Optical properties of sapphire crystals

Optical properties of sapphire crystals

Sapphire is a negative uniaxial optical crystal. Sapphire has a direction along the optical axis in which birefringence does not occur and the refractive index of extraordinary light is less than that of ordinary light. The difference in refractive index in the visible range is approximately 0.008. In the UV region, near the maximum value, the difference increases to 0.012 but decreases to zero near the maximum value.

Ruby is pleochroic, i.e., absorbing ordinary and extraordinary light in different directions. In the direction perpendicular to the optical axis, the crystal has a strong purplish red color. In the direction parallel to the optical axis, the crystal is red It is yellowish and weak in intensity. The absorption spectra of crystals with different impurities are shown in Fig. 2.110.

The absorption coefficient of sapphire has been studied in many articles. Although there are disagreements on some conclusions, they still have something in common [51] (Table 2.47). The general scheme for giving recommended values is as follows. According to the existing The dependence of the absorption coefficient on the temperature in the literature and taking into account the possible error caused by the test method, an average dependence value Kλ(T) is established for each wavelength with an interval of 0.1 μm. These values are used to establish the absorption spectrum Kλ(λ) .

The absorption coefficient of sapphire is proportional to temperature raised to the power (T n ) / n = 2.2 for λ = 4.2 mm, n = 2 for λ = 4.5–5.5 mm, n = 1.5 for λ = 6 mm, n = 1.2 for λ= 6.5 mm, and n = 1 for λ= 7 mm.

Absorption coefficient recommendations derived from literature data analysis are considered literature values only. For some spectral temperatures, these have been extrapolated.

Transmission. Generally speaking, the transmission in sapphire is described by the curve given in Figure 2.36. The curve was obtained on a pure and sufficiently perfect crystal (Linde Cz UV grade) grown by the Czochralski method. The sample is 1 mm thick. In the UV region, the transmission of the thin disk is essentially limited by the Fresnel loss. The values given in the figure do not take this loss into account. The most relevant is the transmission of a pure perfect crystal (UV grade) in the UV region See Figure 2.37.

Transmission in the UV region increases with high-quality polishing of the surface (surface roughness 20/10). Low-quality polishing can result in a decrease of about 10% in transmission values. The effect of surface polishing quality is large for λ < 250 μm. Point Absorption due to the presence of defects and impurities is located at wavelengths 204, 230 and 400 μm. Transmission values for actual samples also depend on the medium in the growth chamber and even the instrument accessories used (Fig. 2.38 [52]).

Kunshan Shuojing Optoelectronics Technology Co., Ltd. is involved in a wide range of new materials, and has complete preparation methods and means for functional ceramics and sapphire crystals, scintillation crystals, and laser crystals, forming optically transparent ceramics, sapphire optical windows, scintillation crystals, lasers and nonlinear crystals Master's four series of high-tech products