Optical Properties of Sapphire Glass
The transmission wavelength range of infrared optical materials is determined by the structure and properties of the material itself, and its short-wave limit value is determined by the energy gap Eg of the material's structure. It can usually be used to calculate the short-wave cutoff limit of infrared optical materials. The long-wave cutoff depends on the crystal structure and lattice thermal vibrations, which are intrinsic properties of the material and cannot be changed by improving the material process.
The energy gap Eg value of sapphire crystal is 9ev, the limit of short-wave cutoff is 0.14um, and the transmission band is 0.14~6um. Since its short-wave cutoff is in the ultraviolet, visible light is also transparent. 
As shown in Figure 1 below, the visible light and infrared transmission curves of a typical sapphire crystal.
Figure 2 below shows the infrared transmission curves of sapphire crystals with different thicknesses.
It can be seen from the above two figures that the transmittance of the sapphire crystal begins to decrease after 4um, and the rate of decrease is related to the thickness. This drop is caused by fundamental lattice vibration absorption, which is an inherent property.
So, is the transmittance of sapphire crystal in the infrared related to other factors? After a lot of experimental research, it is found that the transmittance of sapphire crystal in the infrared is related to temperature.
As shown in Figure 3, under different temperature conditions, the transmittance curves of sapphire crystals, especially after 4.0um, have obvious differences.
As the temperature increases, the transmittance of the sapphire crystal decreases significantly at 4.0um, which is caused by the sharp increase of its absorption coefficient with the increase of temperature. According to the experimental research data, when the temperature rises to 500°C, the absorption coefficient of the sapphire crystal at 4.0um increases by 2.5 times compared with normal temperature. However, the effect in the visible light region is not obvious.
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