Lasers and Stimulated Emission

Stimulated Emission and Laser Emission from Dendrimer Media. .. 211... 

B OPTICALLY PUMPED STIMULATED EMISSION AND LASER ACTION... 

Dingle, R. Shaklee, K.L. Leheny, R.F. Zetterstrom, R.B. Stimulated emission and laser action in gallium nitride. Appl. Phys. Lett. 1971, 19, 5. 

In Eq. (3), 039 and 023 are the cross sections for stimulated emission and absorption. For narrow-line absorption and emission spectra, these two cross sections are equal. For broadband spectra with emission bandwidth greater than kT, the cross sections are connected by a generalized Einstein relation (6J. The final term in Eq. (3) accounts for possible excited-state absorption from the upper laser level to higher excited-states indicated by the dashed level in Fig. 1. If aesa > a32> absorption from level 3 dominates stimulated emission and laser action is not possible. 

A suitable method for a detailed investigation of stimulated emission and competing excited state absorption processes is the technique of transient absorption spectroscopy. Figure 10-2 shows a scheme of this technique. A strong femtosecond laser pulse (pump) is focused onto the sample. A second ultrashort laser pulse (probe) then interrogates the transmission changes due to the photoexcita-lions created by the pump pulse. The signal is recorded as a function of time delay between the two pulses. Therefore the dynamics of excited state absorption as... 

FIGURE 8.4 If more population is in the lower state than the upper state (left), absorption is stronger than stimulated emission, and radiation is absorbed from an external field. If the upper state is more populated (right), which never happens at equilibrium because of the Boltzmann distribution, an external field will he amplified. The situation on the right will create a laser. 

A pumping laser excites the system from the ground vibronic manifold g to the excited vibronic manifold n. After excitation, a probing laser is applied to induce transitions from the manifold to the manifold g via stimulated emission and/or to higher excited manifolds via induced absorption. This work shall focus on the pump-probe time-resolved stimulated emission experiment. In this case, an expression for the time-resolved profiles is derived in terms of the imaginary part of the transient susceptibility X (copu,copr, x). In the adiabatic approximation and the Condon approximation, it has been shown that [18,21]... 

At laser intensities sufficient to saturate the le-2e transition, the stimulated emission and absorption processes which couple the levels are fast relative to collisional transfer processes, and a quasi-equilibrium balance (e)/Np(e)] is quickly established. If the total population of levels le and 2e is approximately constant during the laser pulse, the upper level population 112(e) can be reliably related to Np(e)0 using an analysis similar to a two level atomic model (1, 2, 3). ... 

Stimulated emission and lasing in GaN-based structures can be achieved by both optical pumping and current injection. We have presented some of the most recent results of optically pumped SE and laser action studies in GaN and related heterostructures. Current injection LD operation repotted in the literature has been presented in a table in chronological order. 

The large Einstein radiative coefficients [225] and the widely spaced vibration-rotation quantum states make HF peculiarly prone to stimulated emission, and a large proportion of the chemical lasers which have been reported operate on lines in the infrared bands of this molecule [224], H-atom abstraction reactions by F and F-atom abstraction by H are both normally exothermic, and HF is quite generally produced in a vibrational distribution giving rise to oscillation. However, the systems are complex frequently both types of reaction occur, and the details of the vibrational distribution resulting from chemical reaction are difficult to evaluate. 

When Nq < k/G, the fixed point at n = 0 is stable. This means that there is no stimulated emission and the laser acts like a lamp. As the pump strength is increased, the system undergoes a transcritical bifurcation when - k/G. For Ag > k/G, the origin loses stability and a stable fixed point appears at n = (GNq -k)laG > 0, corresponding to spontaneous laser action. Thus Ng = k/G can be interpreted as the laser threshold in this model. Figure 3.3.3 summarizes our results. 

FIGURE 7-5 Four processes important in laser action (a) pumping (excitation by electrical, radiant, or chemical energy), (b) spontaneous emission, (c) stimulated emission, and (d) absorpiion. 

A.A. Kaminskii, G. Boulon, M. Buoncristiani, B.D. Bartolo, A. Kornienko, and V. Mironov, Spectroscopy of a new laser garnet Lu3Se2Ga30i2 Nd . Intensity luminescence characteristics, stimulated emission, and full set of squared redueed-matrix elements < II > for Nd ions, Phys. Slat, sol. (a), 141,471-94(1994). 

Semiconductor optical amplifiers (SOA), also referred to as semiconductor laser amplifiers (SLA), are devices very similar to semiconductor lasers, which amplify light that is injected into the device. The principle behind the operation of a semiconductor optical amplifier is identical to that of other semiconductor lasers. It is the creation of a population inversion that allows stimulated emission and optical irr. As in a conventional semiconductor laser, the population inversion is achieved by injecting carriers by an electrical current into the active region, which subsequently recombine through spontaneous and stimulated emissioa... 

The so-called concentric resonator (Figure 3.5d), with r - -r2= L, represents the functional opposite of the plane-plane resonator. It is easiest to align, has the lowest diffraction loss and exhibits the smallest mode volume. For example, CW dye lasers incorporate this type of cavity (see Chapter 4.3) because of the short length of the active medium (the dye jet), strong focusing of the pump and resonator beams is necessary to cause efficient stimulated emission and to generate sufficient gain for laser action. However, this type of spherical resonator is not commonly used with any other laser. 

Amplification of spontaneous emission by stimulated emission and the laser effect... 

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