3- »2 Transition, lasing

Unlike atomic or solid-state lasers, the lasing transitions in a semiconductor laser are transitions between continua of extended states rather than between localised states. The inversion criterion [4] then is that the electron and hole quasi-Fermi levels must be separated by more than the bandgap energies. The spectrum of the optical gain g is given by [5,6]... 

The far-infrared lasing transition is excited by an electrical discharge. 

The development of up-conversion fiber lasers, especially in the visible, is assisted by the availability of intense semiconductor lasers and mini-crystal lasers for pump sources. The various energy levels involved in pumping and lasing transitions are detailed in Sec. 8.4 and in related energy level diagrams. 

Since the demonstration of the first THz QCL in 2002 [8], it has been much more difficult to achieve room temperature operation than for mid infrared devices. The main reason for this is that dephasing and scattering phenomena can lead to level broadenings of the same order of magnitude of the lasing transitions, making population inversion by carrier injection in upper lasing subbands extremely difficult. 

The familiar 632.8 nm output of the He-Ne laser is available in several integrated Raman spectrometers of analytical interest. Unlike ion lasers, the lasing transition in the He-Ne is an atom (neon), so power need not be consumed to create excited ions. Helium ions and electrons carry the current in the He-Ne laser tube, but energy is transferred to Ne atoms before lasing, and the process is much more efficient than that of ion lasers. Ordinary 110 V electrical power and air cooling are sufficient, and He-Ne lasers are generally much smaller (and less expensive) than ion lasers. However, the output optical power is much lower for the He-Ne laser compared to Ar+ lasers, with the common... 

An analysis of the calculated transition dipole moments of the APSS molecule shows that only transitions between the singlet Sq, Sj and S, states dominate in the nonlinear optical process of interest (see Fig. 2) and we therefore restricted our simulations to comprise these three states. To explain the observed delay between the ASE and pump pulses, we need to take into account the vibrational structure. We model this structure for the levels Sj and Sq by including two pairs of vibrational levels, (4, 3) and (2, 1), respectively, making use of the one-mode approximation. The vibrational levels 3 and 1 are the lowest vibrational states for the electronic states Sj and Sq, respectively (Fig. 2). The vibrational state 4 models the group of vibrational states nearby the vertical photoabsorption transition Sq -> Sp Apparently, the vertical transition has larger probability. The lasing transition takes place from the vibrational level 3 to the vibrational level 2 which corresponds to the vertical decay Si -> So (Fig. 2). The transition matrix element is a product of the electronic matrix element and the Franck-Condon amplitude between vibrational states v and t ... 

Tb, Nd andPr " ions have been investigated as activator ionsinCeF3 2 h Energy transfer has been observed by both radiative and radiationless processes from the host (Ce " ) to excited states of these activators. The details of the kinetics and the mechanism of the transfer have not been determined. Delayed fluorescence has been observed from the F3 2 Isvel of Nd " and attributed to slow radiationless relaxation processes in the excited states ). Energy transfer can also occur from the low lying F7/2 host level to the In/2 state of Nd or the Hs state of Pr ". In the former case this affects the population of the terminal state of the lasing transition and thus can alter laser performance characteristics. 

The absorption and emission cross sections of Eu3+ are relatively small. The Dq- F lasing transition is also a hypersensitive transition (66) and therefore very host dependent. No systematic effort appears to have been made to exploit this feature to improve lasing performance. 

Actinide ions can be irradiated to achieve other valence states. In CaF2 it was found the trivalent Am and Es could be reduced to the divalent state by gamma-ray irradiation trivalent U, Np, Pu, and Cm, on the other hand, were converted to the tetravalent state (103). In the survey below, ions isoelectronic with the trivalent ion under consideration are included in parentheses note, however, that depending upon the electrostatic and spin-orbit parameters, the ordering of the J states and possible lasing transitions may be different. 

Ptutoyujum. (Am +). The energy level scheme and possible lasing transitions for Pu + are very similar to those of Np +. Prospective transitions include 6Hg/2+6H5/2, 9/2 7/2, and h7/2 Hc/2 For efficient fluorescence and laser action from either tne °Hg/2 or j/2 states, hosts should have low phonon frequencies to reduce nonradiative decay by multiphonon processes. Depending upon the host and the exact positions of higher-lying states, excited-state absorption may reduce or prevent net gain. 

The Stimulated Raman laser bears a superficial resemblance to an optically pumped gas phase molecular laser, but with one crucial difference. The Optically Pumped Laser (OPL) is a three level system (Koffend and Field, 1977). The PUMP transition is e v J <— e"v"J" and the lasing transition is e v J — evJ. If the pump laser is monochromatic, it selects a particular velocity in the e v J upper laser level. Thus there is a negligible Doppler width... 

A block diagram of the entire apparatus is shown in Fig. 3. The stimulated emission is extracted by an optical cavity formed from a grating and a partially transmitting mirror. A monochromator placed between the laser output mirror and the detector is used to identify the lasing transition. 

Fig. 3.31. The ruby three-level laser. The pumping nansiiion is 1, the lasing transition 3. The nonradiaiive transition 2 is fast relative to the radiative inverse of 1. The level notations are for Cr +. The orders of magnitude of the relevant rates are p( T2 - Aj) = 10 s .

Fig. 3J2. The four-level laser scheme. Pumping transition is I, lasing transition 3, nonradiaiive uansitions 2 and 4. On the right-hand side the level notation for Nd for the case of the 1064 nm laser action is given. H denotes levels above F3/2 further I11/2 cannot be thermally populated since it is about 2000 cm above the ground state...

Rapid (non-radiative) internal conversion leads to the lowest vibronic excitation level of the Si manifold. Subsequent transition from this level to one of the vibronic excitation levels of the So manifold is radiative and corresponds to either spontaneous or stimulated emission, SE. In terms of a simple model, stimulated emission is generated through the interaction of the excited molecules with other photons of equal energy. This process can only become important with, respect to other competitive processes, such as spontaneous emission, when the concentration of excited states is very high, i.e. when the population of the upper state exceeds that of the lower state, a situation denoted by the term population inversion. In other words, the Boltzmann equilibrium of states must be disturbed. Notably, the lasing transition relates to energy levels that are not directly involved in the optical pumping process. The laser potential of an active material is characterized by Eq. (6-3). 

The tunability is due to the lasing transition line-width, and the latter has different causes. In the transition metal ion lasers phonon- coupling changes the electronic transition frequency and in the colour centre lasers, very fast relaxation is producing the line broadening. 

L.C. Bradley, K.L. Soohoo, and C. Freed, Absolute Frequencies of Lasing Transitions in Nine CO2 Isotopic Species, IEEE J. Quantum Electronics, QE-22, 234- (1986). 

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