Influence of Nd:Zn codoping in near-stoichiometric lithium niobate

Abstract

Near-stoichiometric lithium niobate (SLN) crystals doped with up to 1.6 mol % Zn and codoped with various Nd concentrations in the melt (0.2, 0.5, 0.9, and 1.5 mol %) (Nd:Zn:SLN) are grown from 58.6 mol % Li(2)O using conventional Czochralski technique. Crystals are pulled at the rate of 0.35 mmh with seed rotation at 9 rpm. Concentrations of Zn and Nd in the crystal are varied by adding appropriate amounts of ZnO and Nd(2)O(3) to the starting composition. Unit cell parameters of the grown crystals are calculated by Rietveld refinement method using FULLPROFF software. Domain structure studies are carried out by chemical etching followed by microscopic examination. Dielectric studies reveal the existence of piezoelectric resonance at high frequencies. Enhancement in dielectric constant and tan delta in Nd doped samples has been attributed to the space charge polarization. Nd doped samples exhibit reduction in the relative permittivity after oxygen annealing. Transmission spectra of Nd:Zn:SLN crystals in the UV region exhibit blueshift in the cutoff wavelength. In Mid Infrared (MIR) region crystals doped with 1.6 mol % Zn have shift in the OH absorption peak from 2873 to 2833 nm. Judd-Ofelt analysis carried out on the absorption spectra of codoped crystal yields the lifetime of 104 mus for the metastable state (4)F(32). The branching ratio for the electronic transition from (4)F(32) to (4)I(112) is high compared to that for (4)F(32) to (4)I(132), indicating a higher emission cross section for the former transition. Laser damage threshold evaluated using 532 nm, 5 ns pulsed neodymium doped yttrium aluminum garnet laser, shows an increase by two orders of magnitude for crystals doped with 1.6 mol % Zn. Photorefractive damage threshold for these crystals shows an enhancement of four orders of magnitude due to increase in the photoconductivity.

Investigations of the OH − absorption bands in congruent and near-stoichiometric LiNbO 3 :Hf crystals

“…They suggested the component at 3466 cm −1 corresponding to H + ions located at the Li vacancies (V − Li ) in perfect lattice and the two near 3484 cm −1 to H + ions which occupy V − Li near antisite niobium ions (Nb 4+ Li ) and are largely affected by these intrinsic defects. It is well known for highly (above the optical-damage threshold) Mg-and Zn-doped LN that no OH − absorption bands near 3484 cm −1 can be observed [2,8,9,[17][18][19]. But the case of Hf-doped LN is completely different: the components near 3484 cm −1 still remain there even when the doping level is far beyond the threshold.…”

“…Though the elimination of Nb 4+ Li ions in nearstoichiometric Mg-doped LN crystals depends not only on the doping level but also on the Li composition, the components near 3484 cm −1 are for ever due only to the Nb 4+ Li ions and can therefore be considered as a good indicator of the optical-damage threshold [6,7,19,22,23]. But in near-stoichiometric Hf-doped LN crystals, the Nb 4+ Li ion is not the unique defect which can influence the OH − stretching vibration, and the large electropositivity makes the Hf 3+…”