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.PACS numbers: 32.80. Wr, 42.50.Hz Transitions between atomic states can arise in two ways: They can be stimulated by an external perturbation such as a laser field, or they can occur "spontaneously" as a consequence of the atom's coupling to the vacuum, Transitions stimulated by an electromagnetic field are reversible; that is, the probabilities of photon absorption and emission are equal. Spontaneous transitions, on the other hand, are irreversible, since they occur only from states of higher energy to states of lower energy.The reversibility of stimulated transitions has important and well-known consequences, such as the impossibility of achieving steady-state population inversion in a driven two-level atom [1] [see Fig. 1(a)]. Optically sustained inversions involving the ground state can, however, be produced in multilevel atomic systems where population transfer channels mediated by irreversible spontaneous decay come into play. A three-level "V" system exhibiting spontaneous emission on all transitions is shown in Fig. 1(b). For certain ratios of spontaneous emission rates, a laser field applied to the 1-3 transition will create a steady-state population inversion on the 2-3 transition [2]. This inversion is made possible through the presence of the irreversible spontaneous decay channel that feeds population from level 1 into level 2. We are interested in three-level "V" systems that have no spontaneous emission coupling between the excited states [see Fig. 1 (c)] and hence have no intrinsic irreversible process whereby either of these states can be populated. We will show that two laser fields, each nearly but not quite resonant with one of the two allowed atomic transitions, act together to create new dynamically irreversible pathways of population transfer that can lead to population inversion relative to the ground state. These processes arise in single atoms unperturbed by incoherent processes such as collisions. Inasmuch as the irreversible processes discussed here, and their generalizations, may lead to the appearance of population inversion in unconventional situations, these processes must be carefully accounted for in the analysis of systems believed to display lasing without inversion [3]. We note that other means of controlling population transfer to excited states have recently been explored and include temporally sequenced pulses [4], colored vacuum [5], and coherent trapping [6].We first define the system of concern and outline our means of analysis. Consider a closed, V-type, three-level atomic system as shown in Fig. 2. Two stron...
We measured the optical decoherence times T2, or, equivalently, the homogeneous line width, in an Er-doped optical fiber at low temperature as a function of external magnetic field and temperature using two-pulse photon echoes. The decoherence times were up to 230 ns at fields above 3 T. The magnitude of the line narrowing induced by a magnetic field of 3 T is 2.5 MHz, which is anomalously large compared to that typical for oxide crystals with similar Er3+ concentration. This is interpreted as evidence for dephasing by coupled spin-elastic tunneling modes where the normal glass tunneling modes acquire a magnetic character by coupling to the Er3+ spin.
We report what is believed to be the first demonstration of laser frequency stabilization directly to persistent spectral holes in a solid-state material. The frequency reference material was deuterated CaF(2): Tm(3+) prepared with 25-MHz-wide persistent spectral holes on the H(6)(3)?H(4)(3) transition at 798 nm. The beat frequency between two lasers that were independently locked to persistent spectral holes in separate crystal samples showed typical root Allan variances of 780+/-120Hz for 20-50-ms integration times.
Laser frequency stabilization giving a 500-Hz Allan deviation for a 2-ms integration time with drift reduced to 7 kHz/min over several minutes was achieved at 1536 nm in the optical communication band. A continuously regenerated spectral hole in the inhomogeneously broadened 4 I 15/2 (1)→ 4 I 13/2 (1) optical absorption of an Er 3ϩ :Y 2 SiO 5 crystal was used as the short-term frequency reference, while a variation on the locking technique allowed simultaneous use of the inhomogeneously broadened absorption line as a long-term reference. The reported frequency stability was achieved without vibration isolation. Spectral hole burning frequency stabilization provides ideal laser sources for high-resolution spectroscopy, real-time optical signal processing, and a range of applications requiring ultra-narrow-band light sources or coherent detection; the time scale for stability and the compatibility with spectral hole burning devices make this technique complementary to other frequency references for laser stabilization.
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