Coupling Efficiency of Surface Plasmon Polaritons for 1D Plasmonic Gratings: Role of Under- and Over-Milling

This paper reports the successful excitation of surface plasmon polaritons (SPPs) through 1D metallic grating on higher refractive index GaP substrate. Coupling efficiency (eta) of a free-space transverse-magnetic (TM) plane-wave mode into a SPP mode is crucial for many plasmonic devices. This eta predominantly depends on the fabrication (milling) parameters and the factors (under- and over-milling) affecting the eta is investigated experimentally and numerically. First of all, eta is estimated by measuring the transmission spectra obtained through the plasmonic grating structures by varying the slit width (a) for a fixed period (I >) and the thickness (t) of the gold (Au) film in which the grating is formed. The wave vector of the incident light is tuned to match the wave vector of the SPP, to get maximum eta. For an optimum Au film thickness, a slit width of half of the periodicity of 770 nm in the grating device yields a maximum eta. Such grating devices support only a fundamental plasmonic mode because the profile/shape of the slit in the grating device is more like a sinusoidal nature. Furthermore, such grating offers intermediate scattering to the incident light and the SPP as well which in-truns couple more incident energy to the SPPs. Moreover, over-milling results in decreased eta where the crystalline plane of the substrate is disturbed. Finite element method (FEM) in COMSOL modeling is used to understand the underlying physics. This study is very useful for the development of the device application in real word.