We have experimentally and theoretically investigated the behavior of Na atoms under the combined influence of resonant laser light and a magnetic field. Linearly polarized light was tuned to the (3s)
2S1/2 (F=2)\ensuremath{\rightarrow}(3p)
2P1/2 (F'=1) component of the
D1 line. The intensity of fluorescent light was observed as a function of the laser intensity for various strengths of the magnetic field B. Trapping in the
MF=\ifmmode\pm\else\textpm\fi{}2 levels of the ground state could be observed to be of decreasing importance for an increasing component of B perpendicular to the laser polarization \ensuremath{\epsilon}^. For laser intensities increasing beyond certain critical values, however, the fluorescence decreased and trapping seemed to become again important. For a quantitative explanation of the observed behavior we describe the system by generalized rate equations for the density matrix. We find that stimulated absorption and emission processes effectively inhibit the Larmor precession, thereby enforcing the trapping effect. Precession can only take place at sufficiently low light intensities.
We observe bimodal fluorescence patterns from atoms in a fast atomic beam when the laser excitation occurs in the presence of a magnetic field and the atoms sample only a portion of the laser profile. The behavior is well explained by competition between the local intensity of the laser, which tends to generate a coherent-population-trapping (CPT) dark state in the
J=1 to ${J}^{\ensuremath{'}}=0$ system, and the strength of an applied magnetic field that can frustrate the CPT process. This work is relevant for understanding and optimizing the detection process for clocks or other coherent systems utilizing these transitions and could be applicable to in situ calibration of the laser-atom interaction, such as the strength of the magnetic field or laser intensity at a specific location.
We have experimentally and theoretically investigated the behavior of excited Na atoms under the combined influence of resonant laser light and a weak magnetic field. The use of weak magnetic fields gives another possibility to vary the polarization of the atoms, additional to variation of the polarization of the exciting laser. We calculated in detail the influence of weak magnetic fields on the density matrix describing the excited atoms, using generalized rate equations. We checked the results of our model experimentally, at the Na(3 2 S1/2, Fl=2) →(3 2 P3/2, Fu =3) transition, excited with circularly polarized light. We determined the polarization of the excited atoms, by measuring the fluorescence light at various angles, as a function of both magnetic field strength and laser irradiance. Model and experiment agreed very well. We applied our model to the associative ionization of the excited Na atoms. We measured the ion signal as a function of magnetic field. Analyzing the results, we could determine a specific associative ionization cross section, which is not accessible in our experimental geometry without the use of magnetic fields. Thereby we showed that the application of magnetic fields in polarization dependence studies can provide us with additional information about the process.
We present a general discussion of the techniques of destabilizing dark states in laser-driven atoms with either a magnetic field or modulated laser polarization. We show that the photon-scattering rate is maximized at a particular evolution rate of the dark state. We also find that the atomic-resonance curve is significantly broadened when the evolution rate is far from this optimum value. These results are illustrated with detailed examples of destabilizing dark states in some commonly trapped ions and supported by insights derived from numerical calculations and simple theoretical models.
We report an experimental observation of a dark region, around the zero-field point of a spheric quadrupole magnetostatic field, in spatial fluorescence emitted by dilute vapor atoms, which were excited by a circularly polarized traveling-wave laser tuned to a $\stackrel{\ensuremath{\rightarrow}}{F}F+1$ transition of
87Rb. By ruling out some possible sources, we show that the fluorescence decrease is associated with the three-dimensional nature of the magnetic field; it is a consequence of a precession of magnetic dipole moments induced by the radial magnetic field, which significantly cancels the orientation caused by optical pumping near the point of level crossing in the presence of the axial magnetic field. In an alternative way, the phenomenon can be understood as resulting from a destructive quantum interference, which is related to a coherent double-quantum coupling of atomic levels with two fields; one is the laser field, the other is the magnetic field. Analytic expressions for the half-width and contrast of the dark region have been obtained with a simplified model in the limit of low laser power. A qualitative agreement between the theory and experiment is found. The relevance of our study to laser cooling and trapping in a widely used magneto-optical trap based on such a quadrupole magnetic field is discussed. Finally, some applications of the phenomenon are also presented.
We have investigated the two basic types of Hanle fluorescence spectra, distinguished by the direction of observation, from a V-type atom with a
Jg =0\ensuremath{\leftrightarrows}
Je =1 transition excited by a linearly polarized laser. In the absence of a magnetic field to lift the Zeeman degeneracy of the sublevels, the incoherent fluorescence spectrum
GXinc(\ensuremath{\omega}) is dark for all Rabi frequencies, while the incoherent fluorescence spectrum
GYinc(\ensuremath{\omega}) exhibits a single peak for small Rabi frequencies and a Mollow-like triplet for large Rabi frequencies. However, if a magnetic field is applied, the incoherent spectra are composed, in general, of five peaks. When ${\mathrm{\ensuremath{\omega}}}_{\mathrm{B}}$ \ensuremath{\ll}\ensuremath{\Omega}, the incoherent spectrum
GXinc(\ensuremath{\omega}) has four peaks, while
GYinc(\ensuremath{\omega}) has a Mollow-like triplet, and the integrated area (i.e., the fluorescence intensity) under the spectrum
GXinc(\ensuremath{\omega}) is much less than that of
GYinc(\ensuremath{\omega}). When ${\mathrm{\ensuremath{\omega}}}_{\mathrm{B}}$ \ensuremath{\gg}:\ensuremath{\Omega}, both incoherent spectra have a two-peak structure similar to that of a two-level atom far off resonance, but the integrated area under the spectrum
GXinc(\ensuremath{\omega}) is much greater than that of
GYinc(\ensuremath{\omega}). When ${\mathrm{\ensuremath{\omega}}}_{\mathrm{B}}$ =\ensuremath{\Omega}\ensuremath{\gg}:\ensuremath{\gamma}, both spectra have a similar five-peak structure, and the same integrated area. The results obtained are interpreted in the dressed atomic state representation. In the strong-field limit, the secular approximation is invoked, and analytical expressions of the resonance fluorescence spectra are derived which demonstrate the dependence of the peak heights and widths of the resonance fluorescence spectra on the intensities of the magnetic and linear polarized laser fields in a more transparent way.