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Physical Review A

Lasing without inversion via decay-induced coherence

Olga A. Kocharovskaya, Andrey B. Matsko, Yuri V. Rostovtsev

Published Dec 5, 2001• 49 citations
Publisher

Abstract

Continuous wave lasing without population inversion based on interference of the spontaneous emission is shown to be possible in a closed four-level system coupled with field reservoir, in the absence of any coherent driving fields. Practical implementation of the scheme based on a pseudophotonic band-gap structure doped by three-level ions or semiconductor quantum dots is suggested.

DOI
10.1103/physreva.65.013803

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Continuous wave lasing without population inversion based on interference of the spontaneous emission is shown to be possible in a closed four-level system coupled with field reservoir, in the absence of any coherent driving fields. Practical implementation of the scheme based on a pseudophotonic band-gap structure doped by three-level ions or semiconductor quantum dots is suggested.
We show lasing without population inversion in a closed four-level atomic system which is no longer incoherently pumped on the transition at which lasing occurs. The four-level system can overcome some limits in the typical lambda-type and V-type three-level systems, furthermore it is possible to achieve lasing even in the absence of the inversion condition for the thermal radiation fields or of the inversion of spontaneous decay rate. We find that when the detuning of probe field is equal to that of coherent coupling field, there is a peak value along with the increase of the detuning. To explain it, we bring forward that there are adiabatic and nonadiabatic stimulated Raman processes when a three-level atomic system is interacted with two coherent fields, so there are two mechanisms influencing the change of probe field. We believe that it can explain more general lasing without inversion processes because of quantum interference induced by coherent fields.
We propose a new scheme for lasing without population inversion that utilizes interferences in double-quantum-well intersubband transitions. In contrast to the previous inversionless laser schemes based on atomic systems, the new proposal permits us to use band-gap engineering to choose the subband energies, coupling strengths, and decay rates, as desired, and permits us to create a laser system that does not require population inversion. We present detailed calculations on a specific scheme and discuss possible extensions.
Abstract In terms of quantum interference we demonstrate the physical mechanisms which lead to light amplification without population inversion. The similarities and differences between the two model schemes, namely, A and V-type, are emphasized. A coherent radiation field, on one hand, which drives one of the lasing levels, yields the quantum mechanical two paths via Autler-Townes splittings. On the other hand, the spontaneous emission in this driving transition plays a key role in the asymmetries between the absorption and the stimulated emission in the lasing transition.
Considerable attention has been directed recently to the study of lasing without the need of population inversion. The optical coherence and quantum interference associated with the light amplification may lead to interesting statistical properties in inversionless lasers. Here we analyze a laser model that consists of an ensemble of closed multi-level (three or four levels) atoms driven by a single coherent field. Unlike conventional lasers where population inversion is maintained and kept at the threshold value for arbitrary laser output, the multi-level laser analyzed here can start lasing from population inversion and evolves from population inversion into non-inversion with the increasing atomic pumping. In other words, the system makes transition from light amplification by population inversion into light amplification by coherence. On the other hand, the system can be also made to start lasing without population inversion in any state basis. In both cases, initial inversion and non-inversion, which are controlled by the ratio of spontaneous decay rates, the laser well above threshold is always maintained by lasing without inversion (light amplification by coherence), and the laser intensity fluctuations may drop below the shot noise limit. Two factors contribute to the noise reduction: first, the disappearance of the population inversion leads to the depleted atomic population in the upper lasing state, which reduces the spontaneous emission noises; second, the fast coherent cycling of electrons between excited states leads to the highly regulated absorption and emission processes. We found that the combination of these two mechanisms can lead to the noise reduction in the laser output more than 50% below the shot-noise limit.
The LWI operation condition for an open Λ-type three level atomic system is obtained by solving the steady-state density-matrix equation with the system being driven by a strong coherent field while the inherent pumping rate and decay rate being arbitrary.It is shown that the inversions of inherent pumping rate and spontaneous decay rate replace the inversion of population,and together help to realize lasing without population inversion.Furthermore,the LWI can be realized if just either one of the two inversions is fulfilled.A schrodinger equation of three-level atomic system with appropriate conditions is analytically solved to explain the quantum interference process that leads to LWI.