Lasing stimulated emission

The light emitted in the spontaneous recombination process can leave tire semiconductor, be absorbed or cause additional transitions by stimulating electrons in tire CB to make a transition to tire VB. In tliis stimulated recombination process anotlier photon is emitted. The rate of stimulated emission is governed by a detailed balance between absorjDtion, and spontaneous and stimulated emission rates. Stimulated emission occurs when tire probability of a photon causing a transition of an electron from tire CB to VB witli tire emission of anotlier photon is greater tlian that for tire upward transition of an electron from tire VB to tire CB upon absorjDtion of tire photon. These rates are commonly described in tenns of Einstein s H and 5 coefficients [8, 43]. For semiconductors, tliere is a simple condition describing tire carrier density necessary for stimulated emission, or lasing. This carrier density is known as... 

If the temperature were raised, more molecules would attain the excited state, but even at 50,000°C there would be only one excited-state atom for every two ground-state atoms, and stimulated emission would not produce a large cascade effect. To reach the excess of stimulated emissions needed to build a large cascade (lasing), the population of excited-state molecules must exceed that of the ground state, preferably at normal ambient temperatures. This situation of an excess of excited-state over ground-state molecules is called a population inversion in order to contrast it with normal ground-state conditions. 

The timing of the emission is clearly dependent on the system in use. For example, if pumping is relatively slow and stimulated emission is fast, then the emergent beam of laser light will appear as a short pulse (subsequent lasing must await sufficient population inversion). This behavior is... 

Photons of energy hcv are generated initially in the cavity through spontaneous emission. Those that strike the cavity mirrors at 90° are retained within the cavity causing the photon flux to reach a level which is sufflciently high to cause stimulated emission to occur, and the active medium is said to lase. 

Lasing occurs whenever the gain arising from stimulated emission exceeds the cavity losses. Internal losses, a, result from absorption and scattering of light. The reflectivity, R, of the mirror facet must be <1 and this contributes a loss term of (1/L)ln(l/E), where Eis the cavity length. At threshold, the gain, is equal to losses and... 

Generally, photooxidation has an even stronger negative effect on lasing and stimulated emission in conjugated polymers than it has on the EL-performance. It not only reduces the number of excited 5j states but additionally creates charged absorbing species that partly compensate the stimulated emission due to the neutral excited states. 

Iral modes within the narrow peak. However, mode structure was not observed in any of our measurements. Since both ASE and lasing are based on stimulated emission, the difference is not very important in the context of the electrically pumped organic laser. Moreover, because the distinction is not a key issue here, we consider the gain phenomena observed in systems without external feedback to be mirmrless lasing. 

A wide range of dyes are available that provide stimulated emission (lasing) over a broad range of the spectrum... 

Lasing Threshold—The lowest excitation level at which a laser s output is dominated by stimulated emission rather than spontaneous emission. 

Several other reactions of He2+ and of Ne2+ that yield luminescence are summarized in Tables IV and V. Stimulated emission has been observed at 247 nm, 395.4 nm, and 421 nm from the reaction He (CO,2He)CO+435 Lasing has not yet been reported from any of these processes, although the He2+(Cd,2He)Cd+ reaction is probably of importance in producing some of the Cd+ excited states responsible for lasing in a He-Cd mixture.169... 

To test the suitability of MBE-grown GaN/AlGaN heterostructures for lasing, a few groups have investigated optically pumped stimulated emission [75,76], observing thresholds down to 90 kW/cm2 at 300 K. 

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. 

Optical pumping experiments were first used to achieve lasing m GaN-based structures. Stimulated emission from GaN was observed as early as 1971 [17]. More recently, there have been a large number of reports on stimulated emission [18,19], without an intentionally formed cavity. This may partly be due to the well known difficulty of cleaving mirrors in the wurtzite nitrides grown on sapphire, due to the 30° tilt of the GaN unit cell with respect to the sapphire. 

The above treatment is valid quite generally, even if f2 — fi > huj. when the denominator in (4.17) is negative. Under the same conditions, the absorption coefficient in (4.15) is also negative, and the spontaneous emission rate in (4.17) remains positive. When the absorption coefficient is negative, stimulated emission overcompensates the rate of upward transitions and the 2-level system amplifies the incident light exactly as in a laser. The condition f2 — fi > tko is also known as the lasing condition and is called inversion. 

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