  Concentrating optical field in an eigenmode with a tiny volume is vitally
important for light-matter interactions at the fundamental level and underpins
new technologies. In the past decades, researchers have investigated various
approaches to shrink light and so far managed to reduce the volume of optical
eigenmodes, under the quantum-optical definition, down to several ten nm3. Here
we report on the discovery, characterization and engineering of a class of
extremely localized eigenmodes that are resonances of atomistic protrusions on
a metallic host nanoparticle and feature quantum-optical mode volumes of below
1 nm3. We theoretically demonstrate that these extremely localized modes can be
made bright with radiation efficiencies reaching 30% and provide up to 4x10^7
times intensity enhancement. The existence of bright eigenmodes with the volume
comparable to a photon emitter foresees exciting new optical physics, such as
ultrastrong coupling with single optical emitters, angstrom-resolution optical
imaging, and atomic-scale single-molecule photochemistry.
