Investigation of the femtosecond optical limiting properties of monoclinic copper niobate

Investigation of the third-order nonlinear optical properties and optical limiting behaviour of micro-structured monoclinic phase copper niobate (CuNb2O6) was performed by the Z-scan technique using femtosecond laser pulses (800 nm, 150 fs, 80 MHz). CuNb2O6 was synthesized by solid-state reaction at a sintering temperature of 700 degrees C maintained at different times of 3, 6, 9 and 12 h. Formation of rods at higher reaction time of 12 h was observed and is attributed to the mass transport and coalescence processes. From the absorption tail of UV-Vis spectrum, the optical band gap was estimated to be 3.5 eV. In the fluorescence spectra, blue emission was observed near 430 nm and was assigned to the charge transfer from oxygen to central niobium of Nb-O-6 octahedra. Open-aperture Z-scan data demonstrated the presence of nonlinear absorption in copper niobate and are ascribed to two-photon absorption process. Closed-aperture data indicated a sign reversal in nonlinear refraction as the sintering time increased. Third-order nonlinear optical coefficients were estimated, and the largest coefficient was observed for the rod-structured CuNb2O6. Copper niobate exhibited optical limiting behaviour, and the limiting threshold was found to be lowest for microrod structures (similar to 0.21 mu J/cm(2)). Due to the top-notch third-order nonlinear optical coefficients and excellent limiting behaviour, monoclinic copper niobate microrods can be used as a potential material for utilization as an optical limiter for femtosecond pulses.