Long-range surface plasmon polariton enhancement in double-electrode structure

Abstract

Surface plasmon polariton (SPP) is a kind of highly confined surface-wave mode associated with collective electron charge oscillation. A remarkable feature of the SPP is its highly sensitive response to change in permittivity or refractive index of the material in the vicinity of the metal surface, and it can be used as a high sensitive sensor. Long-range surface plasmon polariton (LRSPP) is a low-loss surface wave supported by symmetric structure, such as symmetric insulator-metal-insulator (IMI) slab. In most of previous investigations, only the properties of the eigenmodes of LRSPPs are analyzed. In this paper, however, we investigate the phenomena associated with the excitations of LRSPPs which cannot be explained by the eigenmode theory. Double-electrode structures are studied in this paper. For simplicity, we assume that the structures are symmetric if no coupler is introduced. When the coupler is introduced, however, this system can have interesting new properties. The influence of the parameters of the structure on the LRSPP is discussed in detail, and the enhancement effect of the LRSPP excited by the attenuated total reflectance (ATR) method is found. The research on the parameters is based on the reflectivity and the field enhancement calculated by the characteristic matrix technique. Taking the coupler into consideration, there are six media in the double-electrode structure excited by ATR. It turns out that the LRSPP can have new properties other than those of eigenmodes supported by symmetric structures without couplers. This is due to the asymmetry brought by the coupler in the ATR method, thus it is possible to enhance the wanted mode while suppress the other mode. The asymmetry brought by the coupler in the ATR method leads to new and interesting phenomena. If the distance between the coupler and the closer metal film (denoted by s) and that between the two metal films (denoted by t) are properly chosen, the long-range mode will be enhanced while the other mode will be suppressed. It should be emphasized that s is a crucial parameter. When s is small, the long-range mode is suppressed and the other mode is enhanced; when s is large, the energy focuses more on the long-range mode. However, when s is too large, the exciting efficiency is very low. It is found that the appropriate parameters in the ATR-mothod-exciting double electrode structure are s=350 nm, t=(1)/4λ, where λ is the wavelength of the source light in vacuum and is taken to be 546.1 nm, and the thickness of each metal Ag film is taken to be 36 nm. These parameters are important for future experiments to observe this kind of phenomenon.It is also found that both the field enhancement factor and its sensitivity to the refractivity of the output-end medium are very high in LRSPP case, which is possible to be used as a biological or chemical sensor. The asymmetry brought by the coupler in the ATR method makes LRSPP have new and interesting features, one of which is the enhancement of the long-range mode. The present research has heuristic significance for studying the long-range surface plasmon in asymmetric excitation configuration.