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Refractive index of optical material of powder is measured not as easily as a bulk material. Here, the prism coupling technique in combination with the immersion method is proposed to measure the refractive index of an optical material of powder. First, the powder material to be measured was dispersed in α-bromonaphthalene (C 10 H 7 Br) liquid to form a suspension mixture. The refractive index of the mixture, together with that of pure C 10 H 7 Br, was then measured at the wavelengths of 632.8, 1311, and 1553 nm using a commercial prism coupler. From the measured index values of pure C 10 H 7 Br and powder-dispersed mixture, the refractive index of the powder material was obtained on the basis of the Maxwell-Garnett model. Microcrystal powder from a LiNbO 3 single-crystal, which has the known refractive index values, has been exemplified to demonstrate the method. The results show that the method is feasible with an accuracy of ±0.05.

A number of Li-deficient MgO-doped LiNbO 3 (LN) crystals with different Li 2 O contents ranging from 43.4 mol. % to 44.5 mol. % were prepared by carrying out the Li-poor vapor transport equilibration treatment on 5 mol. % (in growth melt) MgO-doped LN crystals. Unclamped electro-optic (EO) coefficients γ 13 and γ 33 of these crystals were measured by Mach-Zehnder interferometry. The results show that γ 13 (γ 33 ) increases linearly by ∼14% (11%) as the Li 2 O content decreases from 44.5 mol. % of the as-grown state to 43.4 mol. % of the Li-deficient state. This feature is desired for the EO application of the Li-deficient MgO:LN crystal.