International Symposium on Antennas and Propagation
The Evolution of Plasmonic States from Visible Light to Microwaves
Minfeng Chen, Hung-chun Chang,
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The guiding of light below the diffraction limit has been an active research in recent years [1,2]. In designing these deep subwavelength photonic devices, the surface plasmon polaritons (SPPs) play a crucial role. The SPP is the coupling between photons and electrons. At a metal/dielectric boundary, the electromagnetic (EM) waves are strongly confined where the surface electrons simultaneously oscillate with electric fields. Extensive studies on light guidance utilizing the SPP have been reported. The simplest structure is known to be the metal/insulator/metal (MIM) heterostructure. The detailed plasmonic dispersion characteristics have been investigated [3-5]. Many complicated designs of plasmon-based waveguide have also been proposed [6,7]. The plasmonic waveguides are often considered in the visible light and near-infrared, whereas the conventional waveguide modes reside in the microwave regime . Despite the widely-studied plasmonic waveguides, previous works barely mentioned the transition of plasmonic modes between these two frequency ranges. To explore this subject, we focus on the MIM heterostructre in this paper. We demonstrate that given the metal dielectric at microwaves, the plasmonic modes over the MIM heterostructure can evolve into the conventional waveguide modes. In addition, the dispersion of a single SPP at a single metal/insulator boundary can be extracted from the transverse magnetic (TM) plasmonic modes. In doing so, the coupling effect of MIM plasmonic modes is clearly revealed. In fact, the symmetric and antisymmetric coupled SPP modes are associated with TEM (TM0) and TM1 guided modes, respectively. It is worth to mention that earlier researches often employed negative dielectric or fitting to the relative permittivity function εr(ω) = 1 ? ωp 2/ω2, where ωp is the angular plasma frequency, for the metal material in solving the plasmonic dispersion [4,5]. This approximation holds for nearinfrared. Nevertheless, the permittivity is complex with extremely large imaginary part in the microwave regime. To truly reflect the reality, we adopt the Drude model with relaxation time (τ), which leads to large imaginary permittivity in microwaves.