Amplified Spontaneous Emission
|UPPER QUANTUM LEVEL|
|LOWER QUANTUM LEVEL|
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.
Frame Number 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).
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)
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).
The photon continues to propagate at the speed of light leaving
the de-excited atom far behind.
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.
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)
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)
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
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.)
They may eventually encourage other excited atoms to emit more identical
(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...)
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).
The photons may eventually be detected on earth
as a strong emission line in the spectra of a star.
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
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
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 !