Bright and dark nonlinear magneto-optical resonances associated with the ground state Hanle effect have been studied experimentally and theoretically for
D1 excitation of atomic cesium. This system offers the advantage that the separation between the different hyperfine levels exceeds the Doppler width, and hence transitions between individual levels can be studied separately. At the same time, the system retains the advantages offered by ordinary glass cells, including simplicity and subnatural width Hanle resonances. Experimental measurements for various laser power densities and transit relaxation times are compared with a model based on the optical Bloch equations, which averages over the Doppler contour of the absorption line and simultaneously takes into account all hyperfine levels, as well as mixing of magnetic sublevels in an external magnetic field. In contrast to previous studies, which could not resolve the hyperfine transitions because of Doppler broadening, in this study there is excellent agreement between experiment and theory regarding the sign (bright or dark), contrast and width of the resonance. The results support the traditional theoretical interpretation, according to which these effects are related to the relative strengths of transition probabilities between different magnetic sublevels in a given hyperfine transition.
Experimental signals of nonlinear magneto-optical resonances at
D1 excitation of natural rubidium in a vapor cell have been obtained and described with experimental accuracy by a detailed theoretical model based on the optical Bloch equations. The
D1 transition of rubidium is a challenging system to analyze theoretically because it contains transitions that are only partially resolved under Doppler broadening. The theoretical model took into account all nearby transitions, the coherence properties of the exciting laser radiation, and the mixing of magnetic sublevels in an external magnetic field and also included averaging over the Doppler profile. The experimental signals were reproduced very well at each hyperfine transition and over a wide range of laser power densities, beam diameters, and laser detunings from the exact transition frequency. The bright resonance expected at the ${F}_{g}=1\ensuremath{\rightarrow}{F}_{e}=2$ transition of
87Rb has been observed. A bright resonance was observed at the ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=3$ transition of
85Rb, but displaced from the exact position of the transition due to the influence of the nearby ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=2$ transition, which is a dark resonance whose contrast is almost 2 orders of magnitude larger than the contrast of the bright resonance at the ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=3$ transition. Even in this very delicate situation, the theoretical model described in detail the experimental signals at different laser detunings.
Nonlinear magneto-optical resonances on the hyperfine transitions belonging to the
D2 line of rubidium were changed from bright to dark resonances by changing the laser power density of the single exciting laser field or by changing the vapor temperature in the cell. In one set of experiments atoms were excited by linearly polarized light from an extended cavity diode laser with polarization vector perpendicular to the light's propagation direction and magnetic field, and laser-induced fluorescence was observed along the direction of the magnetic field, which was scanned. A low-contrast bright resonance was observed at low laser power densities when the laser was tuned to the ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=3$ transition of
87Rb and near to the ${F}_{g}=3\ensuremath{\rightarrow}{F}_{e}=4$ transition of
85Rb. The bright resonance became dark as the laser power density was increased above 0.6 mW/cm
2 or 0.8 mW/cm
2, respectively. When the ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=3$ transition of
87Rb was excited with circularly polarized light in a second set of experiments, a bright resonance was observed, which became dark when the temperature was increased to around 50${}^{\phantom{\rule{0.16em}{0ex}}\ensuremath{\circ}}$C. The experimental observations at room temperature could be reproduced with good agreement by calculations based on a theoretical model, although the theoretical model was not able to describe measurements at elevated temperatures, where reabsorption was thought to play a decisive role. The model was derived from the optical Bloch equations and included all nearby hyperfine components, averaging over the Doppler profile, mixing of magnetic sublevels in the external magnetic field, and a treatment of the coherence properties of the exciting radiation field.
We have measured magneto-optical signals obtained by exciting the
D1 line of cesium atoms confined to an extremely thin cell (ETC), whose walls are separated by less than $1\ensuremath{\mu}\text{m}$, and developed an improved theoretical model to describe these signals with experimental precision. The theoretical model was based on the optical Bloch equations and included all neighboring hyperfine transitions, the mixing of the magnetic sublevels in an external magnetic field, and the Doppler effect, as in previous studies. However, in order to model the extreme conditions in the ETC more realistically, the model was extended to include a unified treatment of transit relaxation and wall collisions with relaxation rates that were obtained directly from the thermal velocities of the atoms and the length scales involved. Furthermore, the interactions of the atoms with different regions of the laser beam were modeled separately to account for the varying laser beam intensity over the beam profile as well as saturation effects that become important near the center of the beam at the relatively high laser intensities used during the experiments in order to obtain measurable signals. The model described the experimentally measured signals for laser intensities for magnetic fields up to 55 G and laser intensities up to
1W/cm2 with excellent agreement.
Abstract\nThe main subject of this thesis is theoretical modeling of bright and dark\nmagneto-optical resonances observable on the D1 spectral line of alkali metals.\nA theoretical model based on the rate equations for Zeeman coherences is em-\nployed in the research. The theoretical model was developed from the optical\nBloch equations, applying the rotating wave approximation and the decorrela-\ntion approach to stochastic dierential equations.\nIn this thesis the theoretical model is adapted to describe magneto-optical\nresonances observed in alkali metal vapor that is conned in either an ordinary\ncm-sized cell or an extremely thin cell, whose thickness is comparable to the ex-\ncitation wavelength. A method for estimating the parameters of the theoretical\nmodel for either of the experimental conditions is developed in the paper.\nThe adapted theoretical model is tested by comparing its results to the ex-\nperimentally registered data. The experimental results are obtained by exciting\natoms with laser radiation and observing the dependence of the \nuorescence sig-\nnal on a magnetic eld. Atomic vapor of 85Rb, 87Rb, and Cs is used. Rubidium\natoms are studied in both types of cells, but Cesium atoms only in ordinary\ncells. Altogether the results of the theoretical model describe the experimental\nresults well.\nAside from describing experiments, the theoretical model is employed as an\nindependent tool to study the saturation of the contrast of the dark magneto-\noptical resonances and the formation of the dark states.
Abstract The absorption of a light wave interacting with optical transitions in the D1 line of an alkali metal atom subjected to microwave radiation that is in resonance with magnetic dipole transitions between hyperfine ground-state components, has been investigated. It is known that when scanning a longitudinal magnetic field (B || k, where k is the wavevector), one may observe a magneto-optical resonance due to the ground-state Hanle effect. In addition, the effect of double radio-optical resonance takes place because of the presence of the resonance microwave field. The joint influence of these effects on the formation of a narrow magneto-optical resonance in light wave absorption has been studied theoretically and experimentally. It has been shown analytically that the effects compete with each other and destructively act on the resonance formation. As a result, the amplitude of the resonance is small and its shape is complicated. However, in the presence of a buffer gas the pressure of which is such that the hyperfine splitting of the ground state remains spectrally unresolved, it becomes possible to observe a magneto-optical resonance with a relatively large amplitude. Experiments have been carried out with the use of a miniature glass cell (V ~ 0.1 cm3) filled with 87Rb vapor and a buffer gas argon (a pressure of about 95 Torr). In particular, the theoretically predicted resonance narrowing with increasing light field intensity has been experimentally observed. A configuration for magneto-optical resonance excitation suggested here may be applied in quantum magnetometry to measure weak permanent magnetic fields and resonance microwave fields using cells filled with alkali metal vapor.