches that of the homogeneous liquid droplet (both experimentally and computationally). However, significant deviations occur when the layer thickness comes within a few optical wavelengths of the core surface. Observations were made for glass, mercury, and polystyrene cores. An exact computational model for concentric spheres with differing indices of refraction accounts for much of the observed spectra. Discrepancies are discussed in terms of the model restrictions of concentricity and step function layer-core boundary.

© 1989 Optical Society of America