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Physical Review Letters

Dynamically induced irreversibility in coherently driven systems

Krishna K. Meduri, Geoffrey A. Wilson, Peter B. Sellin, THOMAS W. MOSSBERG

Published Dec 27, 1993• 36 citations
Publisher

Abstract

We show that continuous-wave, two color excitation of three-level atoms opens new channels of irreversible population transfer between atomic levels. We point out that dynamically induced irreversibility leads to unanticipated phenomena such as the inversion of atomic transitions in cases where the upper atomic level has no conventional source of irreversible population pumping. The widely held view that coherent excitation is reversible is called into question.

DOI
10.1103/physrevlett.71.4311

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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.

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We show that continuous-wave, two color excitation of three-level atoms opens new channels of irreversible population transfer between atomic levels. We point out that dynamically induced irreversibility leads to unanticipated phenomena such as the inversion of atomic transitions in cases where the upper atomic level has no conventional source of irreversible population pumping. The widely held view that coherent excitation is reversible is called into question.
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 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.
Effects of detunings on dynamically induced irreversibility is studied for coherently driven V systems in which there is no conventional source of irreversible population pumping. For atomic barium (γ 1 /γ 2 = 400 >> 1, where γ 1 and γ 2 are the rates of the spontaneous decay from the excited states 6 s 6 p 1 P 1 and 6 s 6 p 3 P 1 to the ground state 6 s 2 1 S 0 , respectively), the strong irreversibility is found to lead to a maximum inversion of 0.77 [only 0.1 in Phys. Rev. Lett. 71 (1993) 4311 ]. The maximum population inversion requires relatively strong fields coupled respectively to two transitions, a disparity in two atomic decay rates, and the atom-field detunings of opposite signs. However, it is also shown that even in the cases where two detunings have the same sign, or where two decay rates are equal, population inversion takes place.
We analyse the physical origin of population inversion via continuous wave two-colour coherent excitation in three-level systems by dressing the inverted transition. Two different mechanisms are identified as being responsible for the population inversion. For V -configured systems and cascade (Ξ) configured systems with inversion on the lower transition, the responsible mechanism is the selective trapping of dressed states, and the population inversion approaches the ideal value of 1. For Λ-configured systems and Ξ-configured systems with inversion on the upper transition, population inversion is based on the selective excitation of dressed states, with the population inversion tending towards 0.5. As the essential difference between these two mechanisms, the selective trapping of dressed states occurs in systems with strong decay into dressed states while the selective excitation appears in systems with strong decay out of dressed states.
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 consider the problem of amplification without inversion in a three-level atomic system. When atoms are driven by a field reservoir and a low-frequency coherent field, amplification of a high-frequency probe field may occur even when there is no population inversion on any atomic transition. We prove that, depending on the ratio of the probe and coherent drive field frequencies, which we term the frequency up-conversion factor, this process may or may not require hidden inversion in the field reservoir. In the special case of a frequency up-conversion factor less than 2, we demonstrate that amplification is possible even if the field reservoir is in complete thermal equilibrium: this represents a new kind of coherent radiation source, distinct from both conventional lasers and lasers without inversion.