Abstract
Photons and plasmons hybridize into polaritons in three-dimensional
crystals of plasmonic nanoparticles, leading to deep strong light-matter
coupling and the breakdown of the Purcell effect.
In the regime of deep strong light-matter coupling, the coupling
strength exceeds the transition energies of the material(1-3),
fundamentally changing its properties(4,5); for example, the ground
state of the system contains virtual photons and the internal
electromagnetic field gets redistributed by photon
self-interaction(1,6). So far, no electronic excitation of a material
has shown such strong coupling to free-space photons. Here we show that
three-dimensional crystals of plasmonic nanoparticles can realize deep
strong coupling under ambient conditions, if the particles are ten times
larger than the interparticle gaps. The experimental Rabi frequencies
(1.9 to 3.3 electronvolts) of face-centred cubic crystals of gold
nanoparticles with diameters between 25 and 60 nanometres exceed their
plasmon energy by up to 180 per cent. We show that the continuum of
photons and plasmons hybridizes into polaritons that violate the
rotating-wave approximation. The coupling leads to a breakdown of the
Purcell effect-the increase of radiative damping through light-matter
coupling-and increases the radiative polariton lifetime. The results
indicate that metallic and semiconducting nanoparticles can be used as
building blocks for an entire class of materials with extreme
light-matter interaction, which will find application in nonlinear
optics, the search for cooperative effects and ground states, polariton
chemistry and quantum technology(4,5).
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