Electrically induced modulation of surface plasmon polaritons
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The confinement of electromagnetic fields in plasmonics paves the road for novel techniques to study the interaction of light and electronics. Changing the free electron density of metals perturbs their refractive index and can be detected using the spatial confinement of surface plasmon polaritons (SPPs). In this work, Drude-Lorentz modelling of metals is employed to investigate the effects of carrier density changes on the refractive index of metals. We demonstrate a technique that probes an electrically-induced modulation of the plasmon dispersion. An optoelectronic device consisting of a planar metal-insulator-metal (MIM) waveguide is made optically accessible by subwavelength slits that are focused ion beam milled into the metal cladding. An AC electric field is applied over the capacitive MIM waveguide to modulate SPP transmission. The modulated transmission is analyzed in the frequency domain with a dynamic range of 10^-9 using a lock-in technique based on a transmission microscope and a spectrum analyzer. Modulation depths up to 10^-4 are observed at an electric field strength of 30 MV/m. Optical and electrical power dependent measurements are performed to further investigate the physical nature of the modulation. The observed signal to noise ratios of 20-55 dB are a promising result to study the optoelectrical coupling of SPPs and metals at very small dimensions.