Uniaxial crystals are anisotropic materials that possess a unique optical axis along which light behaves differently in different directions within the crystal. An anisotropic material means that its physical properties vary with direction.

The single unique optical axis is called the optic axis or the extraordinary axis. The optic axis of a crystal is the direction in which the light propagates through the crystal without facing double refraction. All the light waves that are parallel to this axis do not undergo double refraction. In other words, the light beam passes in this direction with a constant speed that does not depend on the polarization. Perpendicular to the optic axis lies the ordinary axis, where light behaves conventionally. The distinction between the two axes gives rise to intriguing phenomena and enables applications in various scientific domains.

The optic axis of a uniaxial crystal plays a crucial role in determining its optical behaviour. When a light wave propagates parallel to the optic axis, it experiences a constant refractive index, similar to that of ordinary isotropic materials. However, when light enters the crystal at an angle to the optic axis, it splits into two rays: the ordinary ray and the extraordinary ray. The ordinary ray follows Snell's law and behaves similarly to light in isotropic materials, while the extraordinary ray deviates from Snell's law due to the crystal's anisotropic nature.

The separation of light into two rays, each with different refractive indices, is known as birefringence. Uniaxial crystals exhibit birefringence due to the disparity in the refractive indices along the ordinary and extraordinary axes. This property makes them useful in polarization optics. By manipulating the polarization of light and exploiting birefringence, uniaxial crystals find applications in devices such as polarizers, waveplates, and retardation plates.

Uniaxial crystals form through a variety of geological and synthetic processes. Natural examples include minerals like calcite, quartz, and tourmaline. These minerals have unique crystal structures that give rise to their anisotropic properties. Synthetic uniaxial crystals are also commonly produced for specific applications, such as lithium niobate, potassium dihydrogen phosphate (KDP), and potassium titanyl phosphate (KTP). These synthetic crystals are created through carefully controlled growth processes, enabling engineers to tailor their properties to suit various technological requirements.

Uniaxial crystals find extensive use across numerous scientific and technological disciplines. In the field of optics, they are utilized in polarizing microscopes, optical filters, and telecommunications devices. The ability to control the polarization of light is crucial in fields like liquid crystal displays (LCDs) and 3D cinema technology. Uniaxial crystals also play a significant role in nonlinear optics, where they are used in frequency doubling, optical parametric amplifiers, and electro-optic modulators.