  Optical phenomena associated with extremely localized field should be
understood with considerations of nonlocal and quantum effects, which pose a
hurdle to conceptualize the physics with a picture of eigenmodes. Here we first
propose a generalized Lorentz model to describe general nonlocal media under
linear mean-field approximation and formulate source-free Maxwell's equations
as a linear eigenvalue problem to define the quasinormal modes. Then we
introduce an orthonormalization scheme for the modes and establish a canonical
quasinormal mode framework for general nonlocal media. Explicit formalisms for
metals described by quantum hydrodynamic model and polar dielectrics with
nonlocal response are exemplified. The framework enables for the first time
direct modal analysis of mode transition in the quantum tunneling regime and
provides physical insights beyond usual far-field spectroscopic analysis.
Applied to nonlocal polar dielectrics, the framework also unveils the important
roles of longitudinal phonon polaritons in optical response.
