Proposal for Superfocusing at Visible Wavelengths Using Radiationless Interference of a Plasmonic Array

Radiationless electromagnetic interference has been proposed to focus light below the diffraction limit. This has been demonstrated for microwave wavelengths; however, those approaches cannot be extended directly to visible wavelengths due to the material response and limitations from nanofabrication. In this Letter, a different approach is proposed to enable subdiffraction-limit focusing in the visible. The approach is based on the highest-order mode of a metal-dielectric waveguide array. Comprehensive simulations of focusing are provided for two different extreme examples: a structure that gives a 0.21 wavelength focus spot at a relatively large distance of 0.5 wavelength, and a sharp 0.057 wavelength focus spot at distance of 0.1 wavelength. The proposed nanostructures are comparable to those found in recent experiments.

Figure 1

(a) Source distribution required to achieve an extreme subwavelength Gaussian focus. Magnetic field intensity at the focus has a 74.5 nm FWHM [dark gray (blue) curve]. The magnetic field intensity at the source, $\lambda /2$ from the focus [light gray (red) curve]. $\lambda =632.8\mathrm{nm}$. (b) Schematic of waveguide array distribution required to achieve source distribution of (a), with the highest-order guided mode. The parameters used and center locations of the dielectric layers are provided. The refractive-index values and the spacing of the dielectric regions are chosen to produce a mode that best fits the source field distribution in (a). (c) Transverse magnetic field intensity of the highest-order guided mode in (b), to be compared with (a).

Figure 2

(a) Magnitude of Poynting vector, $x$ component, calculated by fully vectorial electromagnetic FDTD simulations. Color map on a dB scale. The metal-dielectric waveguide array is present for $x<0$, and the region $x>0$ is free space. (b) Transverse magnetic field intensity, and $x$ and $y$ components of the electric field intensity (normalized to the maximum of the $y$ component), at a half wavelength away from free-space interface to the metal-dielectric waveguide array. The FWHM of the transverse magnetic field intensity is 136 nm at the free-space wavelength of 632.8 nm.

Figure 3

Magnitude of Poynting vector, $x$ component, calculated by electromagnetic FDTD simulations. The color map is on a dB scale. The FWHM is 36.1 nm at the free-space wavelength of 632.8 nm after propagating a distance of 0.1 wavelength.