Light fields from localized light sources create a variety of interesting glory phenomena when reflected by small glass spheres (microbeads). Starting with single glass spheres, the interplay of the underlying optical phenomena, i.e., refraction, reflection, diffraction, and dispersion, have been studied both theoretically and experimentally. In a second step, we considered retroreflecting screens formed by flat arrays of tiny glass spheres when exposed to divergent illumination. The arising glories show a specific ring pattern and appear freely floating around the light source as their blurred real image. The investigated glory structures will have numerous applications in sophisticated lamp and illumination designs, as demonstration tools in optical education or advertising, and artistic installations, in particular when appearing as three-dimensional objects by stereoscopic observation.
In this paper we present arguments for understanding the phenomenon of optical anisotropy in a perfectly cubic crystal such as CaF2. To simplify the discussion we review the basic arguments which seem to preclude any optical anisotropy in a cubic crystal. We discuss the range of validity and define clear conditions for deviations of optical isotropy in cubic crystals. Length and energy scales involved in the problem of radiation-matter interaction for the DUV wavelength range around 157 nm are discussed. These scaling arguments naturally force us to focus on the role of absorption processes at higher photon energies (i.e. smaller wavelengths). Especially the role of a strong, dispersing absorption, in the case of CaF2 caused by exciton excitation, is emphasized. Recent measurements of the anisotropy of the exciton resonance in CaF2 are described and discussed in terms of the small optical anisotropy.
Properties of a new rare-earth doped heavy metal oxide containing silicate glass are presented. The glass has potential for fabrication of ultra-short wideband fiber and planar waveguide amplifiers. We report specific results for a fiber amplifier geometry, discussing achieved improvements in device compactness (Giles gain g* = 210 dB/m allowing up to 100 times shorter fiber) and amplification bandwidth (50% more bandwidth in C-/L-band) compared to the conventional EDFA. We also access the potential of this material for fabrication of active planar integrated waveguide devices.
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.
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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