Concentric-ring-grating-induced strong terahertz near-field enhancement on a micro-tip

Abstract

A terahertz near-field source, composed of a subwavelength metallic concentric ring grating on a dielectric slab with a micro-tip, is proposed. The field enhancement property and the field distribution near the tip apex are studied theoretically. A volume-averaged energy density enhancement factor of 550 around the tip apex region could be obtained for radial polarized field illumination. Compared with conventional scanning near-field optical microscopies (SNOMs), about 15-fold enhancement of near-field energy density is obtained by using the proposed near-field source. We attribute such a strong enhancement to the guided resonant modes launched by the radial grating, the axial symmetry of the device, and the radial polarization of incident beam. The resonance frequencies of the device are controlled by varying the grating period and the thickness of dielectric substrate. The proposed device is useful for improving the coupling efficiency of SNOMs.