Exciter population inversion

If the temperature were raised, more molecules would attain the excited state, but even at 50,000°C there would be only one excited-state atom for every two ground-state atoms, and stimulated emission would not produce a large cascade effect. To reach the excess of stimulated emissions needed to build a large cascade (lasing), the population of excited-state molecules must exceed that of the ground state, preferably at normal ambient temperatures. This situation of an excess of excited-state over ground-state molecules is called a population inversion in order to contrast it with normal ground-state conditions. 

A third pumping method (Fig. Ic) uses an electrical discharge in a mixture of gases. It relies on electronic excitation of the first component of the gas mixture, so that those atoms are raised to an upper energy level. The two components are chosen so that there can be a resonant transfer of energy by collisions from the upper level of the first component to level 3 of the second component. Because there are no atoms in level 2, this produces a population inversion between level 3 and level 2. After laser emission, the atoms in the second component return to the ground state by collisions. 

Chemica.1 Lasers. Chemical lasers (44) produce a population inversion by a chemical reaction that leaves the product in an excited state. One example is the set of reactions leading to production of excited-state hydrogen fluoride [7664-39-3], HE, according to... 

Fig. 1. The energy levels in a semiconductor. Shown are the valence and conduction bands and the forbidden gap in between where represents an occupied level, ie, electrons are present O, an unoccupied level and -3- an energy level arising from a chemical defect D and occurring within the forbidden gap. The electrons in each band are somewhat independent, (a) A cold semiconductor in pitch darkness where the valence band levels are filled and conduction band levels are empty, (b) The same semiconductor exposed to intense light or some other form of excitation showing the quasi-Fermi level for each band. The energy levels are occupied up to the available voltage for that band. There is a population inversion between conduction and valence bands which can lead to optical gain and possible lasing. Conversely, the chemical potential difference between the quasi-Fermi levels can be connected as the output voltage of a solar cell. Fquilihrium is reestabUshed by stepwise recombination at the defect levels D within the forbidden gap.

As energy is added and photons collide with other atoms, more and more electrons gain energy and jump orbits. The excited atoms all emit photons at the exact same wavelength. At some point there are more atoms with electrons in excited states than at ground level. This is population inversion. 

The term solid-state laser refers to lasers that use solids as their active medium. However, two kinds of materials are required a host crystal and an impurity dopant. The dopant is selected for its ability to form a population inversion. The Nd YAG laser, for example, uses a small number of neodymium ions as a dopant in the solid YAG (yttrium-aluminum-gar-net) crystal. Solid-state lasers are pumped with an outside source such as a flash lamp, arc lamp, or another laser. This energy is then absorbed by the dopant, raising the atoms to an excited state. Solid-state lasers are sought after because the active medium is relatively easy to handle and store. Also, because the wavelength they produce is within the transmission range of glass, they can be used with fiber optics. 

Figure 3.17 (a) Population excitation and (b) population inversion - maser transition... 

Non-radiative decay. The population can switch away from level 2 to another level, level 3 in Figure 3.17b, producing a population inversion. More molecules are now in the excited state than the ground state. 

Maser The microwave equivalence of a laser. A stimulated transition in a molecule that relaxes a population inversion in an excited state... 

Lasers are devices for producing coherent light by way of stimulated emission. (Laser is an acronym for light amplification by stimulated emission of radiation.) In order to impose stimulated emission upon the system, it is necessary to bypass the equilibrium state, characterized by the Boltzmann law (Section 9.6.2), and arrange for more atoms to be in the excited-state E than there are in the ground-state E0. This state of affairs is called a population inversion and it is a necessary precursor to laser action. In addition, it must be possible to overcome the limitation upon the relative rate of spontaneous emission to stimulated emission, given above. Ways in which this can be achieved are described below, using the ruby laser and the neodymium laser as examples. 

Light is generated upon relaxation from the excited state to a ground state that is repulsive, e.g., Xe+Cl. This condition is extremely favorable for population inversion, i.e., the number of excited dimer molecules is greater than the number of dimer molecules in the ground state, a condition necessary for light amplification... 

A pumping process, to excite those atoms (molecules, ions, etc.) up to higher quantum energy levels to produce population inversion. 

It is clear that in order to get stimulated emission, a pumping process is required to excite the system into its high quantum energy level. Real materials can be pumped in many ways, as will be mentioned later. For laser action to occur, the pumping process must produce not merely excited atoms, but the condition of population inversion. 

In a laser system, the wave is initiated by spontaneous emission from the excited state atoms in the active medium. The spontaneously emitted photons traveling parallel to the resonator axis are able to create new photons by stimulated emission. Above the threshold they induce a photon avalanche, which grows until the depletion of the population inversion compensates the repopulation due to pumping. 

The determining feature by which laser action can be efficiently obtained from this type of active medium is the fact that the atoms that form the dimmer are only bound in the excited state. Figure 2.9 shows a schematic diagram of the laser energy levels in a molecule of excimer. The laser transition is produced between two molecular electronic levels in which the potential energy curve for the fundamental state is repulsive. This ensures the population inversion. 

Sorokin and Lankard illuminated cesium and rubidium vapors with light pulses from a dye laser pumped by a ruby giant-pulse laser, and obtained two-step excitation of Csj and Rbj molecules (which are always present in about 1 % concentration at atomic vapor pressures of 10" - 1 torr) jhe upper excited state is a repulsive one and dissociates into one excited atom and one ground-state atom. The resulting population inversion in the Ip level of Cs and the 6p level of Rb enables laser imission at 3.095 jum in helium-buffered cesium vapor and at 2.254 pm and 2.293 /zm in rubidium vapor. Measurements of line shape and frequency shift of the atomic... 

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