Substrate Dependence of Surface Plasmon Resonance Frequency of Silver Island Films
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Substrate Dependence of Surface Plasmon Resonance Frequency of Silver Island Films Gang Xu, Masato Tazawa and Ping Jin National Institute of Advanced Industrial Science and Technology, 2266-98 Shidami, Moriyama-ku, Nagoya 463-8560, Japan ABSTRACT A simple method was demonstrated to tune surface plasmon resonance wavelength of silver island films by introducing an underlayer. The tunability can be much enhanced by varying the underlayer medium. In combination with the size effects of metallic islands the plasmon wavelength can be readily adjusted throughout the visible spectral regions. The experimental results were interpreted based on a generalized Maxwell-Garnett theory. INTRODUCTION Silver island films have attracted much interest for several decades due to their unique optical properties which are strikingly different from the bulk. The films are often produced by thermal evaporation and the islands are nanometer-sized. These nanoparticles exhibit light-induced localized surface plasmon resonance (SPR), resulting in an anomalous absorption in the visible region. The SPR depends on the particle size, shape, filling factor, as well as the surrounding dielectric medium. Based on these properties, a variety of optical devices and chemical and biological sensors have been developed [1,2]. The SPR excitation can greatly enhance the local electromagnetic field, which plays a key role in surface enhanced spectroscopies (SES) [3,4]. It is highly desirable to manipulate and predict the SPR frequency. One of the motivations comes from SES, in which the excitation and emission frequencies must overlap the SPR frequency of the substrate. In the visible region, Ag-coated substrate has the very SPR frequency and has been widely used. Today, however, SES is commonly practiced with a wider range of excitation wavelength from UV to near-IR. Therefore, of great significance is the ability to tune and extend the Ag SPR frequency. Further, the tunability in SPR would also benefit other technological applications. So far, much work has been done for this purpose. Van Duyne group has developed an elegant fabrication technique called nanoshpere lithography to precisely control geometry and spacing of the islands to improve the tunability in SPR [5]. Weimer and Dyer have made an effort to optimize the fabrication conditions of thermal evaporation, substantially to change film morphology parameters [6]. Baba et al. altered the component of metallic islands by alloying Ag with Au [7]. In this presentation, we propose a simple method to tune SPR wavelength of Ag island film in a wide spectral region. The key point is to deposit an underlayer for Ag islands to vary interisland dielectric constant. It is known that, for islands supported by a substrate, the interisland dielectric constant (εav) of metallic islands is usually a geometrical average of the dielectric constants of the substrate (εsub) and air, i.e., εav = (εsub+1)/2. If an underlayer present, the effect of the substrate will be replaced by the underlayer. Thus, we
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