Metal double layers with sub-10 nm channels

Double-layer plasmonic nanostructures are fabricated by depositing metal at normal incidence onto various resist masks, forming an antenna layer on top of the resist post and a hole layer on the substrate. Antenna plasmon resonances are found to couple to the hole layer, inducing image charges which enhance the near-field for small layer spacings. For continued evaporation above the resist height, a sub-10 nm gap channel develops due to a self-aligned process and a minimal undercut of the resist sidewall. For such double layers with nanogap channels, the average surface-enhanced Raman scattering intensity is improved by a factor in excess of 60 in comparison to a single-layer antenna with the same dimensions. The proposed design principle is compatible with low-cost fabrication, straightforward to implement, and applicable over large areas. Moreover, it can be applied for any particular antenna shape to improve the signals in surface-enhanced spectroscopy applications.

Gap plasmons and near-field enhancement in closely packed sub-10 nm gap resonators

Pairs of metal nanoparticles with a sub-10 nm gap are an efficient way to achieve extreme near-field enhancement for sensing applications. We demonstrate an attractive alternative based on Fabry-Perot type nanogap resonators, where the resonance is defined by the gap width and vertical elongation instead of the particle geometry. We discuss the crucial design parameters for such gap plasmons to produce maximum near-field enhancement for surface-enhanced Raman scattering and show compatibility of the pattern processing with low-cost and low-resolution lithography. We find a minimum critical metal thickness of 80 nm and observe that the mode coupling from the far field increases by tapering the gap opening. We also show the saturation of the Raman signal for nanogap periodicities below 1 μm, demonstrating efficient funneling of light into such nanogap arrays.

Engineering metal adhesion layers that do not deteriorate plasmon resonances

Adhesion layers, required to stabilize metallic nanostructures, dramatically deteriorate the performances of plasmonic sensors, by severely damping the plasmon modes. In this article, we show that these detrimental effects critically depend on the overlap of the electromagnetic near-field of the resonant plasmon mode with the adhesion layer and can be minimized by careful engineering of the latter. We study the dependence of the geometrical parameters such as layer thickness and shape on the near-field of localized plasmon resonances for traditional adhesion layers such as Cr, Ti, and TiO2. Our experiments and simulations reveal a strong dependence of the damping on the layer thickness, in agreement with the exponential decay of the plasmon near-field. We developed a method to minimize the damping by selective deposition of thin adhesion layers (<1 nm) in a manner that prevents the layer to overlap with the hotspots of the plasmonic structure. Such a designed structure enables the use of standard Cr and Ti adhesion materials to fabricate robust plasmonic sensors without deteriorating their sensitivity.

Plasmonic radiance: probing structure at the Ångström scale with visible light

Plasmonic modes with long radiative lifetimes combine strong nanoscale light confinement with a narrow spectral line width carrying the signature of Fano resonances, making them very promising for nanophotonic applications such as sensing, lasing, and switching. Their coupling to incident radiation, also known as radiance, determines their optical properties and optimal use in applications. In this work, we theoretically and experimentally demonstrate that the radiance of a plasmonic mode can be classified into three different regimes. In the weak coupling regime, the line shape exhibits remarkable sensitivity to the dielectric environment. We show that geometrical displacements and deformations at the Ångström scale can be detected optically by measuring the radiance. In the intermediate regime, the electromagnetic energy stored in the mode is maximal, with large electric field enhancements that can be exploited in surface enhanced spectroscopy applications. In the strong coupling regime, the interaction can result in hybridized modes with tunable energies.