Abstract

For homogeneous spheres irradiated by infinite, electromagnetic plane waves (Fig. 1), existence of extremely strong sharp maxima in the extinction efficiency curves as a function of size parameter was found analytically by the so-called Mie theory for a long time.¹ Excellent coincidences of the maxima of the calculated extinction efficiency as a function of wavelength for a homogeneous sphere with the spikes on the spectra of a laser power to levitate a homogeneous spherical particle at a fixed height when varying the wavelength of the laser beam have been found.^2,3 Since the volume averaged electric density inside a sphere at a resonant state is several orders higher than that inside a sphere at a nonresonant state,⁴ even a laser beam of small intensity may cause nonelastic emissions. In fact, spikes on spectra of fluorescence emission and stimulated Raman scattering coincide excellently with the maxima of the calculated extinction efficiency spectra by using the Mie theory.³ According to Chylek,⁴ the basic period (Δx) of the double-peak structure on the Q_ext - x curve is a monotonous function of refractive index m, given that x≫1, where the size parameter is defined by x = 2πα/λ, λ is the wavelength. Thereby, the MDRs have been used to measure sizes and refractive indices of spherical particles and optical fibres with an accuracy several orders higher than any other optical methods. The MDRs is seen to be a powerful tool in the field of particle characterisation.

© 1994 IEEE