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Nonlinear Crystal Materials

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Definition: crystal materials exhibiting an optical nonlinearity, usually of χ(2) type

German: nichtlineare Kristallmaterialien

Categories: optical materials, nonlinear optics

How to cite the article; suggest additional literature

Transparent crystalline crystal materials can exhibit different kinds of optical nonlinearities which are associated with a nonlinear polarization. For example, media with a χ(2) nonlinearity are mainly used for parametric nonlinear frequency conversion (e.g. in frequency doublers and optical parametric oscillators) and for electro-optic modulators, whereas χ(3) nonlinearities lead to the Kerr effect, the Raman effect, and four-wave mixing. In essentially all cases, artificial (rather than naturally occurring) crystals are used.

Relevant Aspects for the Choice of Nonlinear Crystals

Many different properties of a nonlinear crystal can be important for an application e.g. in nonlinear frequency conversion:

LBO crystals

Figure 1: A block of LBO and some crystals cut from such material. The photograph has been kindly provided by EKSMA OPTICS.

Additional properties can be relevant for a comparison:

The choice of the most suitable crystal material for a given application is often far from trivial; it should involve the consideration of many aspects. For example, a high nonlinearity for frequency conversion of ultrashort pulses does not help if the interaction length is strongly limited by a large group velocity mismatch and the low damage threshold limits the applicable optical intensities. Also, it can be highly desirable to use a crystal material which can be critically phase-matched at room temperature, because noncritical phase matching often involves the operation of the crystal in a temperature-stabilized crystal oven.

Frequently Used χ(2) Nonlinear Crystal Materials

Lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) are materials with a relatively strong nonlinearity. They are often used for nonlinear frequency conversion and also for electro-optic modulators. Both materials are available in congruent and in stoichiometric form, with important differences concerning periodic poling and photorefractive effects (see below). Lithium niobate and tantalate are the most often used materials in the context of periodic poling; the resulting materials are called PPLN (periodically poled lithium niobate) and PPLT, respectively, or PPSLN and PPSLT for the stoichiometric versions. Both have a relatively low damage threshold, but do not need to be operated at high intensities due to their high nonlinearity. They have a tendency for photorefractive effects, which are detrimental for frequency conversion, but are used for, e.g., holographic data storage in Fe-doped LiNbO3 crystals. The tendency for “photorefractive damage” depends strongly on the material composition; e.g. it can be reduced via MgO doping and by using a stoichiometric composition.

Potassium niobate (KNbO3) has a high nonlinearity. It is used for, e.g., frequency doubling to blue wavelengths and in piezoelectric applications.

Potassium titanyl phosphate (KTP, KTiOPO4) may be flux-grown (cheaper) or hydrothermal (better for high powers, lower tendency for gray tracking → photodarkening). The “KTP family” of materials also includes KTA (KTiOAsO4), RTP (RbTiOPO4) and RTA (RbTiAsPO4). These materials tend to have relatively high nonlinearities and are suitable for periodic poling.

DKDP crystals

Figure 2: KD*P crystals, including one with very large size. The photograph has been kindly provided by EKSMA OPTICS.

Potassium dihydrogen phosphate (KDP, KH2PO4) and potassium dideuterium phosphate (KD*P or DKDP, KD2PO4, exhibiting extended infrared transmission), are available in large sizes at low price. They exhibit good homogeneity over large volumes and have a high damage threshold, but are hygroscopic and have a low nonlinearity.

