Raman spectroscopic study of proton-exchanged LiTaO3 crystals

Abstract

Raman spectra of LiTaO3 crystals with different proton‐exchange times are examined. The oblique phonon at 190 cm−1 and A1(TO) phonons at 356 and 596 cm−1 appear in the transverse A1‐ and E‐symmetry spectra, respectively. Their intensities change with proton‐exchange time and reach a maximum in the spectrum of the sample with about 8 h treatment. In the E‐symmetry spectra, the intensities of E(TO) modes also show a dependence on proton‐exchange time and reach a maximum in the spectrum of the sample with 12 h treatment. These results are attributed to both the internal strain resulting from the order–disorder distribution of protons inside the sample and the enhanced photorefractive effect. In addition, the existence of energy transfer between A1(TO) and E(TO) modes is ascribed to the coexistence of paraelectric and ferroelectric phases in the protonated samples.

The microstructural difference between proton-exchanged and crystals by Raman spectroscopy

“…A similar result can also be obtained in other scattering geometries. Shown in figure 6 is the typical PE time dependence of the normalized intensity for the A 1 (TO) mode at 356 cm −1 relative to E(TO) at 382 cm −1 , measured in x(zx)z geometry [10]. The tendency of the curve is also opposite to that in figure 5 and is in agreement with the change of relative intensity I E(T O) /I A 1 (T O) with PE time.…”

“…From our previous work [9,10], it was known that the PE process is accompanied by accumulation and relief of internal strain. According to the perturbation approach used by Barker [14], the internal strain with the four components (s 1 − s 2 ), s 4 , s 5 , and s 6 can lead to an E mode becoming Raman active for (zz) incident and scattering phonon polarization.…”

“…This would be beneficial to waveguide applications. However, in our recent work [9,10] we drew attention to the changes of bulk phonon properties with PE time by means of a conventional Raman scattering technique. The experimental results found that the E(TO) and A 1 (TO) modes appear in the A 1 -and E-symmetry spectra, respectively, and their intensities show a dependence on the PE time.…”

“…Second, the PELT waveguide shows much less susceptibility to the photorefractive effect than PELN [9,10,12,13] (estimated to be one to two orders of magnitude lower than for PELN). These results imply that the PE in the two ferroelectrics is somewhat different, or that the two crystals are not completely isomorphous.…”

LiNbO 3 proton-exchanged waveguide layers: Phase composition and stress

“…This means that the layer has lost its anisotropy which indeed corresponds to the total loss of electro-optical properties at high level of doping. Such kind of quasi-amorphization (paraelectric state) has been observed when Raman spectroscopy is applied for waveguide characterization showing that all β i -phases (i = 1-4) are paraelectric [13]. Actually, it is well known that at higher values of x the lattice changes from perovskite to cubic [14].…”