This article introduces the properties, application characteristics, life and commonly used nonlinear crystal materials of nonlinear crystal materials.

A crystalline material, usually in the form of x(2), that exhibits optically nonlinear properties.

Transparent crystalline materials can exhibit many different optical nonlinear properties, which are related to the nonlinear polarization of crystalline materials. For example, a material with nonlinearity of the form x(2) is mainly used for parametric nonlinear frequency conversion (eg, in frequency doublers and optical parametric oscillators), and the material with nonlinearity of the form x(2) Also used as an optoelectronic modulator, however with nonlinear properties of the form x(3) leading to Kerr effect, Raman effect and four-wave mixing. Essentially, all of the above use artificial crystals (rather than naturally occurring events).

Many different properties of nonlinear crystals are important in applications such as nonlinear frequency conversion:

• The dispersion and birefringence characteristics of nonlinear crystals determine that nonlinear crystals can be used for phase matching and phase matching bandwidth, angular spectral width (the angular spectral width of critical phase matching), etc.
• The size of the effective nonlinear coefficient depends on the nonlinear tensor part and the type of phase matching, and the size of the effective nonlinear coefficient is important, especially when the achievable light intensity is low.
• In general, crystalline materials should have high transmittance for all wavelengths.

Other related features:

• The potential of the material is periodically polarized to achieve quasi-phase matching.
• Linear absorption, which can cause heating at high optical power values, so that phase matching is broken and thermal lensing occurs.
• Resistance to optical damage, grayscale tracking, photodarkening effects, IR absorption including green light, etc.
• Anti-photorefractive effects (often referred to as photorefractive damage, although photorefractive damage is usually reversible).
• Whether high-quality, large-scale and cost-effective nonlinear crystals are readily available. — Ease of fabricating high-quality antireflection coatings on nonlinear crystals
• Chemical resistance; for example, some crystalline materials are susceptible to moisture, others are susceptible to chemical changes when dielectric coatings are applied in vacuum chambers

It is very important to select the most suitable crystal material for a certain application requirement, and many aspects need to be considered when selecting the appropriate crystal material. For example, the highly nonlinear nature of frequency conversion of ultrashort pulses will not work if the interaction length is strongly limited by large group velocity mismatches and low available light intensities caused by low damage thresholds. Also, since noncritical phase matching usually involves operating the crystal in a temperature-stabilized crystal oven, crystal materials that can critically phase match at room temperature are desirable.

Lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) are materials with relatively strong nonlinear properties. They are commonly used for nonlinear frequency conversion and electro-optic modulation. Both materials are available in consistent and stoichiometric forms that differ substantially in periodic polarization and photorefractive effects (described below). In the case of periodic polarization, lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) are commonly used materials; the resulting materials are referred to as PPLN (periodically polarized lithium niobate) and PPLT (periodically polarized tantalate), respectively Lithium), or PPSLN and PPSLT in stoichiometric form. Both crystals have relatively low damage thresholds, but should not be used at high light intensities due to nonlinear effects. They tend to produce photorefractive effects, which are detrimental for frequency conversion, but useful for holographic information storage such as in iron-doped lithium niobate crystals. The effect of this trend on “photorefractive damage” depends largely on the material composition; for example, by magnesium oxide (MgO) doping and the use of stoichiometric compositions, photorefractive damage can be reduced.

Potassium niobate (KNbO3) has high nonlinear properties. It is used in lightwave and piezoelectric applications such as frequency-doubled blue wavelengths. Potassium titanyl phosphate (KTP, KTiOPO4) can be prepared by hot melt (cheaper) or by hydrothermal method (suitable for high energy and low grayscale tracking → photodarkening effect).

The “KTP family” materials include KTA (KTiOAsO4), RTP (RbTiOPO4) and RTA (RbTiAsPO4). These materials tend to have relatively high nonlinear properties and are suitable for periodic polarization. Potassium dihydrogen phosphate (KDP, KH2PO4) and potassium dideuterium phosphate (KD*P or DKDP, KD2PO4, exhibiting extended infrared transmission) are suitable for low-cost large-scale nonlinear crystals. They exhibit good bulk uniformity and have a high damage threshold, but they are hygroscopic and have low nonlinear coefficients.

