René de Waele

A single-layer wide-angle negative-index metamaterial at visible frequencies

Metamaterials are materials with artificial electromagnetic properties defined by their sub-wavelength structure rather than their chemical composition. Negative-index materials (NIMs) are a special class of metamaterials characterized by an effective negative index that gives rise to such unusual wave behaviour as backwards phase propagation and negative refraction. These extraordinary properties lead to many interesting functions such as sub-diffraction imaging and invisibility cloaking. So far, NIMs have been realized through layering of resonant structures, such as split-ring resonators, and have been demonstrated at microwave to infrared frequencies over a narrow range of angles-of-incidence and polarization. However, resonant-element NIM designs suffer from the limitations of not being scalable to operate at visible frequencies because of intrinsic fabrication limitations, require multiple functional layers to achieve strong scattering and have refractive indices that are highly dependent on angle of incidence and polarization. Here we report a metamaterial composed of a single layer of coupled plasmonic coaxial waveguides that exhibits an effective refractive index of -2 in the blue spectral region with a figure-of-merit larger than 8. The resulting NIM refractive index is insensitive to both polarization and angle-of-incidence over a +/-50 degree angular range, yielding a wide-angle NIM at visible frequencies.

Plasmon dispersion in coaxial waveguides from single-cavity optical transmission measurements.

We determine the plasmon dispersion relation in coaxial waveguides composed of a circular channel separating a metallic core and cladding. Optical transmission measurements are performed on isolated coaxial nanoapertures fabricated on a Ag film using focused ion-beam lithography. The dispersion depends strongly on the dielectric material and layer thickness. Our experimental results agree well with an analytical model for plasmon dispersion in coaxial waveguides. We observe large phase shifts at reflection from the end facets of the coaxial cavity, which strongly affect the waveguide resonances and can be tuned by changing the coax geometry, composition, and surrounding dielectric index, enabling coaxial cavities with ultrasmall mode volumes.

Experimental evidence for large dynamic effects on the plasmon dispersion of subwavelength metal nanoparticle waveguides

We present angle and frequency resolved optical extinction measurements to determine the dispersion relation of plasmon modes on Ag and Au nanoparticle chains with pitches down to 75 nm. The large splitting between transverse and longitudinal modes and the band curvature are inconsistent with electrostatic near-field models. Good agreement with an electrodynamic point-dipole model implies lower propagation loss and a larger signal bandwidth and group velocity than previously predicted. We conclude that for the design of optical nanocircuits coherent far-field couplings across the entire circuit need to be taken into account, even at subwavelength feature sizes.

Negative refractive index in coaxial plasmon waveguides

We theoretically show that coaxial waveguides composed of a metallic core, surrounded by a dielectric cylinder and clad by a metal outer layer exhibit negative refractive index modes over a broad spectral range in the visible. For narrow dielectric gaps (10 nm GaP embedded in Ag) a figure-of-merit of 18 can be achieved at lambda(0) = 460 nm. For larger dielectric gaps the negative index spectral range extends well below the surface plasmon resonance frequency. By fine-tuning the coaxial geometry the special case of n = -1 at a figure-of-merit of 5, or n = 0 for a decay length of 500 nm can be achieved.

Plasmonic Anti-Reflection Coating for Thin Film Solar Cells

We study plasmonic nanoparticle arrays on top of silicon solar cells for efficient light coupling. An optimized array combined with a Si3N4spacer layer shows 8% more incoupling than the standard Si3N4antireflection coating.

Dispersion and Scattering of Resonant Nanoparticle Chains

Coupled resonant nanoparticles may help to realize subwavelength control over photons. We claim that metal nanoparticle chains show an unexpected polariton splitting in the dispersion relation and discuss consequences for scattering and subwavelength optical confinement.

Active plasmonic devices and optical metamaterials

We studied active near-infrared metamaterials based on phase transition of vanadium oxide thin films, asymmetrically coupled split-ring resonators for narrowing resonance line-widths, field effect modulation of plasmon propagation and 3D single layer, plasmonic negative-index metamaterials.