MONTANA BOARD OF RESEARCH AND COMMERCIALIZATION TECHNOLOGY

Electro-optical and nonlinear optical crystals like lithium niobate and lithium tantalate are important materials for light beam control with the use of an applied electric field. They are widely used in optoelectronics and optical communication. Due to significant differences in the observed characteristics, the technical scope of these materials for many applications is limited.

The project was devoted to comparative study of intrinsic and extrinsic defects in nonlinear optical materials for the development of materials suitable for advanced applications. The work was performed in collaboration with Scientific Materials Corporation (SMC).

Lithium Niobate and Tantalate (LN and LT) have been of great interest for many years for both fundamental science and technical applications because of the unusual richness of its ferro-, pyro- and piezoelectric properties. Conventional LNcrystals, grown from a congruent melt with lithium deficiency (Xmelt = XCrystal = 48.4%, where X = [Li]/([Li]+[Nb]), contain some percent of intrinsic (non-stoichiometric) defects and, consequently, have strong structural disorder. Crystalsgrown from melts containing potassium have extremely low intrinsic defect concentrations. These samples, called stoichiometric, have significantly decreased widths of spectral lines. This leads to increased resolution of optical and EPR/ENDOR spectra.

The X- and Q-band EPR investigations of the paramagnetic Nd3+ and Yb3+ ions in the temperature range between 4 and 50 K have shown the existence of several different centers. The presence of nonequivalent centers manifests itself also in the multiband structure of observed optical spectra. Since the EPR line width in the stoichiometric crystal is about 15 G, i.e. 10 times smaller than in the congruent sample (Fig. 1), we were able to obtain additional information:

EPR Nd

Fig. 1. The EPR spectra of Nd3+ in congruent and stoichiometric lithium niobate

Comparison of the EPR spectra for congruent and stoichiometric crystals doped with relatively low concentration of iron (about 0.01 wt.% in the melt) clearly demonstrates several important features that appeared in stoichiometric LT (Fig. 5):

  • all lines become significantly narrower

  • the line shape changes to the shape of symmetrical derivative of the absorption signal

  • many small lines of trace impurities and satellite become resolved

  • new group of lines labeled Fe2 arises in stoichiometric LT samples prepared by VTE treatment.

The lines of the discovered Fe3+ center, Fe2 become stronger in stoichiometric LT samples doped with high concentration of iron (Fig. 2). The crystal field parameter of the Fe2 center (b20 » 2050´10-4 cm-1) is significantly smaller than for Fe1. The ENDOR measurements have shown that hyperfine interactions of the Fe3+ electrons with the surrounding Li nuclei for Fe2 are significantly stronger than for Fe1. Therefore, the conclusion was made that in the case of the Fe2 center the iron ion substitutes for Ta and has Li nuclei in the nearest neighborhood.

EPR in cLN
EPR sLN

Fig. 2. Left: the EPR spectra of congruent (a) and stoichiometric (b) lithium tantalate doped with low concentration of iron. X-band, room temperature, B||x. Right: The EPR spectrum of stoichiometric LT doped with 6.7´1019 cm-3 of Fe. Q-band, room temperature, B||x.

Reference

G.Malovichko, R.Petersen, Ch.Bäuman, V.Grachev. Second axial Fe3+ center in stoichiometric lithium tantalate. Journal of Applied Physics, 100, 023911 (2006)