Optical Crystals

For the application, it is usually required to guarantee an appropriate orientation of the crystal lattice e.g. relative to the propagation direction of a light beam or to the endfaces. This is an additional complication for the fabrication process, where one might have to employ methods like X-ray diffraction for accurately determining the crystal orientation, if the orientation is not already sufficiently well determined by the crystal growth process.

Within a wide transparency range, the propagation losses of light are often quite low in crystalline materials compared with glasses. This is partly due to the high material quality (e.g. with a low concentration of absorbing impurities) and partly due to the uniform crystal lattice. While glasses always exhibit some level of Rayleigh scattering at the unavoidable density fluctuations, crystals can in principle have zero propagation loss — in practice limited by extrinsic factors like crystal defects and impurities.

Low propagation losses can be important not only for maximizing the transmission, but also for minimizing thermal effects, e.g. in high-power laser applications.

Crystals with not too high symmetry of the crystal lattice exhibit birefringence, i.e., a polarization-dependent refractive index. This is important for many applications. See the article on birefringence for details.

Some crystal materials (in contrast to glasses) exhibit a ($\\chi^{(2)}$) nonlinearity. Its strength is characterized not by a single number, but rather by a nonlinear tensor, and varies a lot between materials.

All crystals (like glasses) have a ($\\chi^{(3)}$) nonlinearity, which is also characterized by a tensor.

In many applications, the nonlinear properties are essential. Some examples:

• Nonlinear frequency conversion like frequency doubling and optical parametric oscillation is mostly done with the ($\chi^{(2)}$) nonlinearity, but sometimes (e.g. using stimulated Raman scattering) with ($\chi^{(3)}$).
• Electro-optic modulators (based on Pockels cells) mostly work with ($\chi^{(2)}$) crystals.

The Faraday effect (magnetic field induced polarization rotation) in some crystal materials (e.g. terbium–gallium garnet) is used for Faraday rotators and Faraday isolators. Its strength is characterized by a Verdet constant. Although the Faraday effect also occurs in glasses, it is often much stronger in crystalline materials.

Crystals of low symmetry can exhibit a strong piezoelectric effect: The material changes its lattice constants in response to an applied electric field. This is mostly used in piezoelectric transducers (which have various applications in photonics, although light is not directly involved) and sensors.

The pyroelectric effect in some crystal materials is exploited in pyroelectric detectors. These are used for pulse energy measurements (→ optical energy meters).

Certain crystal materials can be used as scintillator crystals for detecting radiation. The radiation causes flashes of light which can be detected with suitable photodetectors.

Generally, crystalline materials exhibit substantially higher thermal conductivity than amorphous materials like glasses. This is essentially because phonons (quanta of lattice vibrations) can propagate over long lengths in a crystal, while they are subject to substantial scattering in amorphous media. A high thermal conductivity minimizes temperature gradients and thus optical effects like thermal lensing in cases where a substantial amount of heat is deposited in a crystal.

Many crystal materials exhibit substantially anisotropic thermal expansion, i.e., upon heating they expand more in certain directions than in others. That can be particularly relevant when endfaces need to be equipped with dielectric coatings. As the latter generally exhibit isotropic thermal expansion, it is not possible to fully match the expansion coefficients with any choice of coating materials. Therefore, certain coated crystals should be exposed only to limited temperature cycling because otherwise the dielectric coatings may be damaged. This is particularly relevant for nonlinear crystal materials when one uses noncritical phase matching at substantially elevated operating temperatures.

A practical challenge with various optical crystals (e.g. KDP, DKDP, LBO, and certain alkali halides) is hygroscopy: the tendency to absorb moisture from the surrounding air. Absorption of water can fog polished surfaces, degrade optical transmission, and eventually destroy the crystal structure.

For hygroscopic materials, special precautions are required:

• Sealed housings: The crystals are often mounted in hermetically sealed housings, sometimes filled with a dry inert gas.
• Heated mounts: Maintaining the crystal temperature slightly above the ambient temperature prevents condensation.
• Dry environments: Handling and operation must occur in humidity-controlled environments (e.g., purged optical setups).

Conversely, oxide crystals (like quartz, sapphire or YAG) are typically non-hygroscopic and chemically stable, allowing for easier handling and exposure to ambient air.