We observe from simulations that a doubly resonant structure can exhibit spectral behavior analogous to
electromagnetically induced transparency, as well as superscattering, depending on the excitation. We develop a
coupled-mode theory that explains this behavior in terms of the orthogonality of the radiation patterns of the
eigenmodes. These results provide insight in the general electromagnetic properties of photonic nanostructures and
metamaterials.
We consider an aperiodic array of coupled metallic waveguides with varying subwavelength widths. For an incident
plane wave, we numerically demonstrate that a focus of as small as one hundredth of a wavelength can be achieved for a
focal distance that is much longer than the wavelength. Moreover, the focusing behavior can be controlled by changing
either the incident wavelength, or the angle of incidence, thus providing the capability of nanoscale beam steering. We
show that the behavior of such subwavelength focusing can be understood using Hamiltonian optics.
We report the first experimental demonstration of far-field lensing using a plasmonic slit array. We implement a planar
nano-slit lens using a combination of thin film deposition and focused ion beam milling. Our lens structures consist of
optically thick gold films with micron-size arrays of closely-spaced, nanoscale slits of varying widths milled using a
focused ion beam. We demonstrate experimentally that it acts as a far-field cylindrical lens for light at optical
frequencies. We show excellent agreement between the full electromagnetic field simulations of the design, which
include both evanescent and propagating modes, and the far-field, diffraction-limited confocal measurements on
manufactured structures. The flexibility offered by these slit-based planar lenses allows for the design of microlenses
that compensate for oblique illumination in integrated opto-electronic systems, such as complementary metal-oxide
semiconductor (CMOS) image sensors.
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