Simultaneous excitation of long and short range surface plasmons in an asymmetric structure

Abstract

An asymmetric multilayer structure allowing nearly 100% simultaneous excitation of both symmetric and antisymmetric surface plasmons is proposed. It is shown that the two surface plasmons can be excited at a fixed angle of incidence with polychromatic light or with monochromatic light at different angles of incidence, depending on the design. The structure can be of interest in high-performance surface plasmon resonance sensing and in all-optical signal processing.

Introduction

Surface plasmon resonance has been exploited in various fields of modern optics, including thin film characterization [1], sensing [2], and non-linear optics [3]. Besides configurations in which light excites a surface plasmon (SP) localized on a single surface of a thin metal layer, there are many reports on study [4], observation [5] and exploitation [6] of coupled SPs propagating along both surfaces of a thin metal layer. In most reports, a configuration in which the metal layer is sandwiched between index-matched or nearly index-matched dielectrics is used. In this configuration, two bound modes (herein referred as antisymmetric SP (A-SP) (magnetic field inside the metal crosses zero) and symmetric SP (S-SP)) can be excited [5] through a dielectric which has higher refractive index compared to the two dielectrics surrounding the metal layer – typically a high refractive index prism [7] or through a diffraction grating [8]. Besides applications where only one of these coupled SPs is excited, which is the case for most reported applications, there is also an interest in excitation of both these modes simultaneously. For example, in two-SP spectroscopy for metal layer characterization [9] or in optical sensing, simultaneous excitation of two surface waves provides additional information [9], [10]. Another example is the all-optical switching, where light coupled into one surface plasmon modifies the refractive index of a non-linear material deposited on the metal surface, which changes resonant condition for the other SP. Efficiency of excitation of both A/S-SPs through the high refractive index optical prism depends on the thickness of the metal layer and on the thickness of a buffer layer (the dielectric placed between the high refractive index prism and the metal layer). Generally, coupling of entire excitation light into A and S-SP (optimum coupling) requires different buffer layer thickness for S-SP and A-SP, respectively, disabling efficient simultaneous coupling into both S and A-SP. However, in applications like optical sensing or all-optical switching, optimum performance requires efficient simultaneous coupling. In a large group of SP resonance sensors, the resonance is identified as a dip in the reflectivity spectra [2] and the dip position is correlated with the measured quantity. The position of a deeper dip is identified with higher accuracy [11] resulting in higher sensor resolution. For all-optical switching, a SP induces a change in refractive index of a non-linear material adjacent to the metal layer through its high electromagnetic field concentration and, consequently, changes resonant condition of the other SP used for the switching.

In this paper, we demonstrate an optical structure on which the A-SP and S-SP can be excited simultaneously by different spectral components of an incident polychromatic light (spectral configuration) or by monochromatic light [2] at different angles of incidence (angular configuration), if the structure is designed properly. We demonstrate experimentally the spectral configuration, where we target in a sensor operating in aqueous media.