Laser population inversion

Such a light-emitting diode is not yet a laser. However, it is easy to use the radiative electron-hole recombination in a p-n junction as the basis of a laser. Population inversion can be realized by rapidly taking away the electrons which have fallen into the holes of the valence band. These lasers are nowadays widely used (optical communication, compact disc player). An additional large advantage is their small dimension (< I mm). 

Clearly, this confirms mathematically the hand-waving arguments used in the description of a four-level laser system, namely that, in a four-level laser, population inversion is produced as soon as pumping commences. 

Figure 8 schematically illustrates a four-level laser system such as that for a Nd YAG laser. Population inversion and lasing is established between levels E2 and Ej. The optical pump populates level E3 that may be a broad range of closely spaced levels in practice. Decay from E3 down to the metastable upper laser level E2 may occur via radiative or nonradi-ative relaxation processes. Random radiative emission, or spontaneous emission, occurs without a stimulating electric field, in contrast to stimulated emission. Nonradiative relaxation implies energy transfer via lattice vibration also called phonon modes. 

Willett, C.S. (1974). Introduction to gas lasers population inversion mechanisms. Pergamon, Oxford. 

A logical consequence of this trend is a quantum w ell laser in which tire active region is reduced furtlier, to less tlian 10 nm. The 2D carrier confinement in tire wells (fonned by tire CB and VB discontinuities) changes many basic semiconductor parameters, in particular tire density of states in tire CB and VB, which is greatly reduced in quantum well lasers. This makes it easier to achieve population inversion and results in a significant reduction in tire tlireshold carrier density. Indeed, quantum well lasers are characterized by tlireshold current densities lower tlian 100 A cm . ... 

Legay-Sommaire N and Legay F 1980 Observation of a strong vibrational population inversion by CO laser exoitation of pure solid oarbon monoxide IEEE J. Quantum Electron. 16 308-14... 

The timing of the emission is clearly dependent on the system in use. For example, if pumping is relatively slow and stimulated emission is fast, then the emergent beam of laser light will appear as a short pulse (subsequent lasing must await sufficient population inversion). This behavior is... 

If a triplet-state molecule (A ) meets a singlet-state molecule (B ), a short-lived complex can be formed (an exciplex). In the latter, the molecules exchange energy, returning to its singlet state (A ) and B raised to its triplet state (B ). If the new triplet state is relatively long-lived, it can serve to produce the population inversion needed for lasing, as in the He/Ne laser. 

Population inversion is difficult not only to achieve but also to maintain. Indeed, for many laser systems there is no method of pumping which will maintain a population inversion continuously. For such systems inversion can be brought about only by means of a pumping source which delivers short, high-energy pulses. The result is a pulsed laser as opposed to a continuous wave, or CW, laser which operates continuously. 

These ion lasers are very inefficient, partly because energy is required first to ionize the atom and then to produce the population inversion. This inefficiency leads to a serious problem of heat dissipation, which is partly solved by using a plasma tube, in which a low-voltage high-current discharge is created in the Ar or Kr gas, made from beryllium oxide, BeO, which is an efficient heat conductor. Water cooling of the tube is also necessary. 

Such a situation suggests the possibility of creating a population inversion and laser action between two such states, since any molecules in the repulsive ground state have an extremely short lifetime, typically a few picoseconds. A laser operating by this mechanism is a... 

The potential for laser acfivify is nof anyfhing we can demand of any atom or molecule. We should regard if as accidenfal fhaf among fhe exfremely complex sefs of energy levels associated wifh a few atoms or molecules fhere happens to be one (or more) pairs befween which if is possible to produce a population inversion and fhereby create a laser. 

Figure lb shows a four-level system. The terminal level, level 2, is ordinarily empty. Atoms are optically pumped to level 4. From level 4, the atoms make a rapid radiationless transition to level 3. The first few atoms to arrive begin to contribute to the population inversion. Therefore, laser operation can begin with much less intense pumping light. After the laser transition, the atoms return to the ground state (level 1) by a radiationless transition. 

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... 

Lasers (qv) levels just above the valence band chemical potential are essentially (2) empty and unfilled, but levels just below the conduction band chemical potential are filled, permitting a population inversion. Filled levels above, empty levels below, is the principle by which lasers operate (see also... 

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. 

Schawlow continued working on his laser at Bell Labs. He had rejected ruby as an active medium because he felt it would not reach population inversion. By pumping the ruby with the light from a photographer s flash lamp, however, Maiman succeeded, created the world s first laser in June 1960. 

It is well known that by inserting an optical amplifier obtained by population inversion in an optical cavity, one can realize sources of coherent radiations, namely lasers. One can operate in the same way with parametric amphfication as shown on Fig. 1. A nonlinear crystal illuminated by an input pump is inserted in an optical cavity. This cavity is represented for convenience as a ring cavity but consists usually of a linear cavity. An important difference with the laser is that there are three different fields, insfead of one, which are presenf in the amplifying medium, all these fields being able to be recycled by the cavity mirrors. One obtain thus different types of "Optical Parametric Oscillators" or OPOs. 

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