There are a number of borates, the most important ones being lithium triborate (LiB3O5 = LBO), cesium lithium borate (CLBO, CsLiB6O10), β-barium borate (β-BaB2O4 = BBO, strongly hygroscopic, often used in Pockels cells), bismuth triborate (BiB3O6 = BIBO), and cesium borate (CSB3O5 = CBO). Yttrium calcium oxyborate (YCOB) and YAl3(BO3)4 (YAB) are also available in rare-earth-doped form for use as a laser gain medium, and can then simultaneously be used for generating and frequency-converting laser light. Less frequently used are strontium beryllium borate (Sr2Be2B2O7 = SBBO) and K2Al2B2O7 (KAB). LBO, BBO, CLBO, CBO and other borate crystals are suitable for the generation of relatively short wavelengths, e.g. in green and blue laser sources, and for UV generation (→ ultraviolet lasers), because their bandgap energy is relatively high, the crystals are relatively resistant to UV light, and there are suitable phase-matching options. Borates such as LBO and BBO also work well in broadly tunable optical parametric oscillators and optical parametric chirped-pulse amplification.

For mid-infrared (and partly also terahertz) generation, one requires crystal materials with a transparency range extending far into the infrared spectral region. The most important of these media are zinc germanium diphosphide (ZGP, ZnGeP2), silver gallium sulfide and selenide (AgGaS2 and AgGaSe2), gallium selenide (GaSe), and cadmium selenide (CdSe). Gallium arsenide (GaAs) has also become useful for mid-infrared applications, since it is possible to obtain quasi-phase matching in orientation-patterned GaAs [13, 21].

Lifetime of Nonlinear Crystals

In many cases, a nonlinear crystal used for nonlinear frequency conversion has a very long lifetime, which is longer than that of the whole laser system. The crystal material is essentially not modified during operation. However, a reduced crystal lifetime can occur under various circumstances:

Crystal lifetime can also be strongly dependent on the material quality, although certain degradation phenomena appear to be intrinsic limitations of the material.

For high-power UV generation, nonlinear crystals may become consumables: they need to be replaced quite often within the lifetime of the whole laser system (e.g., every few hundred hours of operation). Often, several problematic aspects come together in the regime UV generation: crystal materials are generally more sensitive to ultraviolet light (having high photon energies), exhibit a higher absorption in that regime, and in case of ultrashort pulses the high group velocity mismatch enforces the use of a shorter crystal, which requires high optical intensities for a given conversion efficiency.

Very Thin Nonlinear Crystals

For some applications, nonlinear crystals with a very small thickness of well below 1 mm are used. This may be necessary for minimizing the group velocity mismatch, e.g. in optical autocorrelators for extremely short pulses.

A common method for obtaining ultrathin crystals is to first optically contact a thicker nonlinear crystal with some substrate (e.g. of fused silica) and then to polish the crystal down to the required thickness of e.g. 20 μm. The group velocity mismatch in the thicker substrate material may not matter, as the nonlinear interaction takes place only in the thin crystal. The substrate only serves to mechanically stabilize the thin nonlinear crystal.

It is also possible to fabricate free-standing crystals with a thickness of only 100 μm, sometimes even below 30 μm.


