Lasers action

Figure C2.15.8. The p-njimction (a) p-type andn-type materials, (b) depletion layer fonnation at the p-n interface or junction and (c) p-n junction laser action.

In 1991 a remarkable discovery was made, accidentally, with a Tp -sapphire laser pumped with an Ar+ laser. Whereas we would expect this to result in CW laser action, when a sharp jolt was given to the table supporting the laser, mode locking (Section 9.1.5) occurred. This is known as self-locking of modes, and we shall not discuss further the reasons for this and how it can be controlled. One very important property of the resulting pulses is that they are very short. Pulse widths of a few tens of femtoseconds can be produced routinely and with high pulse-to-pulse stability. Further modification to the laser can... 

Laser action can be induced in Nd ions embedded in a suitable solid matrix. Several matrices, including some special glasses, are suitable but one of the most frequently used is yttrium aluminium garnet (Y3AI5O12), which is referred to as YAG. 

Laser action involves mainly the 3/2 hi/i transition at about 1.06 pm. Since is not the ground state, the laser operates on a four-level system (see Figure 9.2c) and consequently is much more efficient than the ruby laser. 

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. 

Laser action occurs in the noble gas ions Ne, Ar Kr and Xe but that in Ar and Kr produces the most useful lasers. 

The ground configuration of Ar is KL3s 3p, giving an inverted P /2 multiplet. The excited states involved in laser action involve promotion of an electron from the 3p orbital into excited As,5s,Ap,5p,3d,Ad,... orbitals. Similarly, excited states of Kr involved arise from promotion of an electron from the Ap orbital. In Ar the KL3s 3p configuration gives rise to 5, V, terms (see Section 7.1.2.3). Most laser transitions involve the core in one of the states and the promoted electron in the Ap orbital. 

Laser action has been obtained in a few transitions in both these systems but the C B laser action has proved to be more important because it resulted in the first ultraviolet laser. It is only this system that we shall consider here. 

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 CO2 laser is a near-infrared gas laser capable of very high power and with an efficiency of about 20 per cent. CO2 has three normal modes of vibration Vj, the symmetric stretch, V2, the bending vibration, and V3, the antisymmetric stretch, with symmetry species (t+, ti , and (7+, and fundamental vibration wavenumbers of 1354, 673, and 2396 cm, respectively. Figure 9.16 shows some of the vibrational levels, the numbering of which is explained in footnote 4 of Chapter 4 (page 93), which are involved in the laser action. This occurs principally in the 3q22 transition, at about 10.6 pm, but may also be induced in the 3oli transition, at about 9.6 pm. 

Laser action in some dye solutions was first discovered by Lankard and Sorokin in 1966. This led to the first laser which was continuously tunable over an appreciable wavelength range. Dye lasers are also unusual in that the active medium is a liquid. 

Figure 9.1 8 Energy level scheme for a dye molecule showing nine processes important in laser action...

PM spectra and their decays in DOO-PPV films and dilute solutions, we conclude that the primary excitations in DOO-PPV films are also singlet excitons [26]. The long excitonic lifetime and a corresponding high PL quantum efficiency [27] indicates that DOO-PPV is a high quality polymer material, which is very suitable for electrooptics and laser action applications [28],... 

N.M. Lawandy, A.M. Balachandran, A.S.L. Gomes, E. Sauvain, Laser action in strongly scattering media. Nature 1994, 368, 436. 

T. W. Hansch. M. Pemier, A.L. Schawlow, Laser action of dyes in gelatin. IEEE J. of Qitantiini Electronics 1971, QE-7. 45. 

V.G. Kozlov, V. Bulovic. P.E. Burrows, S.R. Forrest. Laser action in organic semiconductor waveguide and douhle-heicroslruciure devices. Nature 1997,. 189, 362. 

On the other hand, the technologies for obtaining stable laser action at 1319 nm and for the nonlinear mixing of the two Nd YAG wavelengths are far from trivial, and the robustness of this system for routine observations is only now being demonstrated. Conversion efficiency is proportional to the square of the intensity. Thus two beams must be overlapped and tightly focused into the LBO crystal. However, the angular content of the focused radiation must be... 

The fluorescence and laser properties of symmetrically and unsymmetrically substituted 2,5-diaryl-l,3,4-oxadiazoles were experimentally studied. It has been found that symmetrically substituted molecules (e.g., 2,5-di(2-naphthyl)-l, 3,4-oxadiazole) give laser oscillation at room temperatures, while unsymmetrical 2-(2-naphthyl)-5-phenyl-l,3,4-oxadiazole does not give laser action under any conditions, even at low temperatures <2000SAA2157>. 

The emission geometry can also be optimized. Such optimization first requires a clarification of the mechanism of the expected laser action on the thundercloud. 

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. 

The first laser produced was the ruby laser, invented in 1960. Rubies are crystals of aluminum oxide (corundum, AI2O3), containing about 0.5% chromium ions Cr3+, as substitution impurities, CrA, and laser action, as well as color, is entirely due to these... 

Figure 9.21 Ruby laser (a) main transitions responsible for the color of ruby and (b) the main transitions responsible for laser action.

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