1 April 2003 Short wavelength optical anisotropy in CaF2 caused by exciton dispersion
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Based on exact symmetry considerations one can show that a cubic system is always optically isotropic. Nevertheless even a perfectly cubic crystal such as CaF2 can show small optical anisotropy when interacting with light. Resolving this seeming contradiction leads to a phenomenon called spatial dispersion, which is an enhancement of optical anisotropy. While the initial tiny anisotropy is caused by the symmetry breaking of light, the enhancement that makes the effect observable is provided by the vicinity of a strong absorption. In semiconductors such an absorption is mainly given by the band gap but in an ionic crystal such as CaF2 the bound electron-hole pair, a deep excitonic two-particle bound state, is an additional strong absorption causing response functions to diverge as ω-ω0–1 in its vicinity, where ω0 is the bound state energy. We show that the exciton dispersion is able to explain in all details the optical anisotropy observed in CaF2 including the spatial-dispersion-induced birefringence, the so-called "intrinsic birefringence." As opposed to normal birefringence, the effect in CaF2 does not show up at large wavelengths and has seven optical axes instead of one.
© (2003) Society of Photo-Optical Instrumentation Engineers (SPIE)
Martin Letz, Martin Letz, W. Mannstadt, W. Mannstadt, Matthias Brinkmann, Matthias Brinkmann, Lutz Parthier, Lutz Parthier, G. Wehrhan, G. Wehrhan, Ewald Moersen, Ewald Moersen, } "Short wavelength optical anisotropy in CaF2 caused by exciton dispersion," Journal of Micro/Nanolithography, MEMS, and MOEMS 2(2), (1 April 2003). https://doi.org/10.1117/1.1563262 . Submission:

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