There are many kinds of borates, the most important one is lithium borate (LiB3O5 = LBO), lithium cesium borate (CLBO, CsLiB6O10), β-barium borate (β-BaB2O4 = BBO, very hygroscopic, often used in Pockels cell), bismuth triborate (BiB3O6 = BIBO), and cesium borate (CSB3O5 = CBO). Yttrium calcium oxyborate (YCOB) and YAl3(BO3)4(YAB) are also suitable for rare-earth doped structures in laser gain media, and can be simultaneously used to generate frequency-converted lasers. Less used are strontium beryllium borate (Sr2Be2B2O7 = SBBO) and K2Al2B2O7 (KAB). LBO, BBO, CLBO, CBO and other borate crystals are suitable for generating relatively short wavelength light waves, such as in green and blue laser light sources, and also for generating ultraviolet light waves (such as UV lasers), because these boron Acid salts have large band gaps, crystals are UV resistant, and have suitable phase matching options. Borates such as LBO and BBO are also suitable for broadband tunable optical parametric oscillators and optical parametric chirped pulse amplifiers.

To generate the mid-infrared (sometimes also referred to as terahertz), crystalline materials with transparent wavelengths extending into the infrared spectral region are required. The most important of these cutoff materials are zinc germanium diphosphate (ZGP, ZnGeP2), gallium sulfide silver gallium selenide (AgGaS2 and AgGaSe2), selenide, and cadmium selenide (CdSe). Gallium arsenide (GaAs) is also useful in mid-infrared applications because quasi-phase matching can be achieved with light pattern-oriented GaAs [13, 21].

In many cases, nonlinear crystals used for nonlinear frequency conversion have long lifetimes, longer than the lifetime of the entire laser. Crystals are basically unchanged during use. However, crystal lifetime reduction can occur in various situations:
As in nonlinear frequency conversion:

• Excessive light intensity during operation will instantly damage the crystal. Unfortunately, to achieve sufficiently high conversion efficiencies, nonlinear crystals often need to operate near their optical loss thresholds. This implies a trade-off between conversion efficiency and crystal lifetime. It is important to note that even if the nominal intensity is below the symbolic damage threshold, there may still be problems. These problems are due to beam strength or local strength (eg a beam profile has a “break point”), or due to isolated imperfections inside the crystal. These flaws are more sensitive than conventional crystalline materials.
• Even though crystals operate well below threshold to prevent transient wear, some crystal materials exhibit continuous degradation in some used parts, for example, in the form of “grayscale tracing”. This phenomenon is especially common when crystals operate in the ultraviolet range. Notably, gradual degradation accumulates heat, and the generation of overheating can cause catastrophic damage in an instant.
• Hygroscopic crystalline materials deteriorate when they cannot be kept in sufficiently dry air (or dry inert gas). This crystal is used in KDP and BBO, and is used less in LBO. It is helpful to keep the crystal at a higher temperature, which makes it easier to keep the crystal dry.
• In order to achieve phase matching, it is often problematic to operate nonlinear crystals at temperatures below room temperature because that may cause water to condense on the crystal surface if the surrounding air is not very dry. Even if the crystal material or coating is not sensitive to water, tiny water droplets can focus the laser more sharply than usual, damaging the crystal material.
• Problems with non-critical phase matched crystals in crystal ovens when the temperature changes rapidly or frequently. In particular, antireflection films can be damaged due to different coefficients of material expansion.

Although the phenomenon of degradation seems to be an intrinsic limitation of the material, the crystal lifetime is largely dependent on the quality of the material.

To generate high-power UV light, nonlinear crystals become consumables: they need to be replaced frequently during the lifetime of the laser system (eg, every few hundred hours of operation). Often, several problematic factors co-occur in UV-generating systems: crystalline materials are generally more sensitive to UV light (with high photon energies), exhibit higher absorption in this system, and in the case of ultrashort pulses, A high group velocity mismatch requires the use of shorter crystals, which require higher light intensities with the same conversion efficiency.

In some applications, nonlinear crystals with smaller thicknesses such as less than 1 mm are used. When using such crystals, it is necessary to minimize group velocity mismatch, eg, for very short pulses in optical autocorrelators.

The conventional way to obtain ultrathin crystals is to first optically contact a thicker nonlinear crystal with some substrate (eg, quartz glass), and then polish the crystal to the desired thickness (eg, 20 microns). The group velocity mismatch in thicker base materials is irrelevant because nonlinear interactions only occur in thinner crystalline materials. The substrate is only used to mechanically stabilize the thin nonlinear crystal.

It is possible to make crystals with a thickness of 100 microns without support, and sometimes it is also possible to make crystals with a thickness of less than 30 microns without support.