Valued for many years as a precious gem, sapphire today is finding new uses as a scientific resource in a host of industrial, military and aerospace applications. Because of its physical, optical and chemical properties, it is widely used to fabricate strong, lightweight lenses, windows and wave plates.
Synthetic versus natural sapphire:
Natural sapphire is a gemstone variety of the mineral corundum, or aluminum oxide (Al2O3), commonly referred to as alumina (α-alumina) or aloxide, one of nature's most abundant compounds. In its natural state, aluminum oxide is a white powdery material used extensively as an industrial abrasive.
Optical properties of sapphire:
Both natural and synthetic sapphire have the same crystalline structure, known as rhombohedral class 3m. Each crystal has three axes of symmetry (a-axis, b-axis and c-axis).Other desirable properties:
Sapphire is well suited for demanding optical applications for a number of other reasons as well. For example:
- Except for diamonds, sapphire is the hardest crystal known to man. On the Mohs scale of hardness, which ranks materials from softest (1) to hardest (10), it is rated 9. Because of its structural strength, circular sapphire optical windows can be made much thinner than other common dielectric optical windows with improved transmittance as a result.
- Because of its high dielectric constant, sapphire is an excellent insulator when used in optoelectronic applications.
- Sapphire optical windows are useful over a wide wavelength range from 0.15 to 5.5µm, and sapphire is resistant to UV radiation darkening.
- Sapphire windows and wave plates are environmentally stable and have excellent resistance to common chemical acids and alkalis.
