LiNbO

The nonlinear optical teclmiques of up- and down-conversion are based on mixing optical beams in a suitable crystal (BBO, LiNbO, KDP, etc) witli tire generation of new optical frequencies tire physical principle is as follows. If two beams having optical frequencies cOp CO2 and wavevectors k, are mixed in a nonlinear optical crystal at tire appropriate angle, a new optical frequency co can be coherently generated witli tire following conditions satisfied ... 

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

Materials. For holographic information storage, materials are required which alter their index of refraction locally by spotwise illumination with light. Suitable are photorefractive inorganic crystals, eg, LiNbO, BaTiO, LiTaO, and Bq2 i02Q. Also suitable are photorefractive ferroelectric polymers like poly(vinyhdene fluoride-i o-trifluorethylene) (PVDF/TFE). Preferably transparent polymers are used which contain approximately 10% of monomeric material (so-called photopolymers, photothermoplasts). These polymers additionally contain different initiators, photoinitiators, and photosensitizers. 

Only certain types of crystalline materials can exhibit second harmonic generation (61). Because of symmetry considerations, the coefficient must be identically equal to zero in any material having a center of symmetry. Thus the only candidates for second harmonic generation are materials that lack a center of symmetry. Some common materials which are used in nonlinear optics include barium sodium niobate [12323-03-4] Ba2NaNb O lithium niobate [12031 -63-9] LiNbO potassium titanyl phosphate [12690-20-9], KTiOPO beta-barium borate [13701 -59-2], p-BaB204 and lithium triborate... 

Lithium Niobate. Lithium niobate [12031 -64-9], LiNbO, is normally formed by reaction of lithium hydroxide and niobium oxide. The salt has important uses in switches for optical fiber communication systems and is the material of choice in many electrooptic appHcations including waveguide modulators and sound acoustic wave devices. Crystals of lithium niobate ate usually grown by the Czochralski method foUowed by infiltration of wafers by metal vapor to adjust the index of refraction. 

Lithium niobate [12031 -63-9] Nb20 or LiNbO, is prepared by the soHd-state reaction of lithium carbonate with niobium pentoxide. After... 

LiNb02F2(solid) = LiF(soiid) + LiNbOs(solid) + NbOF3(gas) (HO)... 

The partially reduced form of niobium accounts for the color change of samples that underwent thermal treatment in vacuum or inert atmospheres. Whereas the thermal treatment of the mixture in air leads to the simultaneous oxidation of Nb4+ by oxygen, this is actually equivalent to the replacement of fluorine ions by oxygen ions in the complex structure of oxyfluoroniobate. Extended thermal treatment of systems containing LiNbOF4 and LiF yields a mixture of LiF and LiNbOs as the final thermal decomposition product. 

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

Fig. 97. Temperature dependence of SHG signals normalized by signal of powdered LiNbO (hJhm LiNbOfl. Curves a and b - synthesis of Li4NbC>4F by in situ interaction between LifZOs and NbC>2F curve c and d — Li4Nb04F after holding at 800 and 1100°C, respectively curve e - LijNb04 synthesized at 600°C. Reproduced from [419], S. Y. Stefanovich, B. A. Strukov, A. P. Leonov, A. I. Agulyansky, V. T. Kalinnikov, Jap. J. Appl. Phys., 24 (1985) 630, Copyright 1985, with permission of Institute of Pure and Applied Physics, Tokyo, Japan.

By comparison with an octadecyloxy stilbazium iodide monolayer (x<2> = 0.51 x 10"6 esu) [15], the value of the effective second-order susceptibility x r at 45° incidence was estimated to be 1.0 x 10"7 esu. The value is fairly large, in comparison with those of the conventional nonlinear optical materials (e.g. LiNbOs). Other pyrazine derivatives (C120PPy and C12SPPy) also gave thick noncentrosymmetric LB films with fairly large second-order nonlinearity by the alternating deposition with arachidic acid. The estimated Xeff values of the pyrazine LB films are listed in Table 3. 

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). 

The cover of this book shows each of these three processes the stimulated emission represented by the blue light of an Ar+ laser the absorption process responsible for the attenuation of the blue laser in a LiNbOs Pr + crystal and the spontaneous emission that corresponds to the red light emitted from Pr +... 

Figure E4.3 shows the room temperature absorption spectra of an insulator (LiNbOs), a semiconductor (Si), and a metal (Cu). (a) Determine the spectrum associated with each one of these materials, (b) From these spectra, estimate the energy-gap values of Si and LiNbOj and the plasma frequency of Cu. (c) What can be said about the transparency in the visible range for each of these materials ... 

Figure 5.9 Two examples of dynamic induced band-shape effects, (a) Weak coupling an absorption single line of the Yb + ion in LiNbOs (denoted by an arrow) is accompanied by the appearance of phonon side bands (reproduced with permission from Montoya et al., 2001) (b) Strong coupling the broadband luminescence of the Cr + ion in LiNbOs (reproduced with permission from Camarillo et al., 1992).

Figure 6.2 The absorption spectium of Nd ions in LiNbOs, taken at room temperature (right-hand side) (registered by the authors). The Dieke diagram levels corresponding to the Nd + ion are shown on the left-hand side.

Figure 6.3 The low-temperature emission spectrum of Eu + ions in LiNbOs. Part of the Dieke diagram for the Eu + ion is included for explanation (reproduced with permission from Munoz et at., 1995).

In Table E7.5, the fluorescence lifetimes and quantum efficiencies measured from different excited states of the Pr + ( Po and D2) and Nd + (" Fs ji) ions in a LiNbOs crystal are listed, (a) Determine the multiphonon nonradiative rate from the 19/2 and In/2 states of the Er + ion in LiNbOs. (b) If a fluorescence lifetime of 535 /us is measured from the excited state Fs/2 of the Yb + ion in this crystal, estimate the radiative lifetime from this state. 

Fig. 11.32 Transmission, 7(%) vs wavelength, X, for the smart window, ITO/WOs/LiNbOs/ V205/In203 (a) bleached state (b) coloured state (Goldner et al, 1988).

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. 

M. Yamada, N. Nada, M. Saitoh, and K. Watanabe, First-order quasi-phase matched LiNbOs wave-guide periodically poled by applying an external-field for efficient blue second-harmonic generation. Applied Physics Letters 62(5), 435-436 (1993). 

Several methods can be used for waveguide fabrication in LiNbOs. Among them, titanium in-diffusion and proton exchange (PE) are the most popular ones since they lead to the formation of well-confined and low-loss layers. PE is mainly applied because it results in a considerable decrease of the photorefractive effect in LiNbOj. However, waveguides obtained by pure PE have reduced EO and NL coefficients and usually a post exchange aimealing (APE) is required for restoration of the EO activity. 

PE layers were formed in Z-cut LiNbOs and LiTaOs congruent crystals by using a variety of proton sources, such as benzoic acid - pure (CeHsCOOH) or diluted with lithium benzoate (CeHeCOOLi) pyro-phosphoric acid (H4P2O7), etc. The plates were immersed in the acid melt for various periods of time (up to 8 h in the case of LiNbOs and up to 42 h for LiTaOs substrates) at a temperature in the range of 200 - 240°C. Some of the samples were subsequently annealed at temperatures up to 420°C for periods of time varying from 10 min to 2 h. 

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