Linear and nonlinear optical properties of Co₃O₄ nanoparticle-doped polyvinyl-alcohol thin films

Thin films made from a composite of the polymer polyvinylalcohol and cobalt oxide (Co₃O₄) nanoparticles were fabricated by spin coating. Linear and nonlinear optical properties of thin films with thicknesses of hundreds of nanometers were investigated. The refractive index and absorption coefficient were measured and two direct band gaps (E_g = 1.38 eV and 2.0 eV) were determined from the absorption spectrum. Reversed saturable absorption and saturable absorption were observed when the films were illuminated with the different fluences. Optical nonlinearities corresponding to reverse saturable absorption were measured by the z-scan technique. A nonlinear refractive index (n₂) of ~10_-10 cm²/W and nonlinear absorption (β) of ~10³ cm/GW have been measured from 425 nm to 675 nm. The experimental results show that the Co₃O₄ nanoparticle/PVA composite is a promising material for nonlinear optical devices in the visible, since it takes advantages of the high optical nonlinearities of transition metal oxides and the superior mechanical properties and convenient fabrication properties of polymers.