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EPJ Web of Conferences

The Relevance of Hanle Effect on Na and Fe Lidars

C. Y. She

Published Jan 1, 2020• 0 citations
Publisher

Abstract

A laser resonant scattering process involves two steps, excitation and emission. That emission occurs spontaneously is well accepted. That the atoms involved in the emission are excited coherently by a laser beam leading to a non-isotropic angular distribution of emission (an antenna pattern) is not well known. The difference between coherent and incoherent excitation leads to the Hanle effect. In this paper, I discuss the physics of Hanle effect, and its influences on the backward scattering intensity of Na, K, and Fe atomic transitions and the associated Na and Fe resonant fluorescence lidar systems.

DOI
10.1051/epjconf/202023704008

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A laser resonant scattering process involves two steps, excitation and emission. That emission occurs spontaneously is well accepted. That the atoms involved in the emission are excited coherently by a laser beam leading to a non-isotropic angular distribution of emission (an antenna pattern) is not well known. The difference between coherent and incoherent excitation leads to the Hanle effect. In this paper, I discuss the physics of Hanle effect, and its influences on the backward scattering intensity of Na, K, and Fe atomic transitions and the associated Na and Fe resonant fluorescence lidar systems.
The problem of light scattering from a monochromatic laser beam by an ensemble of atmospheric atoms or molecules in thermal equilibrium under ambient wind and temperature is revisited with two emphases. First, the essence of and difference between laser Cabannes scattering (CS) and laser-induced resonance fluorescence (LIF), which permit atmospheric temperature and wind measurements, are simply presented. A classical model is shown to yield the same angular distribution in scattered power for both, a factor of two larger Doppler shift for the former (CS), in the backward direction with the same line-of-sight (LOS) wind, and a dramatic 17 order larger backscattering cross section for the latter (LIF). Second, the LIF process is developed from first principles, and we present the relations between absorption cross section, differential scattering cross sections, and LIF emission spectrum for atoms with hyperfine structure. By doing this, we (1) equate the decoherence rate among the mixed excited substates to half of the Einstein coefficient to comply with energy conservation, and (2) present a clear physical insight into the origin of the Hanle effect, whose quantitative evaluation for these atoms requires quantum mechanical treatment. We then derive LIF emission rates for each of the eight and 10 pathways associated with the D1 and D2 transitions, and deduce the temperature and LOS wind dependence in differential backscattering cross sections of atmospheric sodium and potassium atoms. Using these cross sections allows retrieval of atmospheric parameters in the mesopause region (80–110 km in altitude) by a metal resonance lidar.
In Chapter 3, we move to a semiclassical treatment (quantum theory) of light absorption and scattering, specifically from atoms. We start with a description of how lidar measures Doppler shift, and the fundamental difference between the measurement when the laser is in resonance with an atomic transition (resonant) and when it is not (nonresonant). We follow with a treatment of quantum polarizability and the resulting absorption cross section, leading to the differential resonance scattering cross section and its contrast with the classical result. After quantum polarizability, we demonstrate the radiation pattern of coherently excited atoms. This takes us to an interpretation of the Hanle Effect. Following these descriptions of the phenomena that impact resonance lidar, we extend our understanding by closing the chapter with an overview of the rudimentary physics of sodium laser guide stars.
We analyze the Hanle effect on the closed Fg equals 3 yields Fe equals 4 D2-line transition of 85Rb, by both numerical solutions of the optical Bloch equations and by experimental investigations on cell containing rubidium vapor. For laser radiation with linear polarization orthogonal to the applied magnetic field, the Hanle signal is characterized by narrow bright resonances at low laser intensities. A bright resonance Hanle signal is obtained also exciting the rubidium atoms by circularly polarized laser light and scanning a transversely applied magnetic field, at different values of an applied longitudinal magnetic field. We report experimental and numerical evidence of this new bright resonance.
We analyse the Hanle effect on a closed F g → F e = F g + 1 transition. Two configurations are examined, for linearly and circularly polarized laser radiation, with the applied magnetic field collinear to the laser light wavevector. We describe the peculiarities of the Hanle signal for linearly polarized laser excitation, characterized by narrow bright resonances at low laser intensities. The mechanism behind this effect is identified, and numerical solutions for the optical Bloch equations are presented for different transitions.
We present an experimental and theoretical analysis of the mechanical Hanle effect, i.e., the modification of the atomic trajectories following the interaction with laser fields in the Hanle configuration. The case of open transitions is examined. In our experiment a thermal sodium atomic beam is crossed perpendicularly by a single-mode linearly polarized laser beam. The laser field is tuned to resonance with an F g F e transition of the sodium D 1 line. A static magnetic field B is applied collinear to the laser light propagation direction. The atomic beam flux at the position corresponding to its maximum in the absence of laser light is studied as a function of the applied magnetic field. For the F g = F F e = F , F -1 transitions the atomic flux shows a sharp increase at B = 0, as a result of the coherent trapping of the population into the ground state. For the F g = 1 F e = 2 transition this effect is absent and no significant variation of the atomic flux with the magnetic field is observed. A theoretical description of the mechanical Hanle effect in open transitions is given and it is clarified why in our experiment the mechanical Hanle/CPT-resonances associated to different hyperfine transitions have different contrasts.