The Simulation Study Of Field Enhancement Of Metal-coat Probe Tip And Metal Annular Grating Lens

The invention of scattering near-field optical microscope (SNOM) makes us obtain aresolution beyond the diffraction limit. The probe of the conventional SNOM is usually anaperture-probe. However, since the scattering-type scattering scanning near-field microscope(S-SNOM) has the advantages both in the resolution and detection, researchers pay more andmore attentions on both the theoretical and experimental study of it. The tip-induced electricnear-field enhancement is one principal character of S-SNOM. In this paper, finite-differencetime-domain (FDTD) technique simulations are performed to study the enhancement betweena pyramidal probe tip claded with silver film and a silver spherical nanoparticle under theillumination of a p-polarized plane wave. Varying the Ag-film thickness of the tip, thedistance between the tip and the nanoparticle, the Ag-film length of the tip and the tip ofnon-uniform Ag-film we compute the electric field distribution between the tip and thenanoparticle. Keeping the Ag-film length of the tip 180nm and the distance 3nm between thetip and the particle, we change the Ag-film length and find the optimal thickness is 20-50 nmand the enhancement factor is about 70. If the thickness is 30 nm and the length is 180nm, theoptimal distance is 1 nm and the enhancement factor is 298. Then keeping the Ag-filmthickness of the tip 30nm and the distance 3nm between the tip and the particle, we find theoptimal length is 180nm and the enhancement factor is 77. The non-uniform silver filmaffects the enhancement factor.The increasing uses of SNOM and interests in the extraordinary transmissionphenomenon have stimulated the study of the transmission and diffraction from a singlesubwavelength aperture in a metallic film at optical wavelengths. Because of the surfacePlasmon wave, a focusing point can be obtained by a single aperture surrounded by periodiccorrugations. The phenomenon makes the design of metallic annular grating lens possible. Inthe paper, we first analyze the theory of the lens and find that the transmit light with specificphase retardation controlled by the width and length of the slits of the lens. The simulationresults by FDTD are the same as the analysis of theory.