An increase of more than 30% in the electrooptic coefficients of Fe-doped and Ce-doped stoichiometric LiNbO₃ crystals

The composition dependences of the electrooptic coefficients in LiNbO₃ crystals and the influence of doping with Ce-ions and Fe-ions were measured precisely and investigated in detail for the first time. The electrooptic coefficient r₃₃ increases with an increase in the Li/Nb ratio. The r₃₃ in stoichiometric LiNbO₃ crystal was 1.14 times as large as that in conventional congruent LiNbO₃ crystal whose electrooptic coefficient is 32.6 pm/V. The electrooptic coefficient r₃₃ is also increased by doping with some ions. An increase of more than 30% in electrooptic coefficients (from those in nondoped congruent LiNbO₃) can be obtained by doping with Ce- or Fe-ions. The lattice constants are also measured. The lattice constant along the c-axis decreases with an increase in the Li/Nb ratio, or by doping with Ce- or Fe-ions, and in this way the crystal structure changes into a less symmetric structure. This may be the reason why the electrooptic coefficient increases with an increase in the Li/Nb ratio or by doping with Ce- or Fe-ions.