Simultaneous excitation of long and short range surface plasmons in an asymmetric structure
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.
Section snippets
Theory
The proposed structure consists of Teflon AF or magnesium fluoride as the buffer layer, gold as the metal layer, and water as the outer dielectric. The reason for choosing the environmentally stable Teflon AF and magnesium fluoride will be shown later; gold was chosen for its good environmental stability. Light was introduced in the multilayer structure through a standard BK7 glass prism.
Theoretical analysis of light propagation in this geometry was based on the transfer matrix method,
Experimental
We realized the structure for the spectral configuration. Based on the performed theoretical analysis, we prepared the layered structure on a BK7 glass substrate. Due to poor adhesion between Teflon AF and gold layers, a thin (<2 nm) adhesion layer of titanium was employed. Teflon AF was spin-coated from 5% solution, titanium and gold were thermally evaporated at the temperature of 150 °C.
The actual realized thicknesses were determined using ellipsometry (Teflon AF) and a reference quartz crystal
Discussion
We will discuss practicability of the demonstrated device for sensing.
The benefit of using the reported device is, e.g. in discrimination of a thin film sensing event and a bulk refractive index change (due to, e.g. temperature), as the two SPs have different field profiles (more specifically, the penetration depth into the medium under study), Fig. 7, and thus also different sensitivity to a refractive index change close to the surface and in the refractive index of the volume, as demonstrated
Conclusions
We present an asymmetric layered structure in which both symmetric and antisymetric surface plasmons can be excited efficiently by different spectral components of an incident polychromatic light or by a monochromatic light at different angles, depending on the design. For the spectral configuration, we performed an experimental verification, in which a coupling efficiency over 96% into both SPs was achieved. We address two applications where such an efficient simultaneous coupling is strongly
Acknowledgments
This research was supported by the Grant Agency of the Czech Republic under contracts 202/04/P141, 303/03/0249, 203/02/1326, and 102/03/0633, and by European Commission under contract QLK4-CT-2002-02323.
References (12)
- et al.
Sensor Actuator B
(1999) - et al.
Sensor Actuator B
(2005) Zeitschrift fur Physik A
(1968)- et al.
Phys. Rev. B
(1996) - et al.
Phys. Rev. B
(1968) - et al.
J. Mod. Opt.
(1988)
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