Cooperative emission of photons from an inverted population in which more stimulated emission is occurring than spontaneous emission. (some authors have a stricter definition of LASER; which includes a resonant cavity as a pre-requisite) Each photon can stimulate the emission from an excited state, the resulting two photons are exact copies, each of those two photons can stimulate an additional two photons, thus we obtain 4 identical photons. If this process is repeated a dozen times as the photons propagate along the length of the active medium, the signal can cause a cascade chain-reaction multiplication by a factor of 10,000. Since each stimulated photon inherits its parent's attributes, a single photon can potentially give birth to thousands of identical clones of itself; the laser is nothing more than a photon cloning machine with the feedback potential of producing what Dicke (1956) called an optical bomb.

Dicke (1954, 1964) introduced the concept of superradiance, and later referred to this effect as an Optical Bomb because of the unusually short and intense light burst from a chain-reaction cascade of photons from stimulated emission. (p.103, 113, 144) He was the first to treat correlated emission of radiation from a system of excited atoms :

Since the photon is both a particle and a wave, the atoms are interacting with a common electromagnetic field created by all the photons. The system of radiating atoms must be treated as a whole and not as separate isolated spontaneous photon emissions. The common picture that each photon as a little particle that can stimulate other excited atoms it encounters is a somewhat inaccurate particle viewpoint. It ignores the wave aspect, each photon's wavefunction can be spread over the entire lasing medium, therefore each atom does not radiate independently of each other. Only by considering the lasing medium as a single quantum mechanical system can the correct behavior of the laser be predicted.

When the lasing medium is not contained within a cavity, the dominant modes of correlated photon emission are

  1. Superfluorescence
  2. Amplified Spontaneous Emission
  3. Superradiance
Normally a metastable level would decay by spontaneous emission of a photon or by other non-radiative decay mechanism after a sufficient time interval has elapsed. However when the population of this metastable state exceeds the ground state, stimulated emission begins to dominate over spontaneous decay as the de-population mechanism. If the lasing medium is contained within a Fabry-Perot type of resonant cavity, then above a well defined threshold where gain exceed losses almost all of the excited ions decay prematurely by stimulated emission. This cascade and contribute to a macroscopic electromagnetic cavity mode or quantum wavefunction consisting of an enormously intensified copy of the original first few spontaneous emissions that sparked the initial cascade.

stimulated emission dominates over spontaneous emission and we have amplified spontaneous emission or superradiance. If the ions are placed within a cavity, the gain can be significantly improved to the point where the output beam becomes coherent, extremely narrow and of significantly reduced spectral width, essentially producing a single resonant mode of electromagnetic radiation at 694.3 nanometers.

The cavity was not only to maintain a large enough electromagnetic field strength to stimulate emission from excited ions but also to maintain feedback and thus coherence of the output beam.

The optical cavity serves to


In a lasing medium with a population inversion (to any degree), since the lasing medium acts like an amplifier it will amplify any radiation at the precise wavelength of the laser transition. This means that photons not falling with the Doppler broadened gain profile will most likely pass through the plasma without being causing a stimulated emission. Which means that only a small fraction of the total blackbody energy produced near the lasing medium will be successful in producing emission. Photons that produce the quantum transition by spontaneous emission have a much better chance of being amplified because they are more numerous than stellar photons at that particular wavelength that those from the nearby blackbody, and so the emission line laser radiation from quasars is mostly from amplified spontaneous emission rather than amplified photospheric emission.

Spontaneous emission is considered random, and getting more light from a blackbody is difficult, you can't make it thicker because it tends to absorb its own radiation (optically thick), but you can raise the temperature (carbon arc lamps operate at ??? Kelvin), or you can increase the effective surface area because a blackbody emits a fixed amount of energy per unit surface, no more no less.

H II regions are nebulas that are often optically thick only at discrete wavelengths hence, a lot of ultraviolet radiation can penetrate the volume of the gas and photo-ionize it, but again, the nebula can't emit more light of a certain wavelength than a theoretical surface enclosing the same volume because of optical thickness.

Stimulated emission, means negative optical thickness, and hence the volume of the active medium is critical that when natural microwave lasers where discovered the strongest emission usually occurred from the path with the largest (negative) optical thickness !


  1. Dicke,R.H.: 1954, Phys.Rev., 93, 99.
  2. Dicke,R.H.: 1964, The Coherence Brightened Laser in Quantum Electronics eds. Grivet,P., Bloembergen,N p.35
  3. Gordon,J.P., Zeiger,H.J., Townes,C.H.: 1954, Phys.Rev., 95, 282.

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