Femtosecond optical nonlinearities of Au/TiO 2 thin films prepared by a sputtering method
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Akihiro Mito National Institute of Advanced Industrial Science and Technology (AIST), NMIJ, Tsukuba, Ibaraki 305-8563, Japan (Received 14 July 2010; accepted 29 November 2010)
A multitarget sputtering method was applied to embed Au nanoparticles in TiO2 thin films (Au/ TiO2 films) with a high concentration of Au particles (19–41 at%). The absolute values of imaginary part of the third-order nonlinear susceptibility, |Im [v(3)]|, of the Au/TiO2 films, exhibited a peak around the localized surface plasmon resonance absorption peak (around 660 nm), and the maximum value was estimated to be 3.6 107 esu measured by the femtosecond Z-scan technique. The figure of merit, |Im [v(3)]|/a, (a is the absorption coefficient of the film at the corresponding wavelength of the measurement) of the film was calculated to be 1.4 1012 esucm, which was larger than that of the Au/SiO2 film. This is mainly due to the local field enhancement. I. INTRODUCTION
Gold and silver nanoparticles embedded in dielectric matrices (metal/dielectric composite materials) have a wide variety of beautiful colors associated with the localized surface plasmon resonance (LSPR) absorption and are of scientific and technological interest. The composite materials have been widely studied as resonant-type third-order optical nonlinear materials for potential use in photonic devices.1 It is known that the third-order optical nonlinearities of the composite materials are influenced by the kind and size of the embedded metal particles, the dielectric constant (refractive index), and the thermal conductivity of the dielectric matrices. Various kinds of materials such as SiO2, Al2O3, and TiO2 have been investigated for the dielectric matrices. Especially, TiO2 with high refractive index seems to be a desirable dielectric for the matrices. The metal/dielectric composite materials have been prepared by various methods such as melt quenching,2 sol–gel,3 ion implantation,4 sputtering,5,6 and deposition–precipitation.7 Among these methods, multitarget sputtering seems to be suitable for the fabrication of various types of metal/dielectric composite thin films. A wide variety of composite thin films with a high concentration of metal nanoparticles is easily prepared by the appropriate choice of metal and dielectric matrix for the sputtering. The dielectric matrix with a high melting point is also easily prepared by this method. The value of the third-order nonlinear susceptibility, v(3), of the composite materials depends on the pulse
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.85 J. Mater. Res., Vol. 26, No. 6, Mar 28, 2011
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duration of the laser used in the evaluation. The contribution of the heat accumulation to the value of v(3) becomes large with increase in the pulse duration of the laser used in v(3) measurements. The v(3) has been mainly obtained by using the Z-scan and degenerated four-wave mixing method. The Z-scan method is a sensitive and simple method
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