Effect of lanthanum doping on linear and nonlinear-optical properties of Bi3TiNbO9 thin films

Lanthanum doped Bi3TiNbO9 thin films (LBTN-x, La3+ contents x = 5%, 15%, 25% and 35 mol.%) with layered perovskite structure were fabricated on fused silica by pulsed laser deposition method. Their linear and nonlinear optical properties were studied by transmittance measurement and Z-Scan method. All films exhibit good transmittance (>55%) in visible region. For lanthanum doping content are x = 5%, 15% and 25 mol.%, the nonlinear absorption coefficient of LBTN-x thin films increases with the La3+ content, then it drops down at x = 35 mol.% when the content of La3+ in (Bi2O2)2+ layers is high enough to aggravate the orthorhombic distortion of the octahedra. We found that, 25 mol.% is the optimal La3+ content for LBTN-x thin films to have the largest nonlinear absorption coefficient making the LBTN-x film a promising candidate for absorbing-type optical device applications.

Research highlights► LBTN-x thin films with layered perovskite structure were prepared on fused silica substrates by the pulsed laser deposition method. The linear optical constants were determined from the transmittance spectra using the envelope method. With increasing La3+ content, the refractive index of LBTN-x thin film decreases but the bandgap of the thin film increases. The dispersion in the refractive indexes are fitted by the Sellmeier dispersion relation and described by an electronic oscillator model. Average oscillator strength increases with the content of La3+ except L4 sample, in which the La3+ content exceeded the critical value. The nonlinear optical properties are obtained using a Z-scan technique at a wavelength of 800 nm with a 100 fs duration laser. The magnitude of β depends on the population of the impurity band. Our results showed that when x ≤ 25%, the nonlinear absorption coefficient of LBTN-x increases with the La3+ content. For the case of L4 sample, x = 35%, the substituted content of Bi3+ in (Bi2O2)2+ layers is high enough to aggravate the orthorhombic distortion of the octahedra, resulting a remarkable change in linear and nonlinear optical properties. Therefore, x = 25% is the optimal La3+ doping content for LBTN-x to obtain the largest nonlinear absorption coefficient.