Birefringence and dichroism effects in the spin noise spectra of a spin-1 system

Abstract

We perform spin noise spectroscopy experiments in metastable helium atoms at room temperature, with a probe light whose frequency is blue detuned from the D0 line. Both circular birefringence fluctuations (Faraday noise) and linear birefringence fluctuations (ellipticity noise) are explored theoretically and experimentally. In particular, it is shown that in both cases but for different optical detunings, two noise resonances are isolated at the Larmor frequency and at twice the Larmor frequency with a behaviour, which strongly depends on the orientation of the probe field polarization. The simple structure of metastable helium allows us to probe, model and explain the changes in the behavior of these peaks in terms of circular and linear dichroisms and birefringences as well as in terms of spin oscillation modes.

Creation and dynamics of spin fluctuations in a noisy magnetic field

“…In spin 1/2 systems, spin fluctuations are associated to circular birefringence noise, and thus to stochastic FR of the light polarization [16,34]. Spin-1 systems such as metastable Helium exhibit richer dynamics due to higher-order tensor spin degrees of freedom, resulting in extra spectral features such as resonances at twice the Larmor frequency [11,32].…”

“…We focus here on the situation where the light probes spin noise near a J = 1 → J = 0 transition, which corresponds to the experiments presented in figure 2. The general model for simulating the time evolution of the density matrix ρ(t) of the open atomic system is detailed in [11]. We focus in the following on the creation of a measurable spin noise signal due to the magnetic fluctuations solely, that is, without taking any other source of stochastic fluctuations of the density matrix elements into account, with noise levels comparable with the one observed experimentally where transit noise occurs.…”

“…, 0]. Contrary to simulations performed in [11], we do not consider the fluctuations of the populations of the Zeeman sublevels, which are responsible for the standard spin noise. Since we focus on the effect of the fluctuating magnetic field, we just consider an average transit rate: the vector η is constant, so that is does not by itself give rise to population imbalance.…”

“…Such experiments were conducted in condensed structures to probe electron-nuclei spin interactions [3], confinement effects [4] or electron-hole coupling [5]. In thermal vapors, relaxation processes such as spin exchange in a single specie [6,7] or two-species atomic samples [8,9], as well as binary collisions [10] or simply atomic motion [11] were studied.…”

“…The last section discusses higher-order tensor spin noise, probed as ellipticity noise in a standard SNS setup. Such a noise has already been studied as a consequence of transit noise in high spin systems [11,33]. After having studied the creation of circular birefringence noise, we naturally further investigate the possible existence of linear birefringence fluctuations induced by the fluctuating magnetic field.…”