Nonlinear optical studies of the sol-gel derived lithium tetraborate thin film

Abstract

Lithium tetraborate, Li2B4O7 films on glass substrate were prepared by chemical solution decomposition (CSD) method. The spin coated film (2000 rpm, 20 sec) was preheated (200 °C, 20 min) and then annealed at different temperatures (450°-600 °C) to form polycrystalline Li2B4O7 film. By X-ray diffraction (XRD), the prominent planes are found to be (100), (113), (302) and (403). The result shows that the main component of the film is Li2B4O7 crystalline phase. A slow XRD scan was used to calculate the crystallite size (D) and the average crystallite size was found to be 178 nm. The lower cut off wavelength at 325 nm with 75% transparency was measured by the UV-Vis spectra and the spectral dependence of the optical constants (extinction coefficient (K), reflectance(R)) was estimated. The optical energy band gap was found to be 3.62 eV. The nonlinear optical properties were studied by Z-scan experiment using a Q-switched, Nd: YAG laser (532 nm, 7 ns). The nonlinear absorption coefficient (NLA) β was found to be -5.74×10-5 cm/W. Reverse saturable absorption (RSA) is due to the multi photon absorption process. The high value of β makes the film suitable for optical limiting applications.