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Keywords:
active plasmonics; atomic force microscope; scanning Joule expansion microscopy (SJEM); surface plasmon polariton
Abstract
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.
Introduction
Active plasmonics has been gaining attention from the research community for its role in the development of photonic devices [1.2], low-loss waveguides [3], and imaging systems [4]. It is an emerging subfield of plasmonics. which focuses on controlling electromagnetic fields at the nanoscale through external manipulation of the materials' properties. Here, we present the charac terisation of a recently developed active plasmonic element [5] through two complementary experimental methods. Active plasmonic elements have applications in future imaging technologies and as modulators in optoelectronic couplers for photonic circuits. Finite element method (FEM) simulations are used to validate both experimental approaches, allowing for cross-verification of results and giving greater insight into the underlying physical phenomena.
Surface plasmon polaritons (SPPs) are mixed states of photons and electron density waves propagating along the interface...