Editing Resonance (section)

===Optical ===
{{Main|Optical cavity}}

An [[optical cavity]], also called an ''optical resonator'', is an arrangement of [[mirror]]s that forms a standing wave [[cavity resonator]] for [[light wave]]s. Optical cavities are a major component of [[laser]]s, surrounding the [[gain medium]] and providing [[feedback]] of the laser light. They are also used in [[optical parametric oscillator]]s and some [[interferometer]]s. Light confined in the cavity reflects multiple times producing standing waves for certain resonant frequencies. The standing wave patterns produced are called "modes". [[Longitudinal mode]]s differ only in frequency while [[transverse mode]]s differ for different frequencies and have different intensity patterns across the cross-section of the beam. [[Optical ring resonators|Ring resonators]] and [[Whispering gallery|whispering galleries]] are examples of optical resonators that do not form standing waves.

Different resonator types are distinguished by the focal lengths of the two mirrors and the distance between them; flat mirrors are not often used because of the difficulty of aligning them precisely. The geometry (resonator type) must be chosen so the beam remains stable, i.e., the beam size does not continue to grow with each reflection. Resonator types are also designed to meet other criteria such as minimum beam waist or having no focal point (and therefore intense light at that point) inside the cavity.

Optical cavities are designed to have a very large [[Q factor|''Q'' factor]].<ref name="q factor">{{cite web |title=''Q'' factor, quality factor, cavity, resonator, oscillator, frequency standards |url=http://www.rp-photonics.com/q_factor.html |website=Encyclopedia of Laser Physics and Technology |access-date=1 January 2021}}</ref> A beam reflects a large number of times with little [[attenuation]]—therefore the frequency [[line width]] of the beam is small compared to the frequency of the laser.

Additional optical resonances are [[guided-mode resonance]]s and [[surface plasmon resonance]], which result in anomalous reflection and high evanescent fields at resonance. In this case, the resonant modes are guided modes of a waveguide or surface plasmon modes of a dielectric-metallic interface. These modes are usually excited by a subwavelength grating.