Nanofocusing on gold planar nanotip arrays

Nanofocusing of incident light in the visible and infrared regions is achieved using tapered metallic nanostructures (TMNSs). Illumination under 633 nm excitation produces a very small (∼40 nm in the x-direction), very bright spot with a specific geometry, demonstrating near-field nanofocusing of the incident beam to the deep subwavelength scale. The key processes, including a theoretical understanding, numerical calculation, and a near-field optical measurement involving the metallic planar nanotip arrays, are discussed. As in the lightning-rod effect, there is a large number of surface states to accommodate free electrons, resulting in a very high surface density distribution of the free electrons over the nanoapexes. A theoretical model for calculating the free-electron distribution based on the surface energy state of the nanoapex is established. The spread of free electron oscillation, including the guiding of free electrons towards nearby planar nanoapexes to obtain near-field nanofocusing is examined. As the lightning-rod effect is a broadband phenomenon, the TMNSs are also examined in the far-infrared (far-IR) region, at the typical far-IR wavelength of 10.274 μm. It should be noted that the IR radiation can also be focused into a linear light-spot of ∼20 nm, far beyond the IR diffraction limit. A metasurface constructed by the orderly arrangement of TMNSs highlights their potential in applications such as surface enhanced Raman spectroscopy, ray absorbing materials, and low-cost nanolithography.