This study reports on the determination of the optical constants of a lead-free ferroelectric material, Na0.5Bi0.5TiO3. The optical transmission measurements were carried out in the 300–3000 nm wavelength range, on a (100)-oriented Na0.5Bi0.5TiO3 thin film, epitaxially grown by laser ablation on a (100)MgO single crystal substrate. Applying the “method of the envelopes,” developed by Manifacier et al. [J. Phys. E 9, 1002 (1976)] and by Swanepoel [J. Phys. E 16, 1214 (1983)], the analysis of the observed interference fringes allowed extracting some optical data for Na0.5Bi0.5TiO3, namely the linear refractive and extinction indices n and k, the absorption coefficient α, and as well the nature (direct or indirect transition) and value of the optical band gap. It was found that n∞=2.321 and the transmission data suggested a 3.30 eV indirect transition. Our experimental results are thus in opposition to the few data reported so far for Na0.5Bi0.5TiO3, where a direct transition was evoked. Therefore, we have confronted the optical transmission data to supplementary diffuse reflectance and ellipsometry measurements, and finally, to first principles calculations. The diffuse reflectance data, collected for Na0.5Bi0.5TiO3 powders, indicate a 3.26 eV optical band gap. In addition, the ellipsometry measurements reveal a refractive index of 2.346 at 2.066 μm, an energy gap of ∼3.20 eV, and also confirm the thickness of the layer. These additional data are then in very good agreement with the data derived from the optical transmission measurements. Finally, first principles calculations were carried out in the framework of the density functional theory for the three Na0.5Bi0.5TiO3 polymorphs (i.e., rhombohedral, tetragonal, and cubic). For each polymorph, in order to consider the Na/Bi disorder on the perovskite A-site, three different Na/Bi distributions within the unit cell were investigated, leading to nine atomic configurations. The electronic structure of these nine configurations was then calculated after geometry optimization and only one of them did not converge. These calculations complete the above experimental data, as six electronic structures over eight presented an indirect band gap, with a band gap energy Eg falling in the 3.90–4.60 eV energy range. The latter value is higher than the experimental ones but is definitely more compatible with the energy band gap expected for ferroelectric oxide materials.

We report the electrical properties, measured both at the macroscopic and nanometric scales, of epitaxial (110)-Na_0.5Bi_0.5TiO₃ (NBT) thin films grown on (110)Pt/(110)SrTiO₃ by pulsed laser deposition (PLD). The influence of the A-site composition (Na and/or Bi excess) on both the structural/microstructural characteristics and the electrical properties is discussed. Whatever the composition of the NBT target, the final layers are systematically epitaxially grown, with NBT crystallites mainly (110)-oriented, and as well (100)-oriented for some minor proportion. Atomic force microscopy (AFM) images reveal the coexistence of two kinds of grains presenting different shapes: namely flat and elongated grains, corresponding to (100)- and (110)-oriented NBT crystallites, respectively. The macroscopic ferroelectric properties were measured at room temperature. A rather well-defined shape of the hysteresis loops was obtained: the incorporation of a Bi excess in the target clearly improves the saturation of the loops. The ferroelectric performances are a remanent polarization (P_r) value, ranging from 7 to 14 μC/cm², associated with a coercive field (E_c) in the range 68–85 kV/cm. In addition, at 10⁵ Hz, the relative permittivity was about ɛ_r ∼ 255–410 and the dielectric losses (tan δ) were ∼6%–7%. Finally, the electrical properties at the local scale were investigated by coupling piezoresponse force microscopy (PFM) and tunneling AFM (TUNA) measurements. The collected data reveal that the two types of grains behave differently. The PFM amplitude signal of (110)-oriented grains is very contrasted and such grains are often divided in ferroelectric bi-domains of nanometric sizes, whereas the response of (100)-oriented grains is less contrasted and more homogeneous. The interpretation of the PFM signal is provided. The piezoloop recorded on a (110)NBT grain is strongly distorted and shifted along the vertical axis, in agreement with the vertical drift observed for macroscopic ferroelectric data. Finally, TUNA data clearly indicate that flat grains are leakier than elongated grains, highlighting the anisotropy of the electrical properties at the local scale.

