Surface-plasmon polaritons on metal-dielectric nanocomposite films

Abstract

We observe experimentally that the reflectances of metal-dielectric nanocomposite films in the Kretschmann configuration show different characteristics, depending on the metal fill fraction f, that fall into one of three distinct regimes. In the "metallic" regime, in which f is large, the film supports conventional surface-plasmon polaritons (SPPs), and one can tailor the properties of the SPPs by controlling the value of f. In the "dielectric" regime, in which f is small, the film does not support any surface modes. In the intermediate "lossy" regime, the nanocomposite film supports a SPP mode that is different from that of a "metallic" film. These results are explained by using an anisotropic effective medium model and mode analysis. [5][6][7]. It has been shown that the properties of SPPs can be tailored by the use of artificially synthesized periodic or random metaldielectric nanocomposite materials [8][9][10]. Yet most of these studies assume that the metal in the nanocomposite is interconnected. In this Letter, we investigate how the properties of SPPs supported by random nanocomposite films evolve as the metal fill fraction is varied. Specifically, we explore the transition from continuous metal films to semicontinuous films to isolated nanoislands. We fabricated a large collection of gold-air nanocomposite films with gold fill fractions f ranging from 1 to approximately 0.3. All samples are approximately 30 nm thick. The pure gold films were fabricated by using electron-beam evaporation, the films with high values of f were fabricated by using sputter coating, and those with moderate values of f were fabricated by using the pulsed laser deposition method [11]. We measured the reflectances of these nanocomposite films as functions of the incidence angle in glass inc in the Kretschmann configuration at the wavelength of 1550 nm (see Fig. 1). The measured reflectance is calibrated with that from a bare prism under the same conditions to exclude the influence of unrelated interfaces. Five representative reflectance curves are plotted in Fig. 2. Here, sample 1 is a pure gold film, and the value of the gold fill fraction f decreases from approximately 0.85 to 0.4 for samples 2-5.From scanning or transmission electron microscopy (SEM or TEM) studies (see, e.g., the insets of Fig. 1), one sees that the size of the in-plane features of all the nanocomposite films is much smaller than the wavelength. Furthermore, there is very limited structural variation along the thickness dimension. Thus, we here describe the macroscopic optical property of the nanocomposite films in terms of an anisotropic effective dielectric permittivity tensor ជ eff = diag͓ eff,x , eff,y , eff,z ͔. Here, the two in-plane components are equal to each other, eff,x = eff,y , and can be described by using the effective medium approximation (EMA) [12][13][14] aswhere d and m are the dielectric permittivities of the bulk dielectric and metal, respectively, and f is the gold fill fraction. The out-of-plane component, eff,z , is given by eff,z = f...

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