Refractive index of infrared-transparent polycrystalline alumina

The refractive index of polycrystalline α-alumina prisms with an average grain size of 0.6 μm is reported for the wavelength range 0.9 to 5.0 and the temperature range 293 to 498K. Results agree within 0.0002 with the refractive index predicted for randomly oriented grains of single-crystal aluminum oxide. This paper provides tutorial background on the behavior of birefringent materials and explains how the refractive index of polycrystalline alumina can be predicted from the ordinary and extraordinary refractive indices of sapphire. The refractive index of polycrystalline alumina is described by 𝑛𝑛2 − 1 = (A+B [𝑇𝑇2−𝑇𝑇20]) +Dλ2 /λ2−(λ1+C [𝑇𝑇2−𝑇𝑇20])2 + λ2−λ22 where wavelength λ is expressed in μm, To = 295.15 K, A = 2.07156, B = 6.273× 10-8, λ1 = 0.091293, C = –1.9516 × 10-8, D = 5.62675, and λ2 = 18.5533. The slope dn/dT varies with λ and T, but has the approximate value 1.4 × 10-5 K-1 in the range 296–498 K.