Optical Crystals

In most cases, optical crystals are single crystals, i.e., they exhibit a uniform crystal lattice throughout a large piece, apart from some concentration of lattice defects. This uniform orientation can generally not be achieved e.g. simply by cooling down the molten material (as for an optical glass), because that would generally lead to a large number of crystal domains with different lattice orientations. Instead, one needs to employ special crystal growth techniques such as the Czochralski method or the Bridgman–Stockbarger technique. Typically, a small monocrystalline seed crystal is provided, and the growth conditions are optimized such that all added material just extends the lattice of the seed crystal rather than forming new domains. In most cases, the growth rate needs to be kept at a rather low level, as otherwise a sufficiently high crystal material quality could not be achieved.

In many cases, the purity of the used raw materials must be quite high – substantially higher than for frequently used optical glasses.

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 end faces. 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.

Obviously, the explained aspects of material purity, carefully controlled growth conditions and the observation of lattice orientation lead to a fabrication cost which is in most cases substantially higher than for glass materials.

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, avoiding the unavoidable Rayleigh scattering at density fluctuations in glasses.

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.

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 end faces 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 operation temperatures.

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