Tunable light absorbance by exciting the plasmonic gap mode for refractive index sensing.

ABSTRACT

The tunable and narrowband optical response from the surface plasmon resonances usually requires periodic metal nanostructures; however, it is usually expensive and challenging to construct such macroscale and defect-free devices. Herein, we make it possible to obtain a characteristic and sharp absorbance via exciting the plasmonic gap mode, which can be obtained in a large-area sample prepared with relatively low cost. The resonant wavelength can be tuned via changing the bottom-facet area of the top structured metal or the spacer thickness. Furthermore, we design the hexagonal arrangement gold microholes atop the gold continuous film with a spacer, which possesses a sharp reflectance dip from the intense plasmonic gap mode. Numerical calculations show that the resonant wavelength is linearly changed with the varying environmental refractive index (RI). The sensitivity is up to ∼1287 nm per RI unit, and the figure of merit for an RI sensor is over 300.