Notable third-order optical nonlinearities of a Keplerate-type polyoxometalate in solution and in thin films of PMMA

Abstract

The third-order nonlinear optical properties of a Keplerate-type polyoxometalate (NH4)18[(Bu4N]24[Mo132O372(H2O)72(CH3COO)30]⋅ca.7NH4CH3COO⋅ca.173H2O (denoted Bu4N-{Mo132}) in solution and in films of poly(methylmethacrylate) (denoted PMMA) matrix have been studied by the Z-scan method at 532 nm with a pulse duration of 20 ps. Experimental results show that both the solution and the solid films of Bu4N-{Mo132} have nonlinear self-defocusing effects and notable nonlinear reverse saturable absorption. Embedding Bu4N-{Mo132} as inclusion into PMMA host significantly improves the nonlinear absorption coefficient β, while it reduces the nonlinear refractive coefficient as a result of the higher concentration of Bu4N-{Mo132} in PMMA matrix than in solution. The magnitude of β for solid-state Bu4N-{Mo132} in films increases with the increasing amount of incorporated Bu4N-{Mo132}, while the third-order nonlinear refractive index n2 decreases. Experimental results prove the high stability of Bu4N-{Mo132} in PMMA films in inorganic acidic and basic solutions. These studies indicate that Bu4N-{Mo132} in thin solid films of PMMA are potential candidates for fabricating nonlinear optical devices.