[1]R. S. Craxton et al., “Basic properties of KDP related to the frequency conversion of 1 μm laser radiation”, IEEE J. Quantum Electron. 17 (9), 1782 (1981)
[2]C. Chen et al., “New nonlinear-optical crystal: LiB3O5”, J. Opt. Soc. Am. B 6 (4), 616 (1989)
[3]C. Chen et al., “A new-type ultraviolet SHG crystal β-BaB2O4”, Sci. Sin. (Ser. B) 28, 235 (1985)
[4]J. D. Bierlein and H. Vanherzeele, “Potassium titanyl phosphate: properties and new applications”, J. Opt. Soc. Am. B 6 (4), 622 (1989)
[5]S. Lin et al., “The nonlinear optical characteristics of a LiB3O5 crystal”, J. Appl. Phys. 67 (2), 634 (1990)
[6]R. C. Eckardt et al., “Absolute and relative nonlinear optical coefficients of KDP, KD*P, BaB2O4, LiIO3, MgO:LiNbO3, and KTP measured by phase-matched second-harmonic generation”, IEEE J. Quantum Electron. 26 (5), 922 (1990)
[7]D. N. Nikogosyan, “Beta barium borate (BBO), a review of its properties and applications”, Appl. Phys. A 52, 359 (1991)
[8]S. P. Velsko et al., “Phase-matched harmonic generation in lithium triborate (LBO)”, IEEE J. Quantum Electron. 27 (9), 2182 (1991)
[9]C. Chen et al., “Design and synthesis of an ultraviolet-transparent nonlinear optical crystal Sr2Be2B2O7”, Nature 373, 322 (1995)
[10]Y. Wu et al., “CsB3O5: a new nonlinear optical crystal”, Appl. Phys. Lett. 62, 2614 (1993)
[11]D. N. Nikogosyan, “Lithium triborate (LBO). A review of its properties and applications”, Appl. Phys. A 58, 181 (1994)
[12]Y. Mori et al., “New nonlinear optical crystal: cesium lithium borate”, Appl. Phys. Lett. 67, 1818 (1995)
[13]S. J. B. Yoo et al., “Wavelength conversion by difference-frequency generation in AlGaAs waveguides with periodic domain inversion achieved by wafer bonding”, Appl. Phys. Lett. 68, 2609 (1996)
[14]I. Shoji et al., “Absolute scale of second-order nonlinear-optical coefficients”, J. Opt. Soc. Am. B 14 (9), 2268 (1997)
[15]T. Iwai et al., “Crystal growth and optical characterization of rare-earth (Re) calcium oxyborate ReCa4O(BO3)3 (Re = Y or Gd) as new nonlinear optical material”, Jpn. J. Appl. Phys. 36, L276 (1997)
[16]H. Hellwig et al., “Exceptional large nonlinear optical coefficients in the monoclinic bismuth borate BiB3O6”, Solid State Commun. 109, 249 (1998)
[17]Z.-G. Hu et al., “A new nonlinear optical borate crystal K2Al2B2O7 (KAB)”, Jpn. J. Appl. Phys. 37, L1093 (1998)
[18]N. Ye et al., “New nonlinear optical crystal K2Al2B2O7”, J. Opt. Soc. Am. B 17 (5), 764 (2000)
[19]P. Kumbhakar and T. Kobayashi, “Nonlinear optical properties of Li2B4O7 (LB4) crystal for the generation of tunable ultra-fast laser radiation by optical parametric amplification”, Appl. Phys. B 78, 165 (2004)
[20]M. Katz et al., “Vapor-transport equilibrated near-stoichiometric lithium tantalate for frequency-conversion applications”, Opt. Lett. 29 (15), 1775 (2004)
[21]T. Skauli et al., “Measurement of the nonlinear coefficient of orientation-patterned GaAs and demonstration of highly efficient second-harmonic generation”, Opt. Lett. 27 (8), 628 (2002)
[22]Li Wang and Y. Men, “Comparison study of CsLiB6O10 and β-BaB2O4 as nonlinear media for optical parametric oscillators”, Appl. Opt. 42 (15), 2720 (2003)
[23]M. Ghotbi and M. Ebrahim-Zadeh, “Optical second harmonic generation properties of BiB3O6”, Opt. Express 12 (24), 6002 (2004)
[24]S. V. Tovstonog et al., “Thermal effects in high-power CW second harmonic generation in Mg-doped stoichiometric lithium tantalate”, Opt. Express 16 (15), 11294 (2008)
[25]H. Ishizuki and T. Taira, “Mg-doped congruent LiTaO3 crystal for large-aperture quasi-phase matching device”, Opt. Express 16 (21), 16963 (2008)
[26]D. N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey, Springer, Berlin (2005)

(Suggest additional literature!)

See also: nonlinear frequency conversion, frequency doubling, effective nonlinear coefficient, ultraviolet lasers, optical parametric oscillators, periodic poling, phase matching, quasi-phase matching, orientation-patterned semiconductors, electro-optic modulators, Pockels cells, crystal ovens, Spotlight article 2007-05-26
and other articles in the categories optical materials, nonlinear optics

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Dr. R. Paschotta

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