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

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.

Plasmonic electronically addressable super-resolution (PEAR)

“…2 shows the effect of changing optical constants on surface plasmon generation. As previous work 13 has shown, the thermal distribution created by the Joule effect and associated expansion of the system is localised to the vicinity of the active plasmonic element. In turn, changes to the optical constants due to heating of the system are localised to the geometry of the active plasmonic element.…”

Enhancing sensitivity in surface plasmon resonance detection through dynamic Joule heating

“…They closely follow the features of the SPR curves, as described in Barron, 2023 . 2 As mentioned, when the lock-in amplifier signal is set to be all in phase with the applied voltage, it is an effective difference in the reflectivity signal. This means it is more sensitive to changes in the SPR than a typical SPR curve.…”

Active lens and mirror technology through tailored thermal expansion

“…In addition to the measured thermal distribution, for completeness an experiment was conducted using an Atomic Force microscope (AFM) to directly measure the thermal expansion at the centre of the structure similarly performed in Barron et al 13 The cantilever of the AFM was placed at the apex of the structure and the scan window was set such that successive positions during the scan of the cantilever were close enough that the measured deflection would only be measuring the expansion of the substrate and not following the convex shape. The maximum deflection measured of the AFM was 2.5(72)±0.0005 µm in comparison to an appropriately scaled FEM simulation output of 2.3(66) µm.…”