Fabrication and optical nonlinearities of ultrathin composite films incorporating a Keplerate type polyoxometalate

Abstract

Functional ultrathin films ({Mo132}/PEI)n have been fabricated by using the Keplerate-type polyoxometalate (NH4)42[Mo132O372(CH3COO)30(H2O)72].ca.300H20.ca.10CH3COONH4 (denoted by {Mo132}) and polyethyleneimine (PEI) through the electrostatic layer-by-layer self-assembly method. The structure of the {Mo132} in the multilayers is kept intact; the deposition amounts of PEI and {Mo132} remain constant in every adsorption cycle in the ultrathin multilayers assembly process. Nonlinear optical properties of the ultrathin films and aqueous solution of {Mo132} have been investigated by using the Z-scan technique at a wavelength of 532nm and pulse width of 5ns. The results show that both the aqueous solution of {Mo132} and the resulting ultrathin multilayer films have nonlinear self-defocusing effects, while the ultrathin films also have notable nonlinear saturable absorption. The third-order NLO susceptibility χ(3) increases with the increase of film thickness. The ultrathin films of ({Mo132}/PEI)n (n=28) have the nonlinear refractive index coefficient n2=−3.37×10−10esu, nonlinear absorption coefficient β=−5.09×10−6esu and the third-order NLO susceptibility χ(3)=3.8×10−11esu.