Medida de propriedades ópticas não lineares usando Z-scan e termalização do estado excitado em vapores de Césio

Abstract

In this work we approach two themes related to laser eld interaction with atomic vapors, thermalization of the excited state by radiation trapping and non-linear optical properties measurements using z-scan. In the rst topic we will study a two-photon absorption process in which a pump laser excites 6S1{2 Ñ 6P1{2 transition of cesium vapor and a probe laser excites 6P1{2 Ñ 9S1{2 transition. The pump laser produces an atom density in the rst excited state with velocity component given by the frequency of light. However, even at low densities, where collision mechanisms can be neglected, radiation trapping causes the thermalization of the rst excited state. This work presents a theoretical model in which velocity selection can be observed for small pump beam detunings and that there is no velocity selection for large detunings. In this model, excited state thermalization mechanisms are not considered. These mechanisms are discussed based on the experimental results also in this work. In the second topic, where we study nonlinear e ects, we write the refractive index of cesium vapor as n n0 􀀀 n2I 􀀀 n4I2 􀀀 :::, with n0 the linear refraction index, n2 the Kerr coe cient, n4 a combined contribution of the third and fth orders of susceptibility and I the beam intensity. The dependence of the refractive index on the laser intensity is responsible for the self-focusing and self-defocusing phenomena of the beam. Based on this phenomenon, the z-scan technique was developed to measure the Kerr coe cient. We measured the refractive index for the line D1 (6S1{2 Ñ 6P1{2) of cesium, using a variant of the z-scan technique, where we use the fact the vapor is a saturable medium to obtain both the values of n2 and those of n4. Ours measurements of n2 and n4 are in accordance with the theoretical model that we propose for a large frequency range of laser detuning. With this variant of the z-scan technique, applicable in resonant media, we were able to measure variantions of n2 and n4 of about two orders of magnitude for a wide range of frequencies, corresponding to the fundamental structure hyper ne in the D1 transition from the cesium vapor.