When a beam of TM electromagnetic wave with a matched frequency to a periodic nano-pattern array shaped in a metallic film is incident upon the metal interface, the surface free-electrons of the metal film will be stimulated and thus oscillate collectively. The phenomenon above means that the incident electromagnetic waves and surface free-electrons are coupled effectively and then a mixed excitation mode of surface plasmon polaritons (SPPs) are generated and also spreaded over the metal-dielectric or metal-air interface. According to current researches, the surface waves originated from a strong surface free-electron oscillation can propagate along a interface only being a micrometer scaled distance. When the energy and momentum of incident electromagnetic wave are suitable, a SPPs can be guided and thus a part of electromagnetic energy can be converged onto the metal tip, thereby reducing the surface reflectivity of the electromagnetic waves because they are already concentrated or storaged into the periodic nano-pattern array shaped. In order to study the influence based on several factors including metal surface structures, nano-pattern array period, and electromagnetic wave incident angle, an optical frequency SPP device is designed. The device with a gold plating film is fabricated over a silicon substrate, and then the substrate is etched so as to shape a metal nano-pattern array. The structure is called a sub-wavelength gold structure (SWGS). A virtual model is established according to CST microwave studio. The finite element method is also used to simulate the electromagnetic characteristics of the SPPs. Simulations are carried out to obtain reflectance waveforms so as to explore the reflectivity changes of SWGS irradiated by nearinfrared waves under different conditions including metal surface structural characters, nano-pattern array period, and electromagnetic wave incidence angles.
The surface plasmon polaritons (SPPs) is an electromagnetic wave that can be stimulated and then propagates over the surface of the preshaped metallic nanostructures or the interface between the surface of the metallic nanostructures and the substrate media due to a strong coupling of incident light and the surface free-electrons moving on the metallic nanostructure surface with a featured micrometer scale. As shown, through SPPs, incident light energy can be localized effectively in a sub-wavelength region or space, and thus so-called light diffraction limit can be break through easily. Therefore, it has demonstrated a good prospects for developing advanced functioned materials or devices such as light absorbing materials, optical antennas, and optical information storage modules. In this paper, we propose a special metallic nanostructures, which can be used to absorb a certain band of incident light by converting them into a kind of local freeelectron oscillation, which means that SPPs can be generated and processed efficiently. As shown, the metallic nanostructures will present a lower reflectivity in the wavelength range, and through adjusting several key parameters such as the period of the metallic nanostructures, we can achieve an effective control of reflectivity because a valley of the reflectivity curve can be formed, which means a low reflectance at a specific wavelength band has been obtained.