Enhancing Propagation Length of Surface Plasmon Polaritons by Using Metallic Double-Layer Structure

Abstract

In this paper, we propose to use a wedge-shaped plasmonic waveguide with a metallic double-layer structure for improving the propagation length of surface plasmon polaritons. The wedge silicon waveguide is covered by a first layer of Ag and by a second layer of Au outermost in which the former has a good lightwave guiding property but unstable to the operating medium and the latter is more stable to the operating medium. Numerically investigated results show that the propagation length of surface plasmon polaritons using the Au/Ag double-layer structure with an appropriate thickness of the Au covering layer can be increased by a factor of 2.5 compared to that of using the only Au metallic structure at the same propagation mode area. The advantage of the proposed metallic double-layer structure is also preserved in hybrid plasmonic waveguides.

Plasmon Wave Propagation Property of Metal Wedge Plasmonic Waveguides Covered by a Protective Oxide Layer

“…The figure of merit FoM as a trade-off between the propagation attenuation and the mode confinement, which is defined by FoM = LSPP/ (A eff ) [25]. Here, Aeff is the effective propagation mode area, which is defined by [4,26]…”

“…Compared with the second good metal, Au, its propagation length is larger than a factor of 4. However, Ag is easily oxidized in the operating medium, such as air or aqueous environments [4]. The refractive index of AgO x film depends on the component ratio x [27].…”

“…Table 2 shows the complex refractive index of oxides of Ag, Cu and Al [22]. These oxides are fabricated by different methods, including reactive sputtering, thermal oxidation, and sintering, electrodeposition [4,22,[27][28][29]. The propagation lengths and figure of merit of respective plasmonic waveguides are also calculated and shown for comparison.…”

“…To overcome this problem, optical waveguides guiding lightwaves based on surface plasmon polariton (SPP) have been introduced and intensively studied in recent years [1][2][3]. SPP waveguides can be found in applications such as fundamental elements in integrated nanophotonic circuits including lightwave guiding at nanoscale, optical switches, optical modulators, and ©2022 Vietnam Academy of Science and Technology optical sensors [1][2][3][4][5][6][7][8][9][10][11][12]. Various types of SPP waveguide have been proposed for improving the propagation length while keeping the propagation mode area at subwavelength size such as metallic nanowires [13], metal strips [14], and wedge-shaped waveguides [15,16].…”

“…The Ag metal shows a surface wave guiding ability superior to other metals; however, it is unstable to the operating medium and is easily oxidized. To protect the Ag metal layer, an Au metal layer inert to operating medium is proposed for covering on the Ag wedge waveguide [4]. The investigated results show that the propagation length of SPP using the Au/Ag double-layer structure with an appropriate thickness of the Au covering layer can be increased by a factor of 2.5 compared to that of using the only Au metallic structure at the same propagation mode area.…”