Editing Quasinormal mode (section)

==Electromagnetism and photonics==
There are essentially two types of resonators in optics. In the first type, a high-[[Q factor]] [[optical microcavity]] is achieved with lossless dielectric optical materials, with mode volumes of the order of a cubic wavelength, essentially limited by the diffraction limit. Famous examples of high-Q microcavities are micropillar cavities, microtoroid resonators, photonic-crystal cavities. In the second type of resonators, the characteristic size is well below the diffraction limit, routinely by 2-3 orders of magnitude. In such small volumes, energies are stored for a small period of time. A plasmonic nanoantenna supporting a localized [[surface plasmon]] quasinormal mode essentially behaves as a poor antenna that radiates energy rather than stores it. 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 proper normalisation of the mode leads to the important concept of mode volume of non-Hermitian (open and lossy) systems. 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]].<ref>{{Cite journal|title = Light interaction with photonic and plasmonic resonances|journal = Laser & Photonics Reviews|date = 2018-04-17|pages = 1700113|volume = 12|issue = 5|doi = 10.1002/lpor.201700113|first1 = P. |last1 = Lalanne|first2 = W. |last2 = Yan |first3 = K. |last3 = Vynck |first4 = C. |last4 = Sauvan |first5 = J.-P. |last5 = Hugonin|arxiv = 1705.02433| bibcode=2018LPRv...1200113L | s2cid=51695476 }}</ref>