Influence of nano-scale dimension on properties of transmission in metallic gratings with narrow slits

Abstract

On metallic gratings with very narrow slits, the Fabry-Perot-like phenomenon has been found in the SP resonant transmission: Transmission peaks appear periodically according to the increment of grating depth. We study the phenomenon by setting constants of the structure to be at nanometer scale. The rigorous coupled-wave analysis method (RCWA) has been used in this work. The grating structure we examined is composed of silver. Slits are filled with dielectric. For silver, its plasma wavelength λp=110nm. We study the gratings with period of gratings d=3λp, the grating depth h=2.5λp, width of slits is 0.22λp, and slits on the grating is filled with GaP which refraction index is 3.7. Under this situation, there is no excited peak at the wavelength theory predicts. Next we have investigation on the transmission of the SP resonant mode. Wavelength of normally incident TM-polarized plane wave equals period of gratings. It can be seen from the zero-order SP resonant transmission versus the grating depths, that there is no Fabry-Perot-like phenomenon upon the wavelength calculated from the theory, which appears evidently at greater geometry. Transmission value falls quickly via grating depth increases. Fabry-Perot-like phenomenon is caused by energy transmission in the slits, but nanometer scale slits will cut off the energy transmission in the slits. It's concluded that the surface plasmons execute negative effect on transmission anomalies when the grating dimension is at nanometer scale. It's useful for the fabrication of the sub-wavelength optical element.