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Sapphire Emerges as a Valuable Scientific Resource


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. When heated to about 2050°C (almost 4000°F), however, the powder melts and can then be formed into a single crystal using any of several crystal growth methods.

 

Many methods of manufacturing single crystal sapphire today are variations of the Czochralski process, which was invented in 1916. In this process a tiny sapphire seed crystal is dipped into a crucible containing molten alumina and then slowly withdrawn upward at a rate of one to 100 mm per hour. The alumina crystallizes on the end, creating long carrot-shaped boules of large size, up to 400 mm in diameter and weighing up to 500 kg.

 

Because it is a single crystal, neither natural nor synthetic sapphire can be molded, drawn or cast.  Synthetic sapphire, however, can be "grown" into specific shapesto meet the requirements of different applications.  Unlike natural sapphire, which derives its brilliant colors from impurities such as iron and titanium, synthetic sapphire is water clear and extremely pure, making it ideally suited for demanding optical applications such as lenses and wave plates.

 

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).  The most desirable optical properties can be achieved, however, when light is transmitted along the c-axis.  As a result, single crystals are usually cut and polished in such a way that the c-axis is aligned with the light source.

 

The primary reason for this is a key property of sapphire known as birefringence, or ability to shift and redirect polarized light.  Optimum birefringence is achieved when the c-axis serves as the optic axis of the sapphire crystal.  Components of light with linear polarizations parallel and perpendicular to the optic axis have unequal indices of refraction, and will therefore be displaced by different amounts.

 

When the light propagates either along or orthogonal to the optic axis, such a lateral shift does not occur. In the first case, along the optic axis, all components of the light see the same refractive index, so there is no lateral displacement. In the second case, however, the different components propagate at different phase velocities. A crystal with its optic axis in this orientation, parallel to the optical surface, may be used to create a wave plate, in which there is no distortion of the image but an intentional modification of the state of polarization of the incident wave. For instance, a quarter-wave plate is commonly used to create circular polarization from a linearly polarized source.

 

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, 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.
    • Other properties that make sapphire unique are its high compressive strength, high melting point, high thermal stability and high thermal conductivity.
    • Sapphire’s birefringence makes it ideal as material for high quality environmentally stable wave plates that operate from UV into the mid IR.
    • When making sapphire wave plates, the crystal can be grown at an orientation that maximizes its birefringence. Sapphire windows on the other hand can be grown at a crystal orientation that minimizes the birefringence.