Impact of a Dielectric Layer on the Resonant Conditions of Nanograting Structures

We study the resonant wavelength of nanograting structures covered by a dielectric medium. We find that the resonant wavelength oscillates as the thickness of the thin dielectric layer increases due to the cavity formed by the dielectric layer. The amplitude of this oscillation in the resonant wavelength is small when the minimum reflection occurs in the nanograting structure. For a plasmonic sensor covered by a dielectric medium, a small oscillation in the resonant wavelength as the thickness of the dielectric medium changes is preferred. We also study the impact of a rounded corner on the resonant wavelength and find that the rounded corners with a small radius of r effectively reduce the nanogroove depth by about 0.2r. Results from the finite-difference time-domain (FDTD) method agree very well with the phase-matching condition, using parameters calculated from the rigorous coupled-wave analysis (RCWA) method. These results will lead to a better understanding of the accuracy of plasmonic sensors covered by dielectric media.