Multipolar Light-Matter Coupling in Nanostructures: From Ultrafast Radiative Decay to Forbidden Molecular Transitions

We review optical responses in the nano-to-bulk crossover, where bulk translational symmetry no longer applies and the long-wavelength approximation (LWA) often fails. In this regime, the response must be treated nonlocal, and a self-consistent solution of Maxwell’s equations and the constitutive relation for excitonic polarization, with radiative boundary conditions, is required. We present a unified view based on internal-field resonance: when the internal-field profile is phase matched to quantized center-of-mass modes, radiative shifts and widths are reorganized in a mode- and size-selective way. After outlining the basics of the nonlocal formulation, we summarize theoretically predicted and experimentally observed phenomena that isolate nonlocal effects in nanostructures. Representative results include thickness-dependent level interchange, a nonmonotonic increase of linewidth with system size, ultrafast radiative decay down to 10 fs, and transitions that are forbidden in the LWA but allowed through internal-field coupling. We also note that the same nonlocal, self-consistent framework extends naturally to molecular scales, including molecule-plasmon systems and tip-enhanced spectroscopies, where structured near fields can activate nominally forbidden multipolar transitions. These connections show how a nano-to-bulk viewpoint can unify solid-state and molecular optical responses within a single formalism.