We report on the laser emission of the polycrystalline ceramic obtained from the full and congruent crystallization of the parent glass 1Nd³⁺:75TeO₂-12.5Bi₂O₃-12.5Nb₂O₅ composition. In particular, the current work underlines the importance of carefully controlling the heat treatment in order to solely crystallize the Bi_0.8Nb_0.8Te_2.4O₈ cubic phase and consequently avoid the formation of the BiNbTe₂O₈ orthorhombic phase that would be detrimental for optical purpose. The structure, microstructure and photoluminescence properties of the resulting transparent tellurite ceramics are characterized. The continuous-wave and gain-switching laser performances reveal that the emission remains perfectly single transversal mode in the range of pump powers explored. The maximum output power achieved was ~28.5 mW, for a pump power threshold of ~67 mW, and with associated efficiency and slope efficiency of ~22.5% and ~50%, respectively. These data definitely stand among the best results obtained so far for bulk laser tellurite materials and thus demonstrate the potential of such polycrystalline transparent ceramics as optically active materials. Finally, the laser emission characteristics in pulsed regime, at low and high repetition rates, are also provided: more than 6.5 W of peak power at a repetition rate of 728 kHz can be obtained.

The (a,b) plane angular dependence of the third-order nonlinear optical susceptibility, ${\chi }^{(3)}$ , of a c-cut paratellurite (α-TeO₂) single crystal was quantitatively evaluated here by the Z-scan technique, using a Ti:sapphire femtosecond laser operated at 800 nm. In particular, the mean value Re($⟨{\chi }^{(3)}{⟩}_{a,b}$ )(α-TeO₂) of the optical tensor has been extracted from such experiments via a direct comparison with the data collected for a fused silica reference glass plate. A $Re(⟨{\chi }^{(3)}{⟩}_{(a,b)}(\alpha -{\text{TeO}}_2)):Re({\chi }^{(3)})({\text{SiO}}_2\hspace{0.17em}\mathit{\text{glass}})$ ratio roughly equal to 49.1 is found, and our result compares thus very favourably with the unique experimental value (a ratio of ∼50) reported by Kim et al. [J. Am. Ceram. Soc. 76, 2486 (1993)] for a pure TeO₂ glass. In addition, it is shown that the angular dependence of the phase modulation within the (a,b) plane can be fully understood in the light of the strong dextro-rotatory power known for TeO₂ materials. Taking into account the optical activity, some analytical model serving to estimate the diagonal and non-diagonal components of the third order nonlinear susceptibility tensor has been thus developed. Finally, Re(${\chi }_{\mathit{\text{xxxx}}}^{(3)}$ ) and Re(${\chi }_{\mathit{\text{xxyy}}}^{(3)}$ ) values of $95.1\times {10}^{-22}\hspace{0.17em}{\hspace{0.17em}\mathrm{m}}^2{/\mathrm{V}}^2$ and $42.0\times {10}^{-22}\hspace{0.17em}{\mathrm{m}}^2{/\mathrm{V}}^2$ , respectively, are then deduced for a paratellurite single crystal, considering fused silica as a reference.

The influence of the Bi content, controlled by deposition conditions, on SrBi 2 Nb 2 O 9 ( m = 2 phase) thin films synthesis was studied. Single m = 2 phase films were synthesized by pulsed laser deposition on various substrates provided a fine control of deposition conditions. A comparative study of two different epitaxial films on (100)SrTiO 3, a SrBi 2 Nb 2 O 9 stoichiometric one and a Bi-deficient one was performed by X-ray Photoelectron Diffraction. In the second example, results confirm the epitaxial intergrowth of m = 2 and a m = 3 type phases. However the latter could not be obtained as single phase in thin films and, moreover, never appeared in bulk samples, a strong indication of its metastable character and of its stabilization only when sandwiched in a m = 2 matrix.