Recent research highlight: Resonant nano-focusing antenna
Fig. 1 Concept of constructing a resonant nano-focusing antenna with simultaneously utilizing three different light focusing mechanisms.
As shown in Fig. 1, we introduce a novel resonant nano-focusing antenna (RNFA) geometry that simultaneously uses three physical mechanisms: localized surface plasmons (LSPs), SPP edge localization, and TEM field localization. While LSPs on the nano-disk structure already lead to strong field confinement, a significantly smaller field localization spot size can be achieved by introducing the field edge localization by tapering the nanodisk. The gap between two tapered edges operates as a nano-capacitor in a nano-antenna that is known to confine strong fields even in the static electromagnetic regime much smaller than the operation wavelength. The plasmoinc fields on two tapered edges of the RNFA are strongly coupled and they support nearly uniformly distributed TEM type fields regardless of the gap size (theoretically even for vanishing gaps).
Fig. 2 Simulated electric field distribution in a resonant nano-focusing antenna embedded in a Si waveguide, showing a focusing spot size of 25 nm.
Integrated with a lossless Si waveguide to efficiently excite the plasmonic mode on RNFA, our proposed device has been validated by computer simulations. The presence of the tapered edges and the small gap results in drastic enhancement of the field density (Fig. 2) uniformly localized in all 3 dimensions inside the nano-capacitor with a 25 nm gap. Furthermore, this static electromagnetic activity (nano-antenna) does not change significantly the source-excited or source-free field/current distributions as well as eigen-frequencies of the dynamic resonances (LSP modes). This strong field localization is experimentally characterized using our H-NSOM. The field localization estimated from the restored measured data after deconvolution, is about 75 nm, which is 20 times smaller than the wavelength of the optical field in the free space. The presented structure and phenomena are anticipated to have important impacts on new nanophotonic devices, for example, design of metamaterials including photonic activities in a deep subwavelength static electromagnetic regime with remaining metamaterials' macroscopically dynamical optical response.
- L. Feng et al., "Nanoscale optical field localization by resonantly focused plasmons", Optics Express 17, 4824 (2002).