Substrate effects on the optical properties of metal gratings

The transmission and near-field enhancement spectra of thin metal gratings on a semiconductor designed for the telecom band, similar to 1500 nm, were investigated theoretically. It was found that around the wavelength lambda(R), where a diffraction order changes its character between radiative and evanescent, the spectral peaks are pinned at lambda(R). This state is in marked contrast to the gratings in air where the peak wavelength is always longer than lambda(R). We show that the difference is related to the absence/presence of the bound mode (BM) of the system, which corresponds to the Fabry-Perot (FP) resonance of the slit mode. In the thin gratings on a semiconductor, the BM disappears due to the large dielectric asymmetry above and below the grating, which results in different reflection phases for the slit mode at the grating-dielectric interfaces. It is also shown that the plasmonic band gap is large in gratings on semiconductors, enhancing the disappearance of the BM. (C) 2017 Optical Society of America