Transparent glass-ceramics for optical applications
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Introduction Glass-ceramics for optical applications are easy and flexible to manufacture, and are more cost efficient than corresponding single crystals and optical ceramics. Higher optical quality, larger samples, and higher doping concentrations can be achieved in glass-ceramics. Some crystals precipitated in glass-ceramics are difficult or impossible to obtain in single crystal form. Moreover, a variety of metastable and stable crystals can be obtained in the same glass by appropriate heat treatments. Glass-ceramics, in contrast to glasses, have optical properties similar to those of the single crystals, including welldefined structures with long-range order that is missing in glasses, specific properties determined by active ion position in the crystal structure leading to relatively narrow spectroscopic absorption and emission lines, and unique functionality inherent to noncentrosymmetric crystals (e.g., second-harmonic generation, piezoelectricity, electro-optic effects). One drawback of glass-ceramics is that active ions are not only located in the desired crystal phase, but may also be distributed between and at the interfaces of both crystalline and amorphous phases. Optical glass-ceramics are produced by melt-quenching, sintercrystallization,1 and laser patterning.2,3 Glass-ceramic fibers have also been developed.3–5 This article concentrates on
materials that are not only promising optical glass-ceramics, but also within the field of our research for many years.
Light scattering in glass-ceramics Some demanding optical applications requirements can be met by transparent glass-ceramics. All glass-ceramics have inhomogeneous structure since they contain crystals distributed in glass. This inhomogeneity causes light scattering. In glassceramics for transparent optical elements, scattering needs to be reduced to a minimum. However, for diffuse reflection standards, light scattering is the determining property. Previously, it was stated6–8 that the scattering coefficient (turbidity) of glass-ceramics varies in a wide interval of wavelengths λ (e.g., in the range of visible light) according to the power law λ−p, where (−p) is the exponent of constant value, and p ≤ 4. If the crystals are randomly distributed in the glass and are much smaller than the wavelength, then p = 4 (Rayleigh scattering). Our studies9,10 as well as analysis of published data11,12 show that glass-ceramics frequently demonstrate “anomalous light scattering”13 with p > 4 (up to 9). This effect is a result of interference of light scattered by different crystals demonstrating short-range ordering in their mutual arrangement.14 Interference can reduce light scattering by several
O. Dymshits, NITIOM Vavilov State Optical Institute, Russia; [email protected] M. Shepilov, NITIOM Vavilov State Optical Institute, Russia; [email protected] A. Zhilin, NITIOM Vavilov State Optical Institute, Russia; [email protected] doi:10.1557/mrs.2017.29
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• VOLUME 42 • MARCH 2017 • www.mrs.org/bulletin 2017ofMaterials Research Downloaded MRS fromBULLETIN https:/ww
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