Nonlinear Crystal Materials | previous | next | feedback |
You can buy nonlinear crystal materials from:
- Döhrer Elektrooptik, offering BBO, LBO, KTP, LiNbO3, PPLN and other nonlinear optical materials
- Photon Laseroptik is a specialist for the highly precise processing of laser crystals, nonlinear crystals, and other optical elements
- Raicol Crystals Ltd.: offering crystals of KTP, RTP, LBO, BBO, as well as periodically poled LiNbO3, stoichiometric LiTaO3, and KTP
Ask RP Photonics for advice on the best choice of a nonlinear crystal material, or on the optimization of the phase-matching configuration and crystal length.
Definition: crystal materials exhibiting an optical nonlinearity, usually of χ(2) type
Transparent crystalline crystal materials can exhibit different kinds of optical nonlinearities which are resulting from 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.
Relevant Aspects for the Choice of Nonlinear Crystals
Many different properties of a nonlinear crystal can be very important for an application e.g. in nonlinear frequency conversion:
- The chromatic dispersion and birefringence properties determine possibilities for phase matching and the phase-matching bandwidth, angular acceptance (for critical phase matching), etc.
- The magnitude of the effective nonlinear coefficient, which depends on the nonlinear tensor components and on the phase-matching configuration, is important particularly if the achievable optical intensities are low.
- Normally, the crystal material should have a high optical transparency for all involved wavelengths.
However, additional properties can be relevant for a comparison:
- the material's potential to be periodically poled to achieve quasi-phase matching
- linear absorption, which can cause heating at high optical power levels, so that the phase matching is disturbed, and thermal lensing may occur
- the resistance against optical damage, gray tracking, photodarkening, green-induced infrared absorption, and the like
- the resistance against photorefractive effects (which are often called "photorefractive damage", even though this is usually reversible)
- the availability of crystals with consistently good quality, large size and a reasonable price
- the ease of fabricating high-quality anti-reflection coatings on the crystals
- the chemical durability; e.g., some crystal materials are hygroscopic, others undergo chemical changes when heated in a vacuum chamber for application of a dielectric coating
The choice of the best 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 large nonlinearity. They are often used for nonlinear frequency conversion as well as 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 e.g. for holographic data storage in Fe-doped LiNbO3 crystals. The tendency for "photorefractive damage" strongly depends on the material composition; e.g. it can be reduced via MgO doping and/or by using a stoichiometric composition.
Potassium niobate (KNbO3) has a very high nonlinearity. It is used e.g. for 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.
Potassium dihydrogen phosphate (KDP, KH2PO4) and potassium dideuterium phosphate (KD*P, KD2PO4) 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 rather low nonlinearity.
There is a number of borates, the most important ones being lithium triborate (LiB3O5 = LBO), cesium lithium borate (CLBO, CsLiB6O10), beta barium borate (beta-BaB2O4 = BBO, strongly hygroscopic, often used in Pockels cells), BiB3O6 = BIBO, and cesium borate (CSB3O5 = CBO). Yttrium calcium oxyborate (YCOB) is 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.
Zinc germanium diphosphide (ZGP, ZnGeP2), silver gallium sulfide and selenide (AgGaS2 and AgGaSe2), and cadmium selenide (CdSe) have transparency ranges extending far into the infrared spectral region. These crystals are used for mid-infrared and (partially) for terahertz wave generation.
Note that semiconductor materials such as gallium arsenide (GaAs) can have very strong χ(2) (and also χ(3)) nonlinearities. It has been demonstrated that such crystals can be fabricated in periodically poled (orientation-patterned) form for quasi-phase matching [18, 19].
Bibliography
| [1] | R. S. Craxton et al., "Basic properties of KDP related to the frequency conversion of 1 μm laser radiation", IEEE J. Quantum Electron. QE-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", Scientia Sinica (Series B) 28, 235-243 (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, 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] | C. Chen et al., "Design and synthesis of an ultraviolet-transparent nonlinear optical crystal Sr2Be2B2O7", Nature 373, 322 (1995) |
| [8] | Y. Wu et al., "CsB3O5: A new nonlinear optical crystal", Appl. Phys. Lett. 62, 2614 (1993) |
| [9] | D. N. Nikogosyan, "Lithium triborate (LBO). A review of its properties and applications", Appl. Phys. A 58, 181 (1994) |
| [10] | Y. Mori et al., "New nonlinear optical crystal: Cesium lithium borate", Appl. Phys. Lett. 67, 1818 (1995) |
| [11] | I. Shoji et al., "Absolute scale of second-order nonlinear-optical coefficients", J. Opt. Soc. Am. B 14 (9), 2268 (1997) |
| [12] | 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) |
| [13] | H. Hellwig et al., "Exceptional large nonlinear optical coefficients in the monoclinic bismuth borate BiB3O6", Solid State Commun. 109, 249 (1998) |
| [14] | Z.-G. Hu et al., "A new nonlinear optical borate crystal K2Al2B2O7 (KAB)", Jpn. J. Appl. Phys. 37, L1093 (1998) |
| [15] | N. Ye et al., "New nonlinear optical crystal K2Al2B2O7", J. Opt. Soc. Am. B 17 (5), 764 (2000) |
| [16] | 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) |
| [17] | M. Katz et al., "Vapor-transport equilibrated near-stoichiometric lithium tantalate for frequency-conversion applications", Opt. Lett. 29 (15), 1775 (2004) |
| [18] | 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) |
| [19] | 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) |
| [20] | 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) |
| [21] | M. Ghotbi and M. Ebrahim-Zadeh, "Optical second harmonic generation properties of BiB3O6", Opt. Express 12 (24), 6002 (2004) |
| [22] | D. N. Nikogosyan, "Nonlinear optical crystals: a complete survey", Springer, ISBN 0-387-22022-4 |
See also: nonlinear frequency conversion, frequency doubling, ultraviolet lasers, optical parametric oscillators, periodic poling, phase matching, quasi phase matching, electro-optic modulators, Pockels cells, Spotlight article 2007-05-26
Categories: materials, nonlinear optics
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!
