optical crystals (original) (raw)
Definition: crystals for optical applications, usually single crystals, often with polished endfaces
optical materials- optical materials
- dielectric materials
- optical crystals
* nonlinear optical crystals, birefringent crystals - semiconductors
- scintillator materials
- mirror substrates
- ceramics
- metals
- fused silica
- nonlinear crystal materials
- photochromic materials
- index-matching fluids
- optical adhesives
- photonic metamaterials
- (more topics)
Related: optical materialsbirefringencenonlinear crystal materialslaser crystalscrystalline mirrorssingle-crystal fibers
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DOI: 10.61835/27s Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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Contents
Crystalline Optical Materials in Comparison to Glasses
Fabrication of Single Crystals
Properties of Optical Crystals
Summary:
This article provides a comprehensive comparison between crystalline optical materials and optical glasses. It explains the unique properties of single crystals that make them indispensable for many applications, including birefringence for polarization control, ($\chi^{(2)}$) nonlinearity for frequency conversion, and superior thermal conductivity for high-power applications.
Key differences in their use as laser gain media and for transmission in the infrared and ultraviolet spectral regions are detailed. The article also covers the complex fabrication methods for single crystals, such as the Czochralski and Bridgman–Stockbarger techniques, which contribute to their higher cost compared to amorphous glasses.
Finally, a range of specific optical, thermal, and other physical properties like the Faraday, piezoelectric, and pyroelectric effects are discussed.
(This summary was generated with AI based on the article content and has been reviewed by the article’s author.)
Crystalline Optical Materials in Comparison to Glasses
A wide range of optical materials is used in different fields of optics. As transparent materials, one often uses optical glasses, but for various applications one requires optical crystal materials — mostly monocrystalline materials (single crystals) — because of their special properties:
- In contrast to glasses, crystals can exhibit birefringence, as required for various types of polarizers, waveplates, birefringent tuners and other optical components. Often used birefringent crystal materials are quartz, calcite and sapphire.
- Crystal materials with not too high symmetry of their crystal lattice (e.g. trigonal, tetragonal or monoclinic) can exhibit a ($\chi^{(2)}$) nonlinearity. This is used mostly for nonlinear frequency conversion, but also for optical modulators (e.g. Pockels cells).
- The typically much higher thermal conductivity of crystal materials is also relevant for some applications, e.g. for minimizing thermal lensing effects. In some cases, heat conduction is even the primary purpose of an optical crystal; for example, there are diamond heat spreaders.
- A wide range of crystal materials can be used as laser crystals, i.e., as host materials for laser-active dopants — either rare earth ions or transition metal ions. Compared with laser-active glasses, they typically exhibit relatively high transition cross-sections, a small gain bandwidth and good heat conduction. Often, they also allow for higher doping concentrations. Birefringence is also useful in some cases, e.g. for avoiding depolarization losses.
- In some cases, crystal materials are used in spectral regions where glasses do not have sufficiently wide wavelength ranges with high transparency. In particular, various materials such as zinc sulfide, zinc selenide and sapphire are used as infrared crystals, and other materials such as lithium fluoride, calcium fluoride and magnesium fluoride (fluorite) are used as ultraviolet crystals. Fluorite is also used as an optical material with very low dispersion.
Most optical crystals are insulating (dielectric) materials, having a wide band gap and very low absorption in the visible spectral region. However, there are also semiconductors used as optical crystals, e.g. as infrared crystals, where intense absorption in the visible region is not relevant.
Optical crystals are made with a wide range of geometric shapes, including simple cuboids, but also cylinders and other shapes with curved surfaces. A special form of crystals are single-crystal fibers, often having an extreme ratio of length to diameter.
Fabrication of Single 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. Further, such crystals are technically fabricated in most cases; only small quantities of a few crystal materials (e.g. fluorite = CaF2) can be found in nature with sufficient size and optical quality.
Single crystals can generally not be obtained 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. Even then, various kinds of crystal defects can occur, and the achievable crystal size is often limited.
In many cases, the purity of the raw materials must be quite high — substantially higher than for frequently used optical glasses.
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.
Crystal Lattice Orientation
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.
Properties of Optical Crystals
Propagation Losses
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.
Birefringence
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.
Nonlinear Optical Properties
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.
Faraday Effect
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.
Piezoelectric Effect
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.
Pyroelectric Effect
The pyroelectric effect in some crystal materials is exploited in pyroelectric detectors. These are used for pulse energy measurements (→ optical energy meters).
Scintillation
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.
Thermal Properties
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.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What are the main advantages of optical crystals over optical glasses?
Optical crystals offer unique properties not found in glasses, such as birefringence for polarization control, a ($\chi^{(2)}$) nonlinearity for frequency conversion, much higher thermal conductivity, and often wider transparency ranges in the infrared or ultraviolet.
Why is the fabrication of single crystals more difficult and expensive than that of glasses?
Single crystals require special growth techniques, like the Czochralski method, starting from a seed crystal to avoid multiple crystal domains. This process is slow, requires high-purity materials, and needs careful control of lattice orientation, making it substantially more costly.
What is birefringence in optical crystals?
What are laser crystals?
Why can crystals have lower optical losses than glasses?
The perfectly regular lattice of an ideal crystal does not cause Rayleigh scattering, which is unavoidable in amorphous glasses due to their inherent density fluctuations. Therefore, propagation losses in high-quality crystals can be extremely low, limited mainly by defects and impurities.
How does thermal conductivity differ between crystals and glasses?
Crystalline materials generally have a much higher thermal conductivity than glasses. This is because phonons (lattice vibrations) can travel over long distances in a regular crystal lattice, whereas they are strongly scattered in amorphous materials, making crystals better for managing heat in high-power systems.
What is the significance of the ($\chi^{(2)}$) nonlinearity in some crystals?
Suppliers
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Nonlinear crystals of either β-barium borate (BBO) or lithium triborate (LBO) are used for frequency conversion of laser sources. BBO crystals feature thicknesses from 0.2 mm to 0.5 mm to minimize group velocity mismatch and are ideal for frequency doubling or tripling of pulses from Ti:sapphire and Yb-doped lasers. The critical and noncritical phase matching LBO crystals are ideal for second or third harmonic generation of Nd:YAG and Yb-doped lasers. Nonlinear crystals with 20-10 surface quality and λ/10 (LBO) or λ/8 (BBO) surface flatness provide the broad transparency range and large nonlinear coefficient needed for the harmonic generation of fundamental laser frequencies. Each crystal features a protective anti-reflection (AR) coating that minimizes reflection and limits fogging from ambient conditions.
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With a high degree of expertise, Hangzhou Shalom EO excels at offering a diverse catalog of optical crystals, including:
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UM Optics offers optical crystals of CaF2, BaF2, MgF2 and LiF. UM Optics is China's largest manufacturer of fluoride crystals.
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GWU-Lasertechnik offers all standard optical crystals with a broad variety of specifications. This includes laser crystals like Nd:YVO4, Nd:YAG and Yb:YAG as well as nonlinear optical materials like LBO and BBO. Beside the well-established materials, innovative crystals with outstanding properties are available. No matter if individual pieces for R & D purposes are required or cost-efficient numbers in small, medium or large batches with in-time delivery for the production line are needed: GWU’s dedicated service helps to find the best core components for your application. GWU-Lasertechnik has more than 30 years of experience in distributing laser crystals. Choose GWU to benefit from our wide knowledge and in-field experience!
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