Redistribuição de frequência e efeitos de colisões na fluorescência de um vapor atômico

Abstract

In this work we demonstrate experimentally and numerically the effects of partial frequency distribution on the transport of light in an atomic vapor. This transport is described by a random travel of photons in the vapor due to the successive spontaneous absorption and emission events, a phenomenon known as radiation trapping. The frequency of the emitted photon in the laboratory may differ from the absorbed frequency due to the Doppler effect following a scattering event. The emitted spectrum depends on the incident frequency, the direction of emission, and the probability that the excited atom will undergo a collision before emission. Our interest is in studying these effects arising from frequency redistribution. To do so, we excite the atomic vapor with a laser beam of wavelength 894 nm resonant with the 6S1/2 →6P1/2 cesium transition (line D1), with scanning around the 6S1/2 (F = 3)→ 6P1/2(F’= 3, 4) transitions. Thus, we measured the fluorescence in a direction perpendicular to the laser excitation for different atomic densities. Therefore, we analyzed the experimental measurements of fluorescence as a function of detuning. The experimental results show that the fluorescence maxima occur around the center of the line at low density. At high density and high detuning, the fluorescence increases. In the center of the line, the fluorescence decreases, which means that the photon emitted around the center of the line "sees" a high optical density and has a low probability of scattering. We also analyzed the measurements of fluorescence, eliminating the influence of the spacial excitation distribution, i.e., we compared the signals detected for different densities and detunings for which the absorption coefficient is constant α ≈ 1/L. For this value of α, the excitation is uniformly distributed over the cell. This results in an approximately constant behavior far from resonance, which is related to the scattering of a single photon for which the emitted frequency is equal to the incident one. Accordingly, we use a numerical simulation to interpret the experimental data. The numerical data show good agreement with the experimental data in the region of interest (far from resonance).