Nonlinear Faraday effect and spin noise in rare-earth activated crystals

TL;DR

This paper explores how nonlinear Faraday effects can enhance spin noise spectroscopy in rare-earth crystals, enabling measurement of optical transition widths and advancing understanding of impurity crystal physics.

Contribution

It introduces a theoretical framework and experimental method to use nonlinear Faraday effect for characterizing inhomogeneous broadening in rare-earth activated crystals.

Findings

Demonstrated giant spin-noise gain in rare-earth crystals

Measured homogeneous widths of optical transitions

Linked nonlinear optics with spin-noise properties

Abstract

The spin-noise spectroscopy (SNS) method implies high efficiency of conversion of the spin-system magnetization to the Faraday rotation angle. Generally, this efficiency cannot be estimated using the characteristics of the regular magneto-optical activity of a paramagnet. However, it may be drastically enhanced in systems with strong inhomogeneous broadening of the optical transitions. This enhancement leads to the giant spin-noise gain effect and previously allowed one to apply the SNS to rare-earth-activated crystals. We show that the nonlinear resonant Faraday effect can be used to measure the homogeneous width of the inhomogeneously broadened transition and, thus, to estimate the applicability of the SNS to this type of paramagnet. We present the theoretical description of the effect and perform measurements on intraconfigurational ($4f$-$4f$) transitions of the trivalent rare-earth ions of neodymium and ytterbium in fluorite-based crystals. The proposed experimental approach establishes new links between the effects of nonlinear optics and spin-noise characteristics of crystals with paramagnetic impurities and offers new ways of research in the physics of impurity crystals.