Spin noise in birefringent and inhomogeneous media

TL;DR

This paper demonstrates that spin noise spectroscopy can effectively measure spin fluctuations in birefringent and inhomogeneous media, overcoming previous limitations posed by optical anisotropy.

Contribution

It provides both theoretical and experimental evidence that strong optical anisotropy does not hinder spin noise measurements, expanding the applicability of SNS to new materials.

Findings

Spin noise measurement is unaffected by strong birefringence.

Birefringent media allow measurement of spatial spin correlations.

Experimental results on Nd$^{3+}$ ions agree with theoretical predictions.

Abstract

It is known that linear birefringence of the medium essentially hinders measuring the Faraday effect. For this reason, optically anisotropic materials have never been considered as objects of the Faraday-rotation-based spin noise spectroscopy (SNS). We show, both theoretically and experimentally, that strong optical anisotropy that may badly suppress the regular Faraday rotation of the medium, practically does not affect the measurement of the spatially uncorrelated spin fluctuations. An important consequence of this result is that the Faraday-rotation noise should be also insensitive to spatially nonuniform birefringence, which makes the SNS applicable to a wide class of optically anisotropic and inhomogeneous materials. We also show that the birefringent media provide additional opportunity to measure spatial spin correlations. Results of the experimental measurements of the spin-noise spectra performed on 4$f$--4$f$ transitions of Nd$^{3+}$ ions in the CaWO$_4$ and LiYF$_4$ crystals well agree with the theory.