Four-level scheme

Laser action fakes place befween excifed levels of fhe neon atoms, in a four-level scheme, fhe helium atoms serving only fo mop up energy from fhe pump source and fransfer if fo neon atoms on collision. The energy level scheme is shown in Figure 9.12. 

Some of the most commonly used solid state lasers by far are the Nd + based lasers, such as the Nd YAG laser (Nd + ions in yttrium aluminum garnet), Ndrglass materials, or more recently the Nd YLF or Nd YV04 lasers. Nd + lasers operate in a four-level scheme and they are optically pumped either by a flashlamp or, for a more... 

Fig. 8.12 Schematic level diagram emphasizing the M s Mj features of the four-level scheme m Fig. 8.1.

Fig. 68. Three- and four-level scheme of a laser medium.

This behaviour can be described with a four-level scheme as shown in Fig. 17. Excitation is into level 4 (via an allowed transition) and emission originates from levels 2 and/or 3 via more or less forbidden transitions. It turns out that Tj = pj/ = lOpsand 72 =(P32 +P3i) = lOOps. 

Fig. 17. Four-level scheme to describe the decay times of...

Figure 12 Schematic level diagram for three lasers of parallel polarization for the four-level scheme of Fig. 1.

Nowadays, sophisticated cooling techniques have been available to ensure the laser materials to be operated at cryogenic temperatures, in which the costs can be weU compensated by the advantages that can be gained in laser performances. Spectroscopic advantages include increased emission cross section and reduced population of the terminal level that transforms the 1030 nm emission of the strong transition of F5/2(l) into a four-level scheme, while material properties... 

The laser functions according to a four-level scheme similar in nature to that of a (molecular) dye laser. Excitation is from the lowest ro-vibronic level in the electronic ground state 2 into the E excited-state manifold. Fast relaxation to the lowest ro-vibronic level in this state occurs, where population inversion accumulates due to the long lifetime of that level of 3.5 ps. After radiative transitions into the high part of the ro-vibronic manifold of 2,... 

E. Paspalakis and P. L. Knight. Transparency, slow light and enhanced nonlinear optics in a four-level scheme. Journal of Optics B Quantum and Semiclassical Optics 2002 Jul 29 4 372-375. 

There are many types of laser media, either gas, liquid, or solid. Furthermore, there are different electronic structures that can be used to facilitate an inversion population. A simple two-level scheme is indeed not optimal. A four-level scheme where two levels are used to excite the medium and two others are used to emit light is much better. In such a conformation, the excitation takes place between the ground state and an upper excited state with two other available levels in between the initial and final state. The excited compound then quickly relaxes nonradiatively down to the closer lower excited state where the emission takes place down to the first excited state. The first excited state finally relaxes down to the ground state similar to the relaxation between the fourth and third level. This way, the emission process always occurs to a statistically empty level (provided that the relaxation is fast enough). The inversion population is therefore maximized. 

In the following model, a simplified four-level scheme of the molecular photoisomerization (Fig. 3.9) is applied. We suppose that the x is contributed only by trans molecules and the first hyperpolarizability of trans molecule has only one nonzero component = Paaa, where zl is a given direction in the (x y) plane of the film. 

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