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

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 n ₂ 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.