The relation between the polarization density P of a dielectric medium and the electric field E is linear when E is small, but becomes nonlinear as E acquires values comparable with interatomic electric fields (105 to 108 V/cm). Under these conditions the relation between P and E can be expanded in a Taylor’s series
P = ε0χ(1) E + 2χ(2) E 2 + 4χ(3) E 3 + … (1)
where εo is the permittivity of free space, while χ(1) is the linear and χ(2), χ(3), etc., the nonlinear optical susceptibilities.
If we consider two optical fields, the first Ejω1 (along the j-direction at frequency ω1) and the second Ekω2 (along the k-direction at frequency ω2) one can write the second term of the Taylor’s series as follows
When ω1 ≠ ω2 the (parametric) mixing of the two fields gives rise to two new polarizations at the frequencies ω3 = ω1 + ω2 and ω3´ = ω1 – ω2. When the two frequencies are equal, ω1 = ω2 = ω, the result is Second Harmonic Generation (SHG): χijk (2ω, ω, ω), while equal and opposite frequencies, ω1 = ω and ω2 = –ω lead to Optical Rectification (OR): χijk (0, ω, –ω). In the SHG case the following convention is adopted: the second order nonlinear coefficient d is equal to one half of the second order nonlinear susceptibility
dijk = 1/2χ(2)
Because of the symmetry of the indices j and k one can replace these two by a single index (subscript) m. Consequently, the notation for the SHG nonlinear coefficient in reduced form is dim where m takes the values 1 to 6. Only noncentrosymmetric crystals can possess a nonvanishing dijk tensor (third rank). The unit of the SHG coefficients is m/V (in the MKSQ/SI system).
In centrosymmetric media the dominant nonlinearity is of the third order. This effect is represented by the third term in the Taylor’s series (Equation 1); it is the result of the interaction of a number of optical fields (one to three) producing a new frequency ω4 = ω1 + ω2 + ω3. The third order polarization is given by
Third Harmonic Generation (THG) is achieved when ω1 = ω2 = ω3 = ω. In this case the constant g4 = 1/4. The third order nonlinear coefficient C is related to the third order susceptibility as follows:
Cjklm = 1/4χjklm
This coefficient is a fourth rank tensor. In the THG case the matrices must be invariant under permutation of the indices k, l, and m; as a result the notation for the third order nonlinear coefficient can be simplified to Cjn . The unit of Cjn is m2·V–2 (in the MKSQ/SI system).
Applications of second order nonlinear optical materials include the generation of higher (up to sixth) optical harmonics, the mixing of monochromatic waves to generate sum or difference frequencies (frequency conversion), the use of two monochromatic waves to amplify a third wave (parametric amplification) and the addition of feedback to such an amplifier to create an oscillation (parametric oscillation).
Third order nonlinear optical materials are used for THG, self-focusing, four wave mixing, optical amplification, and optical conjugation. Many of these effects — as well as the variation and modulation of optical propagation caused by mechanical, electric, and magnetic fields (see the preceding table on “Elasto-Optic, Electro-Optic, and Magneto-Optic Constants”) are used in the areas of optical communication, optical computing, and optical imaging.
Material | Other names | Formula | CAS Reg. No. | Symmetry class | dim×1012/m V-1 | λ/µm |
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Aluminum phosphate | Aluminum orthophosphate | AlPO4 | 7784-30-7 | 32 | d11 = 0.35 ± 0.03 | 1.058 |
Ammonium dihydrogen phosphate | ADP | NH4H2PO4 | 7722-76-1 | 4̅2 m | d36 = 0.53 | 1.064 |
Ammonium dihydrogen phosphate | ADP | NH4H2PO4 | 7722-76-1 | d36 = 0.85 | 0.694 | |
Barium borate (β) | BBO | Ba(BO2)2 | 13701-59-2 | 3 m | d22 = 2.22 ± 0.09 | 1.06 |
Barium borate (β) | BBO | Ba(BO2)2 | 13701-59-2 | d31 = 0.16 ± 0.08 | 1.06 | |
Barium sodium niobate | BSN | Ba2Na(NbO3)5 | 12323-03-4 | mm 2 | d33 = –17.6 ± 1.28 | 1.064 |
Barium sodium niobate | BSN | Ba2Na(NbO3)5 | 12323-03-4 | d31 = –12.8 ± 1.28 | 1.064 | |
Barium strontium niobate | Barium niobium strontium oxide | BaSr(NbO3)4 | 37185-09-4 | 4 mm | d33 = 11.3 ± 3.3 | 1.064 |
Barium strontium niobate | Barium niobium strontium oxide | BaSr(NbO3)4 | 37185-09-4 | d31 = 4.31 ± 1.32 | 1.064 | |
Barium strontium niobate | Barium niobium strontium oxide | BaSr(NbO3)4 | 37185-09-4 | d15 = 5.98 ± 2 | 1.064 | |
Barium titanate | Barium metatitanate | BaTiO3 | 12047-27-7 | 4 mm | d33 = 6.8 ± 1.0 | 1.064 |
Barium titanate | Barium metatitanate | BaTiO3 | 12047-27-7 | d31 = 15.7 ± 1.8 | 1.064 | |
Barium titanate | Barium metatitanate | BaTiO3 | 12047-27-7 | d15 = 17.0 ± 1.8 | 1.064 | |
Benzil | Diphenylethanedione | (C6H5CO)2 | 134-81-6 | 32 | d11 = 3.6 ± 0.5 | 1.064 |
Bismuth germanate | BGO | Bi4(GeO4)3 | 12233-56-6 | 4̅3 m | d14 = 1.28 | 1.064 |
Cadmium germanium arsenide | CdGeAs2 | 4̅2 m | d36 = 351 ± 105 | 10.6 | ||
Cadmium selenide | Cadmoselite | CdSe | 1306-24-7 | 6 mm | d33 = 54.5 ± 12.6 | 10.6 |
Cadmium selenide | Cadmoselite | CdSe | 1306-24-7 | d31 = –26.8 ± 2.7 | 10.6 | |
Cadmium sulfide | Greenockite | CdS | 1306-23-6 | 6 mm | d33 = 25.8 ± 1.6 | 1.058 |
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