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Journal of Physics B Atomic and Molecular Physics

Two-photon Hanle effect

W. Jastrzȩbski, M. Kolwas

Published Dec 28, 1984• 5 citations
Publisher

Abstract

Two-photon Hanle resonances were observed in a three-level atomic system interacting with two resonant light fields. The choice of suitable polarisation of the light beams and shape analysis of the resonances enabled one to distinguish the effects connected with two-photon coherent transitions from the effects due to stepwise transitions. The experiment was performed with Ne atoms simultaneously interacting with 632.8 nm and 3392 nm radiations from two He-Ne lasers. The mutual coupling of these lasers made it possible to observe the influence of the mutual correlation of both light fields on the two-photon Hanle effect.

DOI
10.1088/0022-3700/17/24/004

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Two-photon Hanle resonances were observed in a three-level atomic system interacting with two resonant light fields. The choice of suitable polarisation of the light beams and shape analysis of the resonances enabled one to distinguish the effects connected with two-photon coherent transitions from the effects due to stepwise transitions. The experiment was performed with Ne atoms simultaneously interacting with 632.8 nm and 3392 nm radiations from two He-Ne lasers. The mutual coupling of these lasers made it possible to observe the influence of the mutual correlation of both light fields on the two-photon Hanle effect.
The optical Hanle effect (light-shift-induced zero-field level crossing) has been investigated theoretically and experimentally. In the case of a $J=0\ensuremath{\rightarrow}1\ensuremath{\rightarrow}0$ three-level system and of a $J=0\ensuremath{\rightarrow}1$ two-level system, the resonance line shape has been obtained using a nonlinear-response-function method. Approximate expressions, valid under the conditions for the observation of light shifts, are obtained and compared with exact calculations as well as with the corresponding expression for the magnetic Hanle effect. As concerns the weak laser beam which produces the coherent excitation of atomic substates, we have calculated the linear response in the limiting case of monochromatic and of broad-band excitation. The experimental investigation has been performed for the Ba resonance line using two cw dye lasers at $\ensuremath{\lambda}=5535$ \AA{} and an atomic beam. The absorption- and dispersion-shaped level crossing resonances have been observed, for narrow- and broad-band excitation, by varying the power of the strong nonresonant laser beam. We have checked that the line shapes are only dependent upon the reduced variable $\frac{{P}_{L}}{\ensuremath{\delta}}$ (PL{P}_{L}PL​; beam power; $\ensuremath{\delta}$: frequency detuning). Using the theoretical expressions, we have tried to fit the experimental curves. Provided that the Gaussian distribution of the beams is taken into account, we have obtained a good agreement between theory and experiment for the whole set of experimental data.
The optical-pumping cycle in the three-level system e,g,f (J e =1=J f , J g =0), irradiated by two monochromatic sigma -polarized laser light waves is analysed theoretically by the successive approximation (iteration) method up to fourth order. The solutions of the density-matrix evolution equations describe (i) in second order, the influence of the Zeeman structure of the e and f states on the saturation resonances in the intermediate level g (second-order population effect) and (ii) in fourth order, the influence of the Hanle effects and the saturation phenomena on the shape of the double-quantum Hanle resonances and on the two-photon transition probabilities.
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The authors investigate resonant two-step laser excitation in a cascade three-level system using an atomic-uranium beam. Both single-photon and two-photon processes are studied by varying the time delay between the two different excitation pulses. The experimental data are compared to the results of simple theoretical models. The calculations include the effects of the magnetic sublevels. The major theoretical prediction of a two-photon resonance is verified experimentally by using simultaneous irradiation from two lasers. The agreement between theory and experiment is found to be satisfactory.
We have investigated atomic coherence effects in the ladder scheme of the triplet transitions 2S-2P-3S of helium. A powerful dressing laser at 706 nm in resonance with the 2P-3S transition induces a transparency on the 2S-2P transition at 1083 nm probed by a weak laser. We have characterised this induced transparency also known as Electromagnetically Induced Transparency (EIT), demonstrating the possibility of reaching 100% of bleaching. Also, we have evidenced the presence of the two photon effect on the EIT process by studying its dependence on the mutual probe-dress polarisation angle. With such a simple level scheme a quantitative comparison with the theory has been made possible and many of the predicted effects have been investigated. Furthermore, polarisation of the dressing laser field has allowed for the first time the study of the electromagnetically induced birefringence which causes a change in the probe laser polarization. The experiments were performed not only in the clean environment of a metastable atomic beam but also in a weak radiofrequency discharge; this configuration could be suggestive for non inverted gain phenomena through incoherent population of excited levels.
The two-photon absorption by a pair of two-level atoms interacting with an incident field is examined. If the laser field is weak, then the existence of the dipole-dipole interaction-induced two-photon resonance follows from the higher-order Fermi golden rule. The effects of spontaneous emission and intense fields on the dipole-dipole interaction-induced two-photon resonance is studied using density-matrix methods. The presence of an intense field is shown to result in a significant enhancement of the two-photon resonance. Numerical results for the probability of simultaneous excitation of two atoms, for the atomic correlations, and for the radiated field intensity are presented.