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30 July 2002 Spatial dispersion in CaF2 caused by the vicinity of an excitonic bound state
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The microscopic mechanism beyond the optical anisotropy of an ionic crystal which occurs for short wavelengths is investigated. The electron-hole, two particle propagator and its analytical behavior close to the band edge of the one particle continuum plays a major role for the mechanism of this optical anisotropy. Especially for an ionic crystal the two particle bound state, the exciton, is of special importance. In this way we argue that the so called intrinsic birefringence in CaF2 is neither intrinsic to the material nor it is birefringence. Instead it is spatial dispersion caused by the vicinity of a dispersive optical absorption given by the excitonic bound state. We propose a model which connects the bound state dispersion with the band structure and a model potential for a screened coulomb interaction. Based on these considerations we predict a wavelength dependence of the dielectric function approaching close to the bound state level (epsilon approximately ((lambda) - (lambda) 0)-1, where (lambda) 0 is the wavelength of the excitonic bound state level.
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Martin Letz, W. Mannstadt, Matthias Brinkmann, and Ewald Moersen "Spatial dispersion in CaF2 caused by the vicinity of an excitonic bound state", Proc. SPIE 4691, Optical Microlithography XV, (30 July 2002);

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