Lithium niobate, LiNbO

Oxyfluoride compounds and highly densified ceramics that are related to lithium niobate, LiNbOs, were obtained in the system ... 

Raman spectra are usually represented by the intensity of Stokes lines versus the shifted frequencies 12,. Figure 1.15 shows, as an example, the Raman spectrum of a lithium niobate (LiNbOs) crystal. The energies (given in wavenumber units, cm ) of the different phonons involved are indicated above the corresponding peaks. Particular emphasis will be given to those of higher energy, called effective phonons (883 cm for lithium niobate), as they actively participate in the nonradiative de-excitation processes of trivalent rare earth ions in crystals (see Section 6.3). 

It is worth repeating that the relatively low efficiency for the appKTP crystal is due to the fact that Je (KTP) < Je (KNb03). Performing the same assessment with lithium niobate (LiNbOs) should yield up to four times the efficiency, because dg/f (LiNbOs) = 17.6 pmV. Unfortunately, insufficient power was available to measure the duration of the blue pulses from the bulk appKTP crystal. However, our calculations show that the generated blue pulses would be characterized by an uncompensated duration of 370 fs. These pulses could be compressed to around 270 fs in order to access higher peak powers. 

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

Ferroelectric lithium niobate (LiNbOs) has been of considerable interest because of its nonlinear optical properties. Conversion of infrared into visible radiation in LiNb03 crystals has been observed (Midwinter and Warner, 1967 Arutyunyan and Mkrtchyan, 1975). Electro-optic coefficients of LiNbOs have been determined for a wide range of frequencies ranging from the visible (Smakula and Claspy, 1967) to the millimeter-wave portion of the spectrum (Vinogradov et al., 1970). Other nonlinear optical properties such as photovoltaic effects (Kratzig and Kurz, 1977) and optically induced refractive index changes (Ashkin et al., 1966 Chen, 1969) have also been observed. 

Photorefiractivity was discovered by Ashkin and coworkers in 1966 with Lithium niobate LiNbOs and other compounds. Photorefiractive materials change their index of refraction when irradiated by light. The change of index of refraction continues from milliseconds to years. The basic aspects of photorefractivity are treated in textbooks. ... 

Intrinsically acentric organic films are becoming a valuable alternative to poled-polymms and inoiganic matmials such a lithium niobate (LiNbOs) for die formation of electixKqitic devices. The ability to ftbricate noncentro mmetric structures in a molecular layer-by-layer approach offers nanoscale control over film dimensions and allows tailoring of microstructural and optical i operties at die molecular level. 

Since the electro-optic tensor has the same symmetry as the tensor of the inverse piezoelectric effect, the linear electro-optic (Pockels) effect is confined to the symmetry groups in which piezoelectricity occurs (see Table 8.3). The electro-optic coefficients of most dielectric materials are small (of the order of 10 m V ), with the notable exception of ferroelectrics such as potassium dihydrogen phosphate (KDP KH2PO4), lithium niobate (liNbOs), lithium tantalate (LiTaOs), barium sodium niobate (Ba2NaNb50i5), or strontium barium niobate (Sro.75Bao.25Nb206) (Zheludev, 1990). For example, the tensorial matrix of KDP with symmetry group 42m has the form... 

Lead zirconate titanate (PZT), barium titanate (BaTiOs), lead titanate (PbTiOs), potassium niobate (KNbOa), lithium niobate (LiNbOs), lithium titanate (LiTaOs), sodium tungstate (Na2W03) and zinc oxide (ZnO) are some of the most typical piezoceramics. Of these, PZT is the most widely used due to its superior performance. However, the toxicity of lead has raised concerns over the use of PZT. A restriction on the amount of lead present has been placed and is focused at eliminating its use eventually. Nevertheless, PZT has no rival at present. 

For a widely used electro-optics crystal such as lithium niobate (LiNbOs), we have the electro-optics coefficients 1-33=30.8 (in units of 10 m/V), 1-13=8.6, 1-22=3.4, and 1-42=28, with n =2.29 and rig=2.20 (at X=550 nm). It is important to note that these are typical values, and they conld vary quite considerably depending on the presence of impurities (or dopants) and the method of growing these crystals, among other factors. For these values of electro-optics coefficients ( 10 " mAO, an applied dc voltage of 10,000 V is needed to create a phase shift of —n in a crystal of centimeter length. By comparison, in hquid crystal electro-optics devices, the typical ac voltage needed is aroimd 1V and the hquid crystal thickness is on the order of a few microns. 

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