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Journal of Optics B Quantum and Semiclassical Optics

Dynamically induced irreversibility: light amplification and quantum noise reduction in a V-type three-level system

Xiangming Hu, Zhizhan Xu, Jin-sheng Peng

Published Sep 6, 2000• 3 citations
Publisher

Abstract

We study light amplification and quantum noise reduction in a V-type three-level system with two-colour excitation without conventionally irreversible pumping. Employing the equivalent V system in the picture dressed by the pump field coupled to the probe transition, we show that this system exhibits two new features. Lasing with inversion occurs in the regime where the Mollow laser is impossible. The output-intensity fluctuation is suppressed up to 64% below the shot noise limit, which is in contrast to the absence of noise squeezing in the V inversionless laser. More importantly, we identify one irreversible pathway of population transfer to be responsible not only for lasing with inversion between bare states but also for quantum noise reduction. Coherent excitation turns out to be a new way in which one can provide the irreversible pathway for quantum noise reduction.

DOI
10.1088/1464-4266/2/5/302

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We study light amplification and quantum noise reduction in a V-type three-level system with two-colour excitation without conventionally irreversible pumping. Employing the equivalent V system in the picture dressed by the pump field coupled to the probe transition, we show that this system exhibits two new features. Lasing with inversion occurs in the regime where the Mollow laser is impossible. The output-intensity fluctuation is suppressed up to 64% below the shot noise limit, which is in contrast to the absence of noise squeezing in the V inversionless laser. More importantly, we identify one irreversible pathway of population transfer to be responsible not only for lasing with inversion between bare states but also for quantum noise reduction. Coherent excitation turns out to be a new way in which one can provide the irreversible pathway for quantum noise reduction.
We present a theory of light amplification in a cavity based on irreversibility of coherent excitation. It is predicted that the irreversibility of coherent excitation leads to a new way of lasing which is different from lasing with and without inversion in the conventional sense. Lasing with dynamically induced inversion is possible in coherently driven systems in which there is no conventional source of irreversible population pumping. This laser displays two interesting and important features: the maximum gain with inversion is substantially greater than gain without inversion, and in spite of the presence of the inversion, the noise intensity is reduced below the shot-noise limit.
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.
We show that continuous-wave, two colour excitation of three-level atoms opens new channels of irreversible population transfer between atomic levels. This process, which we describe as dynamically induced irreversibility, leads to unanticipated phenomena including the inversion of atomic transitions whose upper atomic level has no conventional source of irreversible population pumping. We also study the effect of excitation-field bandwidth on the gain exhibited by driven, three-level, V-type atomic systems. Gain, which may or may not be inversionless in the atomic-state basis, is produced through strong monochromatic driving of one atomic transition and weaker pumping of a coupled transition. It is found that monochromatic pumping frequently yields greater optical gain than incoherent (wide bandwidth) pumping. This is particularly true when system gain can be attributed to hidden inversions among the eigenstates (semi-classical dressed states) of the coupled atom+driving field quantum system. Last, we report the observation of continuous-wave lasing in a strongly driven barium cascade system and interpret its properties in terms of the models presented.
We present a detailed analysis of the noise properties of a potential lasing scheme employing a coherently driven three-level system in the \ensuremath{\Lambda} configuration inside an optical cavity. We derive expressions for the steady-state laser intensity and the width of the intensity fluctuations and show that the output fields exhibit sub-Poissonian statistics for suitable operating conditions. The fundamental mechanism responsible for noise suppression is identified as increased intensity dissipation inside the cavity through emission and absorption by the atoms, which counteracts the intensity fluctuations.
Light amplification without population inversion and absorption with inversion driven by quantum coherent interferences in a closed three level V-type system is studied from the point of view of two color dressed state basis. Approximate analytic time dependent solutions, for coherences and population distribution are derived, and compared with full numerical solutions. Steady state quantities are also calculated, and the conditions under which the system exhibits gain with and without inversion, in the dressed state representation are derived. It is found that for a weak probe laser field light amplification without inversion may occur, for a range of system parameters, in both the bare and the dressed state representations.