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Plasmonic modes in metallic nano-apertures and aperture arrays (VIS/near-IR)

My research on metal-based nanophotonics involves the exploration of new mechanisms that allows efficient transport of light through subwavelength structures. The goal of this research is to overcome the fundamental size incompatibility between micro-photonics and nano-electronics and to pursue the quest for more efficient nano-scale opto-electronic devices.

Figure: Dispersion relation of the lowest-order waveguide mode of a cylindrical hole. The dashed blue line assumes a perfect electric conductor (PEC) model for the metal and corresponds to the TE11 mode. The solid red line assumes a Drude plasmonic model, and corresponds to the HE11 mode. The radius of the hole is 0.36λp, where λp is the plasma wavelength of the metal.

Nano-aperture structures in metallic thin films have received significant attention in recent years with the hope of overcoming efficiency problems associated with the scaling of light “below the diffraction limit”. Conventional wisdom in plasmonics dictates that no propagating modes exist in sub-wavelength apertures. Hence, most research has focused on enhancing the coupling of external light into the apertures. Recently, we showed theoretically that in contrast to perfect electric conductors (PEC), plasmonic metals can in fact support propagating modes in apertures with deep sub-wavelength dimensions (e.g., radius = 50 nm << wavelength = 600 nm). These modes provide an efficient mechanism to guide light through nano-scale holes and hole arrays. This research should significantly impact optical applications, such as imaging, optical data storage, nonlinear optics and optical sensing.

Figure: Transmission of an array with 50-nm cylindrical holes in a 250-nm thick silver film. Silver is modeled as a plasmonic material (solid red line) and as a perfect electric conductor (PEC, dashed blue line). Insets show propagating field profiles for two peaks in the plasmonic curve (solid red).

The transmission spectrum of an individual subwavelength hole can be completely explained by the properties of these propagating plasmonic modes. In the case of subwavelength hole arrays, the dispersion relation of a single hole remains very relevant provided that the hole separation is such that the propagating modes inside nearest-neighbor holes do not interact. In fact, this is can be shown to hold for thickness of separating metal walls as small as 40 nm.

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