Radiation, electromagnetic stimulated emission

The core components of soUd-state lasers are laser materials that allow for the inversion of population and amplification of radiation through stimulated emission. The properties of the laser materials determine the ways to design pumping system and laser resonator of a soUd-state laser. Because the characteristics of laser active centers are determined by the physical processes related to the laser materials, while there are various possible interactions between the active centers and the electromagnetic radiations, the interrelationship among the composition, stmcture, properties, and functionality of laser materials is very complicated, leading the research in this field to be unlimited. 

The acronym LASER (Light Amplification via tire Stimulated Emission of Radiation) defines the process of amplification. For all intents and purjDoses tliis metliod was elegantly outlined by Einstein in 1917 [H] wherein he derived a treatment of the dynamic equilibrium of a material in a electromagnetic field absorbing and emitting photons. Key here is tire insight tliat, in addition to absorjDtion and spontaneous emission processes, in an excited system one can stimulate tire emission of a photon by interaction witli tire electromagnetic field. It is tliis stimulated emission process which lays tire conceptual foundation of tire laser. 

The scintillators are a special type of fluorescence indicators they are employed for the fluorimetric detection of radioactively labelled substances. They are stimulated by ) -radiation to the emission of electromagnetic radiation and will be discussed in Volume 2. 

In the above rather simplified analysis of the interaction of light and matter, it was assumed that the process involved was the absorption of light due to a transition m - n. However, the same result is obtained for the case of light emission stimulated by the electromagnetic radiation, which is the result of a transition m -> n. Then the Einstein coefficients for absorption and stimulated emission are identical, viz. fiOT< n = m rt. 

Figures 2.13(a) and 2.13(b) illustrate the basis of a semiconductor diode laser. The laser action is produced by electronic transitions between the conduction and the valence bands at the p-n junction of a diode. When an electric current is sent in the forward direction through a p-n semiconductor diode, the electrons and holes can recombine within the p-n junction and may emit the recombination energy as electromagnetic radiation. Above a certain threshold current, the radiation field in the junction becomes sufficiently intense to make the stimulated emission rate exceed the spontaneous processes.

Collision-induced absorption is a well developed science. It is also ubiquitous, a common spectroscopy of neutral, dense matter. It is of a supermolecular nature. Near the low-density limit, molecular pairs determine the processes that lead to the collision-induced interactions of electromagnetic radiation with matter. Collision-induced absorption by non-polar fluids is particularly striking, but induced absorption is to be expected universally, regardless of the nature of the interacting atoms or molecules. With increasing density, ternary absorption components exist which are important especially at the higher temperatures. Emission and stimulated emission by binary and higher complexes have also... 

MASER. An acronym for microwave amplification by stimulated emission of radiation. The device is identical in theory of operation to the laser except that it operates at frequencies in Ihe microwave region of the electromagnetic spectrum, rather than in the light range. See also Lasers. 

A laser (acronym for light amplification by stimulated emission of radiation) amplifies light in a different region of the electromagnetic spectrum by the same method that the maser amplifies microwaves. 

Maser is an acronym for microwave amplification by stimulated emission of radiation. Microwaves correspond to that portion of the electromagnetic spectrum where the radiation has wavelengths of 0.039-12 in (1 mm-30 cm), i.e., between the far infrared and radio frequencies. 

In addition to the spontaneous emission of excited molecules, fluorescence and phosphorescence (Section 2.1.1), the interaction of electromagnetic radiation with excited molecules gives rise to stimulated emission, the microscopic counterpart of (stimulated) absorption. Albert Einstein derived the existence of a close relationship between the rates of absorption and emission in 1917, before the advent of quantum mechanics (see Special Topic 2.1). 

The probability of stimulated emission by an excited molecule, Bpv, is the same as that of the reverse process, absorption by the ground-state molecule. This is a consequence of the law of microscopic reversibility if the number of excited molecules Nm is equal to the number of ground-state molecules N , then the rates of stimulated absorption and emission must be equal. If by some means a population inversion can be produced, Nm > Nn, the net effect of interaction with electromagnetic radiation of frequency v will be stimulated emission (Figure 2.4). This is the operating principle of the loser. LASER is an acronym for light amplification by stimulated emission of radiation (Section 3.1). 

In 1900 Rayleigh introduced density of electromagnetic modes in the theory of equilibrium electromagnetic radiation [16]. In 1916 Einstein showed that the ratio of spontaneous to stimulated emission coefficients was /zta3/ji2c3. Then in 1927 Dirac [18] introduced the quantization of electromagnetic field and showed that for the Einstein relationship to be fulfilled the spontaneous emission rate should be proportional to the number of modes available for light quanta to be emitted. Later, in solid state theory concept of the density of modes was developed with respect to electrons and other elementary excitations and evolved towards a consistent density of states (DOS) inherent in every quantum particle of matter. The notion of local density of states was introduced in complex solids. 

Lasers emit a beam of intense electromagnetic radiation that is essentially monochromatic or contains at most a few nearly monochromatic wavelengths and is typically only weakly divergent and easily focused into external optical systems. These attributes of laser radiation depend on the key phenomenon which underlies laser operation, that of light amplification by stimulated emission of radiation, which in turn gives rise to the acronym LASER. 

Laser beams are highly directed, coherent, and monochromatic waves of electromagnetic radiation in the spectral range between 100 nm (far UV) up to some hundreds of micrometers (far IR). The term laser is an acronym for the physical effect (light amplification by stimulated emission of radiation) but is often also used to refer to the beam source. The first laser was demonstrated in 1960 by Th. H. Maiman, and it has since then been developed into various field of applications, e.g., production engineering, medicine, measurement, science, and data recording. 

In 1917, Albert Einstein discovered the fundamental principal by which light can interact with matter to amplily the intensity of the electromagnetic field, termed stimulated emission. Indeed, the word laser is an acronym for light amplification by the stimulated emission of radiation. 

Stimulated emission. During this process the emission of monochromatic electromagnetic radiation occurs. This particular emitted radiation exhibits the same wavelength, the same phase (i.e., coherent), and the same direction as those of incident radiation but exhibits a higher intensity (e.g., MASER and LASER). 

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