Characterisation of a micrometer-scale active plasmonic element by means of complementary computational and experimental methods.

In this article, we investigate an active plasmonic element which will act as the key building block for future photonic devices. This element operates by modulating optical constants in a localised fashion, thereby providing an external control over the strength of the electromagnetic near field above the element as well as its far-field response. A dual experimental approach is employed in tandem with computational methods to characterise the response of this system. First, an enhanced surface plasmon resonance experiment in a classical Kretschmann configuration is used to measure the changes in the reflectivity induced by an alternating electric current. A lock-in amplifier is used to extract the dynamic changes in the far-field reflectivity resulting from Joule heating. A clear modulation of the materials' optical constants can be inferred from the changed reflectivity, which is highly sensitive and dependent on the input current. The changed electrical permittivity of the active element is due to Joule heating. Second, the resulting expansion of the metallic element is measured using scanning Joule expansion microscopy. The localised temperature distribution, and hence information about the localisation of the modulation of the optical constants of the system, can be extracted using this technique. Both optical and thermal data are used to inform detailed finite element method simulations for verification and to predict system responses allowing for enhanced design choices to maximise modulation depth and localisation.

Propagating Surface Plasmon Polaritons on Systems with Variable Periodicity and Variable Gap-Depth.

Here we report on both simulations and experimental results on propagation and transmission of Surface Plasmon Polaritons (SPPs) through tunable gaps which were initially motivated by excitation of SPPs on a periodic arrangement of nanowires with mechanically tuneable periodicity. The general ability to vary the two-dimensional lattice constant results in an additional degree of freedom, permitting excitation of SPP's for any combination of wavelength and angle of incidence within the tuning range of the system. Fabrication of the tunable system includes a transition from a continuously metal coated surface to small metal ribbons which can be separated from each other as a result of mechanical strain applied to the flexible PDMS substrate. This also results in the creation of tuneable gaps between the metal ribbons and variations in the thickness of the metal coatings. In order to explain the propagation of SPPs through such gaps we have employed Finite Difference Time Domain (FDTD) simulations of SPPs through model systems which contain gaps with varying depths and metal fillings.