Hybrid Electron Spin Resonance and Whispering Gallery Mode Resonance Spectroscopy of Fe3+ in Sapphire

TL;DR

This paper introduces a novel spectroscopic technique combining ESR and Whispering Gallery modes to precisely characterize Fe3+ ions in sapphire at millikelvin temperatures, revealing anisotropic Zeeman splitting.

Contribution

It presents a new method for detailed impurity ion characterization in crystals, enabling high-precision measurements of Zeeman effects across a range of magnetic fields.

Findings

Determined Fe3+ concentration in ultra-high purity sapphires.

Measured anisotropic Zeeman splitting and different g-factors.

Demonstrated tracking of ion resonance frequency from zero to arbitrary magnetic fields.

Abstract

The development of a new era of quantum devices requires an understanding of how paramagnetic dopants or impurity spins behave in crystal hosts. Here, we describe a new spectroscopic technique which uses traditional Electron Spin Resonance (ESR) combined with the measurement of a large population of electromagnetic Whispering Gallery (WG) modes. This allows the characterization of the physical parameters of paramagnetic impurity ions in the crystal at low temperatures. We present measurements of two ultra-high purity sapphires cooled to 20 mK in temperature, and determine the concentration of Fe3+ ions and their frequency sensitivity to a DC magnetic field. Our method is different to ESR in that it is possible to track the resonant frequency of the ion from zero applied magnetic field to any arbitrary value, allowing excellent measurement precision. This high precision reveals anisotropic behaviour of the Zeeman splitting, which has not been previously reported. In both crystals, each Zeeman component demonstrates a different g-factor.