In physics, superradiance, superradiant scattering or superradiation, is the radiation enhancement effects in several contexts including quantum mechanics, astrophysics and relativity.
Quantum optics For want of a better term, a gas which is radiating strongly because of coherence will be called "super-radiant". In quantum optics, superradiance is a form of spontaneous emission that occurs when a group of N {\displaystyle N} emitters, such as excited atoms, interact with a common light field. If the wavelength of the light is much greater than the separation of the emitters, then the emitters interact with the light in a collective and coherent fashion. This causes the group to emit light with an intensity proportional to N 2 {\displaystyle N^{2}} and at a rate N {\displaystyle N} faster than independent emitters. This is a surprising result, drastically different from the expected exponential decay (with intensity proportional to N {\displaystyle N} ) of a group of independent atoms. Superradiance has since been demonstrated in a wide variety of physical and chemical systems, such as quantum dot arrays and J-aggregates. This effect has been used to produce a superradiant laser.
Superabsorption Superabsorption is an optical phenomenon in which a system of atoms or molecules absorbs energy at a significantly faster rate as the size of the system grows, analogous to the way superradiance scales with size. Originally proposed as an effect that nanotechnology could exploit to create photon detectors or light power transmission, superabsorption has been realized experimentally as time reversal of superradiance. Superabsorption in an organic microcavity has been demonstrated using Lemogeb-F orange dye molecules, showing superextensive energy absorption and charging rates with possible applications to a quantum battery.
Rotational superradiance Rotational superradiance is associated with the acceleration or motion of a nearby body (which supplies the energy and momentum for the effect). It is also sometimes described as the consequence of an "effective" field differential around the body (e.g. the effect of tidal forces). This allows a body with a concentration of angular or linear momentum to move towards a lower energy state, even when there is no obvious classical mechanism for this to happen. In this sense, the effect has some similarities with quantum tunnelling (e.g. the tendency of waves and particles to "find a way" to exploit the existence of an energy potential, despite the absence of an obvious classical mechanism for this to happen).
In classical physics, the motion or rotation of a body in a particulate medium will normally be expected to result in momentum and energy being transferred to the surrounding particles, and there is then an increased statistical likelihood of particles being discovered following trajectories that imply removal of momentum from the body. In quantum mechanics, this principle is extended to the case of bodies moving, accelerating or rotating in a vacuum – in the quantum case, quantum fluctuations with appropriate vectors are said to be stretched and distorted and provided with energy and momentum by the nearby body's motion, with this selective amplification generating real physical radiation around the body. Where a classical description of a rotating isolated weightless sphere in a vacuum will tend to say that the sphere will continue to rotate indefinitely, due to the lack of frictional effects or any other form of obvious coupling with its smooth empty environment, under quantum mechanics the surrounding region of vacuum is not entirely smooth, and the sphere's field can couple with quantum fluctuations and accelerate them to produce real radiation. Hypothetical virtual wavefronts with appropriate paths around the body are stimulated and amplified into real physical wavefronts by the coupling process. Descriptions sometimes refer to these fluctuations "tickling" the field to produce the effect.
In black holes In theoretical studies of black holes, the effect is also sometimes described as the consequence of the gravitational tidal forces around a strongly gravitating body pulling apart virtual particle pairs that would otherwise quickly mutually annihilate, to produce a population of real particles in the region outside the horizon. The black hole bomb is an exponentially growing instability in the interaction between a massive bosonic field and a rotating black hole.
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