​Optical properties of sapphire crystals (2)

Optical properties of sapphire crystals (2)

X- and γ-irradiation of sapphire increases the optical absorption; n- and γ-irradiation produces color centers and corresponding absorption bands, which can be eliminated by high temperature annealing.

In the infrared region of the spectrum, sapphire has high transmission in the wavelength range of 1.0-5.5 μm. In the range of 1.0-4.0 μm, the transmission value is about 85~86% (Fig. 2.39). From >3 μm, the transmission value increases with It decreases and decreases with increasing temperature (Fig. 2.40). At T<80K, the transmission of sapphire increases in the boundary portion of the IR (Fig. 2.40). Due to this effect, sapphire can be used as a cool inner window in IR measurements.

Because sapphire has high transmission without considering Fresnel losses, it is used in other optical materials where an anti-reflection coating is required. These coatings can reduce reflection losses and thus increase transmission (Fig. 2.42). From >3 μm Initially, the transmission value decreases with increasing temperature (Fig. 2.42).

Scattering in sapphire is wavelength dependent. The relative scattering decreases with increasing wavelength and reaches a maximum at λ=1.3 μm.

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