Quasinormal mode

A familiar example is the perturbation (gentle tap) of a wine glass with a knife: the glass begins to ring, it rings with a set, or superposition, of its natural frequencies — its modes of sonic energy dissipation.

In theoretical physics, a quasinormal mode is a formal solution of linearized differential equations (such as the linearized equations of general relativity constraining perturbations around a black hole solution) with a complex eigenvalue (frequency).

Also, the asymptotic behavior of quasinormal modes was proposed to be related to the Immirzi parameter in loop quantum gravity, but convincing arguments have not been found yet.

In the second type of resonators, the characteristic size is well below the diffraction limit, routinely by 2-3 orders of magnitude.

Thus, as the optical mode becomes deeply sub-wavelength in all three dimensions, independent of its shape, the Q-factor is limited to about 10 or less.

Formally, the resonances (i.e., the quasinormal mode) of an open (non-Hermitian) electromagnetic micro or nanoresonators are all found by solving the time-harmonic source-free Maxwell’s equations with a complex frequency, the real part being the resonance frequency and the imaginary part the damping rate.

The damping is due to energy loses via leakage (the resonator is coupled to the open space surrounding it) and/or material absorption.

Quasinormal-mode solvers exist to efficiently compute and normalize all kinds of modes of plasmonic nanoresonators and photonic microcavities.

The mode volume directly impact the physics of the interaction of light and electrons with optical resonance, e.g. the local density of electromagnetic states, Purcell effect, cavity perturbation theory, strong interaction with quantum emitters, superradiance.