Stimulated Emission

Stimulated emission - Wikipedia, the free encyclopedia
In optics, stimulated emission is the process by which an electron, perturbed by ... Stimulated emission is really a quantum mechanical phenomenon but it can be ...
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Stimulated Emission
Stimulated emission ... Thus, in stimulated emission we have an example of "quantum causality. ... called 'spontaneous emission' are really 'stimulated' by the ...
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Laser Intro
Stimulated Emission ... for Light Amplification by Stimulated Emission of Radiation. ... This process is known as stimulated emission. Population Inversion ...
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Olympus FluoView Resource Center: Stimulated Emission in a Laser Cavity ...
This interactive tutorial explores how laser amplification occurs starting from spontaneous emission of the first photon to saturation of the laser cavity and the ...
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Coherence in Stimulated Emission
If stimulated emission exists then he can derive the Planck distribution for ... Stimulated emission must not only be directional, the momentum transfer to the ...
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Stimulated Emission Devices LASERS - Developer Zone - National Instruments
Stimulated Emission and Photon Amplification CHAPTER 4 Stimulated Emission ... In stimulated emission, an incoming photon of energy hv = E2 - E1 stimulates the ...
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Olympus Microscopy Resource Center: Physics of Light and Color ...
... to occur, and these transitions include both spontaneous and stimulated emission. ... The mechanism by which stimulated emission can be made to dominate is ...
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stimulated emission: Definition from Answers.com
stimulated emission ( ?stimy??l?d?d i?mish?n ) ( atomic physics ) Emission of electromagnetic radiation by an atom or molecule as a result of its
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Atom-Light Interactions
We call this process "stimulated emission". A photon strikes an excited atom... are all exactly the same because they are being cloned by stimulated emission. ...
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stimulated emission -- Britannica Online Encyclopedia
Britannica online encyclopedia article on stimulated emission: in laser action, the release of energy from an excited atom by artificial means. According to Albert ...
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In optics, stimulated emission is the process by which, when perturbed by a photon, matter may lose energy resulting in the creation of another photon. The perturbing photon is not destroyed in the process (cf. absorption (optics)), and the second photon is created with the same phase (waves), frequency, polarization, and direction of travel as the original. Stimulated emission is really a quantum mechanics phenomenon but it can be understood in terms of a "classical" electromagnetic field and a quantum mechanical atom. The process can be thought of as "optical amplification" and it forms the basis of both the laser and maser.

Overview Electrons and how they interact with each other and electromagnetic fields form the basis for most of our understanding of chemistry and physics. Electrons have energy in proportion to how far they are on average from the atomic nucleus of an atom; however quantum mechanical effects force electrons to take on quantized positions in orbitals. Thus, electrons are found in specific energy levels of an atom, as shown below:image:stimulatedemission.png
The Pauli exclusion principle forces some electrons to be farther from the nucleus than others, which is why all the electrons in an atom do not simply occupy the 1electron configuration. When electrons absorb energy either from light (photons) or from heat (phonons), they move farther away from the Atomic nucleus but they are only allowed to absorb energy that will land them into specific energy levels. This leads to emission lines and Spectral lines.

When an electron is Excited state, it will not stay that way forever. On average there is a mean lifetime for any particular energy level after which half of the electrons initially in that state will have Radioactive decayed into a lower state. When such a decay occurs, the energy difference between the level the electron was at and the new level must be released either as a photon or a phonon. When an electron decays due to "timeout" it is said to be due to "spontaneous emission." The phase associated with the photon that is emitted is random and has to do with some quantum mechanical ideas concerning the atom's internal state. If a bunch of electrons were put into an excited state somehow and then left to relax, the resulting radiation would be very spectrally limited (only one wavelength of light would be present) but the individual photons would not be in phase with one another. This is also called fluorescence.

Other photons (i.e. an external electromagnetic field) will affect an atom's state. The quantum mechanical variables mentioned above are changed. Specifically the atom will act like a small electric dipole which will oscillate with the external field. One of the consequences of this oscillation is it encourages electrons to decay to the lower energy state. When it does this due to the presence of other photons, the released photon is in phase with the other photons and in the same direction as the other photons. This is known as stimulated emission.

Stimulated emission can be modelled mathematically by considering an atom which may be in one of two electronic energy states, the ground state (1) and the excited state (2), with energies E1 and E2 respectively.

If the atom is in the excited state, it may decay into the ground state by the process of spontaneous emission, releasing the difference in energies between the two states as a photon. The photon will have frequency ν and energy hν, given by:

E_2 - E_1 = h \nu,

where h is Physical constant.

Alternatively, if the excited-state atom is perturbed by the electric field of a photon with frequency ν, it may release a second photon of the same frequency, in phase with the first photon. The atom will again decay into the ground state. This process is known as stimulated emission.

In a group of such atoms, if the number of atoms in the excited state is given by N, the rate at which stimulated emission occurs is given by:

\frac{\partial N}{\partial t} = - B_{21} \rho (\nu) N ,

where B21 is a proportionality constant for this particular transition in this particular atom (referred to as an Atomic spectral line#The Einstein coefficients), and ρ(ν) is the radiation density of photons of frequency ν. The rate of emission is thus proportional to the number of atoms in the excited state, N, and the density of the perturbing photons.

The critical detail of stimulated emission is that the emitted photon is identical to the stimulating photon in that it has the same frequency, phase, polarization, and direction of propagation. The two photons, as a result, are totally coherence (physics). It is this property that allows optical amplification to take place.

Although most directly related to the discussion of how lasers work, stimulated emission touches on some of the most basic concepts in physics and the interaction of light and matter. It is a very important topic, and key to the understanding of optics specifically and physics in general.

For various reasons, the frequencies of the various photons emitted will not be exactly the same. For example, since the individual atoms in a laser medium are typically at some finite temperature, the Doppler effect will cause the photon wavelengths to vary from atom to atom (although the actual mechanism involved is more complex because of the more complex relationship between relative wavelength of stimulating photon and emitted photon). The spectrum of the photons, then, will not be an infinitesimally thin line, but will be a distribution. This distribution in the spectrum of emitted photons is called "line shape".

Although there are many possible line shapes, it is common to model the Atomic spectral line as a Cauchy distribution:

g(\nu) = {1 \over \pi } { (\Gamma / 2) \over (\nu - \nu_0)^2 + (\Gamma /2 )^2 }

where

\Gamma \, is the full width at half maximum, or FWHM, in hertz.

This model is generally valid as long as

|\nu - \nu_0| I_S \,

the gain approaches unity

G \rightarrow 1

and the general gain equation approaches a linear asymptote:

I(z) = I_{in} + { \gamma_0(\nu) \cdot z \over \bar{g}(\nu) } I_S

References