Research stoichiometric of heterogeneity of lithium niobate crystals by IR spectroscopy Paranin V.D., Pantelei E.

The crystals lithium niobate (LiNbO3) is a ferroelectric material with nonlinear optical properties. Lithium niobate is one of the most widely used crystal for electro-optic modulator, integrated optic scheme and second harmonic generator.

In plate lithium niobate hydrogen can be found in the form of OH- molecular ions. The OH- ions can get to the crystal with the raw material and during the crystal growth. The concentration of OH-molecular ions in volume lithium niobate is about 1018-1019 cm-3. In the surface layers of the concentration reaches values of 1021-1021 cm-3. The incorporation can be promoted by applying an electric field. On the surface of the crystals are created the proton-exchange waveguide. The main volume waveguide is placed in a layer depth of 6 µm. Because the waveguide has a small size, therefore the optical properties and the quality of the crystal must be constant throughout the plate. However, the chemical heterogeneity can be present in the volume and the surface layers of the lithium niobate. The properties depend on the chemical composition of the crystal and its impurities.

Control methods optical emission spectroscopy X-ray fluorescence method mass spectroscopy physico-chemical method X-ray diffraction analysis Raman spectroscopy optical spectroscopy

To investigate the defects, connected with the presence of OH- groups in LiNbO3 crystal, can use infrared spectroscopy method. The most accessible method for hydrogen impurities investigation is registration of absorption spectra at about 3450 cm-1, in the range, where vibration of O-H bonds become apparent. For the basic understanding of the OH- absorption spectra in lithium niobate, one can see the review papers published by Kovacs (1984) and Schirmer (1991). The form of the spectrum absorption band and its maximum position are dependent on the crystal composition and presence of impurities. This fact allowed considering the OH- absorption band as an indicator of crystal composition.

Figure 1. TENSOR II FTIR Spectrometer with module HOPERION 1000 Series The aim of present work was investigator of changes in OH- absorption band in various points of the surface to find differences in the chemical composition. For this purpose, used a spectrophotometer Tensor Bruker 27 which has an external module FT-IR microscopy HOPERION 1000 Series (Figure 1). Figure 1. TENSOR II FTIR Spectrometer with module HOPERION 1000 Series

The samples for investigation were made from congruent LiNbO3 single crystal x-cut grown by the Czochralski technique. The sample were polished to optical grade. Sample dimensions were 10x30x1 mm. LiNbO3 crystals were proton-exchange channel waveguides on the surface to a thickness of 6 microns (Figure 2). Before the study the crystals were cleared from organic pollution by chemical means. The measurements were in the range of 3400 ... 3550 cm-1. Figure 2 - Proton-exchange channel waveguide in a crystal of lithium niobate congruent

Type crystal, direction Z-axis and measuring the path shown in Fig. 2 Type crystal, direction Z-axis and measuring the path shown in Fig. 2. Number of scans 16; the signal \ noise: 8692;   spectral resolution of 1 cm-1; measuring step: 2mm. Background was measured on aluminum mirrors. After a warm contrast to the spectra taken at one point was ∓0,1%. The aperture value constitutes 35% of the maximum value (20 microns). Figure 2 . View of the crystal and the direction of measurement

At the beginning the crystals were measured in an area where no waveguides (Figure 3). As seen in Figure 3, the depth of the absorption band of the sample in pure area is not more than 0.5% at each measurement point. Received data is not enough to detect a change of the absorption band. Figure 3. The reflection spectrum of lithium niobate X-cut in the field without the waveguide perpendicular to the Z-axis with a pitch of 2mm

The reflection spectra of the absorption band OH channel proton-exchange waveguide NL crystal X-cut (Figure 4) have different depths depending on the point in which the measurement have occurred. The total depth of the band OH vibrations from 5 to 7%, which is sufficient for reliable analysis. The result is reproducible. The difference in the spectra have differences in depth of 0.5-2%, it indicates that the heterogeneous composition of the crystal

Figure 4. The reflection spectrum of lithium niobate X-cut in the area of the channel proton-exchange waveguide perpendicular to the Z-axis with a pitch of 2 mm

The reflection spectra of the absorption band OH channel proton-exchange waveguide NL crystal X-cut (Figure 4) have different depths depending on the point in which the measurement have occurred. The total depth of the band OH vibrations from 5 to 7%, which is sufficient for reliable analysis. The result is reproducible. The difference in the spectra have differences in depth of 0.5-2%, it indicates that the heterogeneous composition of the crystal

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