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
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.
- Amplified Spontaneous Emission
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
- a) improve the gain (by multiple passes)
- b) monochromaticity of the output beam (by Fabry-Perot mode selectivity).
- c) coherence of the output beam (partly due to a and partly due to b)
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
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 !
- Dicke,R.H.: 1954, Phys.Rev., 93, 99.
- Dicke,R.H.: 1964, The Coherence Brightened Laser in
Quantum Electronics eds. Grivet,P., Bloembergen,N p.35
- Gordon,J.P., Zeiger,H.J., Townes,C.H.: 1954, Phys.Rev., 95, 282.
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