The interaction of light with sub-wavelength nanostructures can show various novel behaviors. If the nanostructures are made from metals, incident light with certain wavelengths can lead to the excitation of surface plasmons. In this case the electromagnetic fields in the vicinity of the nanostructures can show complicated characteristics with the possibility of strong field enhancement. The optical transmission at these wavelengths also show characteristic features. These properties of metal nanostructures have potential applications in single molecule detection and in development of nano-biosensors.
In this work, we report on a theoretical study of near-field distribution of optical transmission of a metal film patterned periodically with sub-wavelength nano-holes.The studied system consisted of a thin gold plate of thickness 15-25 nm, patterned periodically with nano-holes of 140 nm in diameter, on a thick silicon dioxide. The nano-holes as well as the top side of the metal plate were filled with water or solvent. Light was sent in toward the plate from the silicon-dioxide side. The optical extinction and the near-field electromagnetic field distribution were calculated for different wavelengths by solving the Maxwell equations using a novel scattering matrix method. It was found that the extinction shows a strong peak at a certain wavelength and the position of the peak is red shifted both when the thickness of the gold plate was decreased and when the refractive index of the solvent was increased. The calculated electromagnetic field distribution shows that strong field intensities occur inside the gold plate at wavelengths corresponding to the extinction peaks, indicating a strong interaction between the incident light and the gold plate as a result of plasmon resonances. We have also made a detailed study of the field distributions in the nano-holes at different wavelengths, and have predicted various hot-spot patterns in the nano-holes as well as the dependences of the hot-spot patterns on the geometrical structure and light polarization. The results shed light on the physical insight of single molecule detection experiments by surface plamon resonances and could be employed for development of molecular sensors with single molecule sensitivity. |