Frequency and spatial filtering by means of advanced optical funneling

Summary form only given. Enhancement of transmission of electromagnetic power through deeply subwavelength holes has gained a lot of attention during the past decade, due to the counter-intuitive nature of the phenomenon and potential applications in sensing and near-field microscopy. One structure where such a giant transmission enhancement is observed is a parallel-plate waveguide with a pair of deeply subwavelength slits at the opposite walls of the waveguide [R. Merlin, Phys. Rev. X, vol. 2, p. 031015, 2012]. In particular, it can be theoretically shown that, at a frequency just below the cutoff frequency of the waveguide´s fundamental mode, a diffraction-limited incident beam can be perfectly transmitted through this structure, no matter how small the slits are, as a result of the excitation of a Fabry- Perot resonance in the cavity. A particularly interesting aspect of this Fabry-Perot resonance is that it extends with identical phase over an area much larger than the area occupied by the slits, resembling the field distribution in an epsilon-near-zero (ENZ) slab. Here, we use this concept and earlier work on power funneling through ENZ-coated subwavelength holes [A. Alù, F. Bilotti, N. Engheta, and L. Vegni, IEEE Trans. Antennas Propag., vol. 54, no. 6, pp. 1632-1643, June 2006], to dramatically increase the effective aperture of the parallel-plate structure described before. In particular, we show that by using an array of slits at one wall of the waveguide, instead of a single slit as in [R. Merlin, Phys. Rev. X, vol. 2, p. 031015, 2012], we can capture incident power along the entire resonance excited between the walls, achieving full transmission of beams even 10λ long. Furthermore, we show that such a funneling behavior is not restricted to the fundamental Fabry-Perot resonance, studied in [R. Merlin, Phys. Rev. X, vol. 2, p. 031015, 2012], but it also applies to higher order resonances, thus allowing multiband operation. An exciting a- plication enabled by this property is the realization of frequency-spatial filters, similar to the Bayer filters used in digital photography, but with much enhanced efficiency. In particular, instead of opening one slit in the output wall as in [R. Merlin, Phys. Rev. X, vol. 2, p. 031015, 2012] we open as many as the frequency components of the incident wave we want to transmit through and at the spatial locations where these frequency components need to be transmitted. We are able to enforce transmission of a single frequency component through a particular slit by attaching to this slit a cavity resonating at this frequency. In such a way we manage to efficiently route different frequency components of the incident wave to different spatial locations.