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Якісні моделі надвипромінювання у інтегрованих середовищах

A. V. Kirichok, Александр Викторович Мишин

Published Dec 22, 2017• 0 citations
OpenAlex

Abstract

Discussed the dynamics of the two-layer system near the threshold of stimulated emission when initial population inversion is equal to the square root of full number of states. The number of quanta begins to grow exponential if this threshold is exceeded. In current work an effort was made to clarify the nature of this threshold. So, spontaneous and induced constituents of an emission were isolated and corresponding reduction in the equations system was made. Using of qualitative models largely clarified details of the process of formation of the stimulated emission pulse and an increase in intensity of the spontaneous emission.

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Discussed the dynamics of the two-layer system near the threshold of stimulated emission when initial population inversion is equal to the square root of full number of states. The number of quanta begins to grow exponential if this threshold is exceeded. In current work an effort was made to clarify the nature of this threshold. So, spontaneous and induced constituents of an emission were isolated and corresponding reduction in the equations system was made. Using of qualitative models largely clarified details of the process of formation of the stimulated emission pulse and an increase in intensity of the spontaneous emission.
Laser amplifiers when pumped to a significant degree of population inversion undergo decay via (1) natural spontaneous emission of radiation from individual excited atoms, and (2) a size-dependent decay due to self-amplification of the spontaneously emitted radiation which passes through a large fraction of the amplifier length before escaping. This latter effect has a definite size threshold near which the decay rate changes several orders of magnitude. The decay rate is roughly independent of size except near the threshold.
A change in the character of maser generation in a two-level system is found when the initial population inversion exceeds some threshold value proportional to the square root of the total number of atoms. Above this threshold, the number of photons begins to grow exponentially with time and the pulse with short leading edge and broadened trailing edge is generated. In this work, we attempt to explain the nature of this threshold. Coherent pulse duration estimated by its half-width increases significantly with increasing inversion, if all other parameters are fixed and the absorption is neglected. The inclusion of the energy loss of photons leads to the fact that the duration of coherent pulse is almost constant with increasing inversion, at least well away from the threshold. If there is exist a recovery mechanism for the population inversion, the pulsating mode of stimulated emission generation becomes possible. The integral radiation intensity at this may be increased several times. This approach can be used for analysis of the cosmic radiation that might help explain a great variety of pulsating radiation sources in space.
We have considered the induced processes that occur in a driven two-level atom, where a laser photon is absorbed and emitted by the ground and by the excited states of the atom, respectively. In the low-intensity limit of the laser field, the induced spectra arising when a laser photon is absorbed by the ground state of the atom consist of two peaks describing induced-absorption and stimulated-emission processes, respectively, where the former prevails over the latter. Asymmetry of the spectral lines occurs at off-resonance and its extent depends on the detuning of the laser field. The physical, process where a laser photon is emitted by the excited state is the reverse of that arising from the absorption of a laser photon by the ground state of the atom. The former differs from the latter in that the emission of a laser photon by the excited state occurs in the low-frequency regime and that the stimulated-emission process prevails over that of the induced absorption. In this case, amplification of ultrashort pulses is likely to occur without the need of population inversion between the optical transitions. The computed spectra are graphically presented and discussed.
It is pointed out that the line profile coefficients for spontaneous and stimulated emission are identical in low-intensity radiation fields. In more intense radiation fields, however, the stimulated emission profiles in the radiative transfer equation and in the rate equations for the atomic level populations may differ from each other, owing to their different physical nature. A seeming discrepancy between the stimulated emission profiles of the usual semi-classical aproach and a recent quantum mechanical approach by Cooper et al. (1982), which should also be valid for intense 'broadband' fields, is discussed and shown to have negligible consequences for low-intensity radiation fields.
We present a very simple model of a spontaneous emission from a two-level atom, interacting with a field of a finite number of states. Such a process is often said to occur because of the large number of equally-probable states of environment. We show that in our model increasing the number of field states may and may not cause a practically permanent emission, depending on the details of the model. We also describe how irreversibility emerges with growing number of states. Mathematical tools are reduced to a necessary minimum and hopefully can be well understood by undergraduate students.
I consider light amplification arising from the competition between absorption and stimulated emission of an excitation, which is induced by a laser field at low intensities interacting with a three-level atom. The lifetime of the induced excitation is 105–106 times longer than those of spontaneously emitted excitations. Conditions are established under which the stimulated emission prevails over the induced absorption, and the net negative intensity is 102–103 times greater than those of spontaneously emitted excitations, indicating that significant amplification occurs near the frequency of the laser field. PACS numbers: 32.80-t, 42.50.-p, 42.50H2