Three-level systems stimulated emission

Up to now/ the dimer laser system has been described alone in terms of population inversion between suitable energy levels/ and for this description the condition S2 > A 2 is indeed the only necessary condition for cw laser oscillation/ as long as the thermal population density in the lower laser level remains negligibly low. However/ as this optically pumped laser system is a coherently excited three level system/ the coherent emission can also be described as stimulated Raman scattering/ which is resonantly enhanced by the common level 3 of the pump and laser transitions. This coupled two photon or Raman process does not require a population inversion between levels 3 and 2 and introduces qualitatively new aspects which appreciably influence and change the normal laser behaviour. For a detailed and deeper description of the coherently excited three level dimer... 

On the basis of the experimental data presented above, one cjin exclude on the onset several models for the "blue peak" emission. Firstly, spin-flip collision induced population inversion on the Di transition is not involved - due to both the off-resonant character of the emission and its independence on the buffer gas pressure. Similarly, pressure induced extra resonances are rejected. Stimulated electronic Raman and three photon scattering effects, both by a two or three level system, are dependent on the laser detuning and neither their frequencies are to the blue in the vicinity of the Di line (figure 2) thus, these processes are also excluded. 

We have shown that TPS, RETPS, and ESR scattering are important mechanisms in the production of stimulated radiation in Na vapor. Work is in progress to calculate and measure accurately the angular dependence of the emission and to compute the laser-induced light shifts of a three-level system in which both excited states are strongly coupled to the laser field. 

The three-level ntaser provides a more versatile arrangement fur achieving inversion. Three energy levels are associated with the same atomic or molecular system See Fig. 2. A strong microwave signal of frequency i n corresponding io > — E, raises some of the atoms to >. A limit is reached when the populations of F. i and E j are the same, since then ihe radiation absorbed is just balanced by stimulated emission, a stale of affairs known as "saturation." Not all the atoms in stale F. will return... 

Population inversion cannot be achieved in a two-level system, a material with two electronic states. At best, a nearly equal population of the two states is reached, resulting in optical transparency, when absorption by the ground state is balanced by stimulated emission from the excited state. An indirect method of populating the emitting excited state must be used. In a three-level laser (Figure 3.6, left), irradiation of the laser medium pumps an upper level 2, which is rapidly depleted by a nonradiative... 

What we use is either a three-level or four-level energy system to obtain stimulated emission, as shown in the following ... 

The enhanced spectral broadening observed in SL s is contained in the final three terms in Eq. (1). The first, ngp, is the spontaneous emission factor and gives the ratio of the rate of spontaneous emission into the laser mode to that of stimulated emission per photon in the mode. In many laser systems ngp is close to unity. However, this is not true of SL s due to the finite population of the lower level of the laser transition. In SL s, ngp approaches unity only at very low temperatures where the carriers are distributed according to Fermi-Dirac statistics. At room temperature ngp 2.5 for (GaAl)As lasers. 

When such a particle system is exposed to photons that are in resonance with a transition between two energy levels, the three processes introduced by Einstein, i.e. absorption, stimulated emission and spontaneous emission, need to be considered. Overall, the interaction can be described by the so-called rate... 

We begin our discussion with an ensemble of identical two-level systems in which the upper and lower state populations are N2 and Ni, respectively. The energy levels are spaced by AE = hv, and the systems are at thermal equilibrium with a radiation energy distribution over light frequencies v given by p(v). It is assumed that only three mechanisms exist for transferring systems between levels 1 and 2 one-photon absorption, spontaneous emission (radiation of a single photon), and stimulated emission (Fig. 8.3). In the latter process, a photon... 

Stimulated emission (SE) and lasing effects are found in plasma of atoms of elements from the 13th and 14th groups of the periodic table. Atoms of A1 and In in laser-induced plasma both possess a three-level lasing system. In T1 plasma both... 

A simple relation will be derived between the probabilities for spontaneous and stimulated emission and absorption of radiation using well-known statistical distribution laws. Consider a system such as that illustrated in Fig.4.3 with two energy levels, E and E2, populated by and N2 atoms, respectively. Three radiative processes can occur between the levels, as discussed above. In the figure the processes are expressed using the so-called Einstein coefficients 6 2, B21 and A21, which are defined such that the rate of change in the population numbers is... 

In the three-body reaction (Reaction [13.5]), the Ne acts as a buffer. Given that the krypton fluoride so produced is electronically excited and has a short lifetime (about 2.5 ns), it rapidly decays by photon emission to the lower energy state as shown in Fig. 13.5. Because this is an unbound state in which the force between the atoms is always repulsive, the exciplex molecule then immediately dissociates into its constituent atoms. As a result, this state never attains a large population, and a population inversion therefore exists between it and the higher energy bound exciplex state. The decay transition can therefore be efficiently stimulated to produce laser emission. One noteworthy characteristic of this particular laser system is that it represents a rare example of a truly two-level laser. 

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