Espectroscopia Ãptica nÃo linear em sistemas atÃmicos envolvendo nÃveis de Rydberg

AUTOR(ES)
DATA DE PUBLICAÇÃO

2002

RESUMO

In this thesis we study nonlinear optical effects in alkali vapors, in which Rydberg levels play the role of near resonant intermediate levels, focusing our attention in processes such as: four wave mixing (FWM), two photon absorption, sum frequency generation (SF) and difference frequency generation (DF). We analyze a FWM process with two-photon transitions involving Rydberg levels of rubidium atoms, in which we have observed interference fringes in the nonlinear signal. We show that itâs possible to have a coherent control over the quantum pathways of the atomic system, through the dependence of the nonlinear signal with the polarization of the incident beams. We measure the visibility of the interference fringes, with 100% contrast in the two-photon resonance with a Rydberg level. The experimental results are in good agreement with the theoretical calculations, based on the density matrix formalism applied to a four-level system. The population distribution of a four-level atom was calculated in the stationary regime, by solving the density matrix equations in high orders in the incident fields. We show that an interference between the quantum pathways causes the cancellation or enhancement of the population of an atomic state, depending on the polarizations of the incident beams. We also studied transient effects, solving the time dependent SchrÃdinger equation for a three-level system in the V configuration, and we have predicted the occurrence of two-photon Rabi oscillations, and quantum beats in the populations of the atomic system. Finally, we have measured, for the first time, the difference frequency generation (DF) in a dense sample of rubidium atoms, through a forbidden process in the electric dipole approximation, involving Rydberg levels. Using a dc electric field model, induced by photoionization, as the dominant mechanism of symmetry breaking, we developed theoretical calculations which allowed us to obtain the spatial profile of the generated nonlinear signal and to compare them with existing results for the second harmonic generation in atomic vapors

ASSUNTO(S)

interferÃncia quÃntica fisica absorÃÃo de dois fÃtons mistura de quatro ondas Ãptica nÃo - linear estados de rydberg geraÃÃo de segundo harmÃnico

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