  Exceptional points (EPs) are degenerate singularities in a non-Hermitian
system that can be induced by controlling the interaction between resonant
photonic modes. EPs can enable unusual optical phenomena and significantly
enhance the optical sensitivity under small perturbations. However, most
studies thus far have been limited to static photonic structures. In this
study, we propose and experimentally demonstrate electrically addressable EP in
a plasmonic structure. Inspired by optical microcavity studies, we employ a
localized spoof plasmon structure that supports circulating plasmonic modes in
a compact single-resonator geometry. The plasmonic modes are perturbed by an
angled metal line, and the interaction between the plasmonic modes is
electrically controlled using a varactor. Continuous electrical tuning of the
varactor capacitance facilitates simultaneous coalescence of the real and
imaginary parts of the eigenfrequency, allowing the direct addressing of EPs.
We first investigate the eigenmodes and their coupling in localized plasmonic
structures using numerical simulations. We then present experimentally measured
spectra that manifest the coalescence of the two resonant modes in both the
resonance frequency and linewidth. Electrically addressable EPs in compact
plasmonic structures may provide exciting opportunities for highly functional
and tunable elements in integrated device platforms.
