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Optics and Spectroscopy

Polarization phenomena upon excitation by the Λ-scheme and at an anisotropic population of lower levels

A. G. Petrashen, N. V. Sytenko

Published Jan 1, 2014• 1 citation
Publisher

Abstract

We have investigated the possibility of the formation of dark states and of the change in the sign of the alignment of an ensemble of atoms with the filled ns-shell the state (n + 1)3 S 1 of which is excited by the Λ-scheme from states n 3 P 1 and n 3 P 2, which, in turn, are populated by means of an axially symmetric interaction.
DOI
10.1134/s0030400x14010159

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We know now how alignment and orientation of angular momenta arise in the excited state as a result of optical excitation from the primarily isotropic distribution of the ensemble of angular momenta. This phenomenon reflects, in essence, the anisotropy of absorption probability G (θ,ϕ). Since spatial structures of the type shown in Fig. 2.2 appear in the excited state the same structure must be ‘eaten out’ from the angular momentum distribution in the ground (initial) state. If the gap created is not closed quickly enough, the ensemble of the remaining angular momenta of the absorbing molecules in the ground state must necessarily also be polarized. If, from another point of view, fluorescent decay leads to a considerable population of the ground state level c, (see, e.g., Fig. 1.2), then molecules on this level may exhibit angular momenta polarization, being particularly noticeable for thermally unpopulated, high lying levels. In the present chapter we intend to discuss the conditions necessary for the creation of ground state angular momenta polarization, the possibilities of its experimental observation, and to develop the theoretical description of broad line radiation interaction with molecules further. Angular momenta polarization via depopulation For many years there existed the widespread opinion that in molecules we obtain an anisotropic distribution of angular momenta only in the excited state in the absorption process, whilst in the ground state isotropy is conserved [124]. This conjecture is based essentially on the assumption of the existence of a so-called ‘weak’ excitation. At first sight it may appear, for instance, from Eq. (2.20) that, as is usual in non-linear spectroscopy, such an admission is correct if condition Γ p /Γ ≫ 1 is valid.
We observe coherent population trapping (CPT) in a two-electron atom— 174 Yb—using the transition. CPT is not possible for such a transition according to one-electron theory because the magnetic sublevels form a V-type system, but in a two-electron atom like Yb, the interaction of the electrons transforms the level structure into a Λ -type system, which allows the formation of a dark state and hence the observation of CPT. Since the two levels involved are degenerate, we use a magnetic field to lift the degeneracy. The single fluorescence dip then splits into five dips—the central unshifted one corresponds to coherent population oscillation, while the outer four are due to CPT. The linewidth of the CPT resonance is about 300 kHz and is limited by the natural linewidth of the excited state, which is to be expected because the excited state is involved in the formation of the dark state.