The long-wave optical modes of vibration in an ionic crystal sphere have been determined, including retardation of the Coulomb forces. These modes, which correspond to coupled excitations of phonons and photons, are also known as polaritons. Their frequencies are complex, the imaginary parts arising from both anharmonic and radiative damping; hence they are virtual modes. It is found that the mode frequencies depend on the radius of the sphere only if retardation is included. The absorption and extinction cross sections for spheres of various sizes are calculated as a function of the frequency of the incident light, and it is shown how the structure in the cross sections is related to the properties of the virtual modes. The theory is used to explain the position and width of an optical absorption peak measured in a polyethylene film containing UO2 particles.
© 1968 Optical Society of America
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