Homogenization of Doppler broadening in spin-noise spectroscopy

Abstract

The spin noise spectroscopy, being a nonperturbative linear optics tool, is still reputed to reveal a number of capabilities specific to nonlinear optics techniques. The new effect of the Doppler broadening homogenization discovered in this work essentially widens these unique properties of spin noise spectroscopy. We investigate spin noise of a classical system-cesium atoms vapor with admixture of buffer gas-by measuring the spininduced Faraday rotation fluctuations in the region of D2 line. The line, under our experimental conditions, is strongly inhomogeneously broadened due to the Doppler effect. Despite that, optical spectrum of the spin noise power has the shape typical for the homogeneously broadened line with a dip at the line center. This fact is in stark contrast with the results of previous studies of inhomogeneous quantum dot ensembles and Doppler broadened atomic systems. In addition, the two-color spin-noise measurements have shown, in a highly spectacular way, that fluctuations of the Faraday rotation within the line are either correlated or anticorrelated depending on whether the two wavelengths lie on the same side, or on different sides of the resonance. The experimental data are interpreted in the frame of the developed theoretical model which takes into account both kinetics and spin dynamics of Cs atoms. It is shown that the unexpected behavior of the Faraday rotation noise spectra and effective homogenization of the optical transition in the spin-noise measurements are related to smallness of the momentum relaxation time of the atoms as compared with their spin relaxation time. Our findings demonstrate novel abilities of spin noise spectroscopy for studying dynamic properties of inhomogeneously broadened ensembles of randomly moving spins.

Spin-alignment noise in atomic vapor

“…Specifically, the SNS made it possible to observe resonant magnetic susceptibility of nanosystem (quantum wells, quantum dots), inaccessible for the conventional EPR spectroscopy [10,11], to examine magnetization dynamics of nuclei [12,13] and to investigate nonlinear phenomena in such systems [14][15][16][17]. The SNS technique also allows one to distinguish between homogeneously and inhomogeneously broadened lines of optical transitions [18,19] or to measure homogeneous linewidth in an inhomogeneously broadened system using multiprobe noise technique [20]; intensity modulation of the probe beam made it possible to expand the frequency range of signals detected in SNS up to microwave frequencies [21,22]. With the use of tightly focused light beams, the SNS allows one to investigate noise signals with high spatial resolution and even to realize tomography of magnetic properties of bulk materials [23].…”

“…In our system, this could be related to the fact that the collisions between the buffer gas and Cs atoms result in the efficient relaxation of the alignment while the spin polarization is conserved [27]. Also, the effects of Doppler broadening could be different for the ellipticity and Faraday rotation noise [19].…”

Nonlinear spectroscopy of high-spin fluctuations

“…Here we demonstrate that the renormalization of the spin S > 1 /2 states by the electromagnetic field causes a number of nonlinear effects, which include: the light-induced fine splittings of the spin states, induced anisotropy of the spin system, i. e., the dependence of the spin noise spectra on the orientation of the probe beam with respect to the magnetic field, and appearance of high harmonics of the Larmor precession frequency in the spin noise. We illustrate experimentally theoretical predictions taking Cs atomic vapour as testbed, because in this case the multiplet corresponding to the total spin S = 3, 4 is easily accessible by the spin noise spectroscopy [31,32]. The results presented in the work can open up a way for system sublevels optical control, which can be executed by combination with radiofrequency addressing of the states [33] or 'active' spin noise spectroscopy [34].…”

Raman scattering model of the spin noise

“…and low-dimensional media (e. g. [16,17]) can be addressed by SNS, but also the nuclear dynamics [18][19][20], light-matter interaction effects [21,22], spatial properties distribution [23,24], and even noise of valley redistribution of electrons [25]. Still the atomic systems attract the significant attention of researchers [26][27][28][29][30][31][32][33] as model objects which allow to reveal the fundamental pecularities of spin noise formation mechanics. In typical SNS experiments with polarimetric detection of the signal, one measures, in fact, the magnetization-noise power spectrum of the studied sample associated, in accordance with the fluctuation-dissipation theorem, with the spectrum of its magnetic susceptibility [34].…”