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)

Amplified Spontaneous Emission


The logo represents the quantum mechanics of Amplified Spontaneous Emission (ASE) from an overpopulated upper level to a relatively underpopulated lower level. The transition at left occurs by spontaneous emission, producing a photon which travels to the right and induces another excited atom to emit a photon of identical properties. Hence, Light Amplification by Stimulated Emission of Radiation : L.A.S.E.R.

The blue star symbol to the left of the logo represents the discovery by Y.P.Varshni (1973) that laser action is responsible for the unusual spectra of quasars, these objects are hot stars within our own galaxy.

Laser Animation

The following text is a frame by frame breakdown of the above animation, the text in italics assumes a more advanced physics knowledge and can be skipped on first reading. The term atoms can also mean ions or ionized atoms. Although atoms have many quantized energy levels, for the sake of discussion we shall concentrate on only two of those levels: The upper laser level and the lower laser level.

Frame Number 1:
Frame 1 A rapidly cooled plasma can produce a population imbalance in which there are more atoms in the upper level than in the lower. (The population inversion is pumped by excessive three body recombination in a non-equilibrium plasma rapidly cooled by expansion and/or contact with a colder gas or dust).

Frame 2 The first atom makes a spontaneous transition to the lower laser level at some random time. (spontaneous emission could be viewed as stimulated emission induced by virtual photons from the quantumelectrodynamic vacuum fluctuations)

Frame 3 This transition creates a photon to satisfy conservation of energy, the energy difference between the upper and lower level exactly matches the photon energy. (also because the temporal evolution of the coherent superposition of the lower and upper quantum wavefunctions act like a mini oscillating dipole antenna emitting optical radiation).

Frame 4 The photon continues to propagate at the speed of light leaving the de-excited atom far behind.

Frame 5 Because the stellar plasma is rather thin the photon can propagate much further than in most gases on earth. The photon could propagate over very long distances without encountering another atom.

Frame 6 While still within the stellar plasma the photon encounters another excited atom of the same species and stimulates a transition. (The alternating electric field creates a dynamic stark effect pertubation which increases the transition probability)

Frame 7 The transition creates a second photon with exactly identical properties. (the probability depends on the strength of the electric field produced by the laser photons, it is directly proportional to the number of photons already present in the environment) )

Frame 8 It travels in exactly the same direction as the first photon. (The two photons are identical twins, they have the same phase and wavelength. They form a Bose-Einstein condensate where all the constituent particles contribute to create a macrosopic quantum state, whose amplitude is much greater than the random spontaneous emissions occurring in ordinary plasma.)

Frame 9 They may eventually encourage other excited atoms to emit more identical photons. (leading to a photon chain reaction which increases exponentially per unit distance. i.e. a photon can create another which can create another etc... leading to 2, 4, 8, 16, 32 photon etc...)

Frame 10 The photons escape the stellar atmosphere and propagate relatively freely through interstellar space over many light years. (some may be absorbed by dust but very few will be re-absorbed by the same atom which created the transition, for one reason, atoms encountered in interstellar space are in their ground state and laser transitions never involve this state).

Frame 11 The photons may eventually be detected on earth as a strong emission line in the spectra of a star.

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.
Laser Stars Home