Hybrid electron spin resonance and whispering gallery mode resonance spectroscopy of Fe3+in sapphire

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 spectroscopic technique which uses traditional electron spin resonance (ESR) combined with the measurement of a large population of electromagnetic whispering gallery modes. This allows the characterization of the physical parameters of paramagnetic impurity ions in the crystal at low temperatures. We present measurements of two ultrahigh-purity sapphires cooled to 20 mK in temperature, and determine the concentration of Fe 3+ ions and their frequency sensitivity to a dc magnetic field. Our method is different from 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 behavior of the Zeeman splitting. In both crystals, each Zeeman component demonstrates a different g factor.

Spin bath maser in a cryogenically cooled sapphire whispering gallery mode resonator

“…The broadening of the ion transitions, which is measured to have a full width at half maximum of 27 MHz, 14 confirms that there is a strong coupling of the ions to the crystal lattice. In addition, ESR spectroscopy measurements of the crystal under study 15 confirm that Fe 3+ is the only ion present with an electron spin resonance in the frequency range studied in the present case (12 GHz), so no other available energy levels exist for the individual ions.…”

Controlling a whispering-gallery-doublet-mode avoided frequency crossing: Strong coupling between photon bosonic and spin degrees of freedom

“…To experimentally demonstrate coupling between the spin and bosonic degrees of freedom of a photon, a sapphire WGC was cooled to approximately 110 mK. The sapphire is a cylinder of 50 mm diameter × 30mm height, characterised using a transmission method with two loop antennae in a setup described in detail elsewhere 21 . The probing signal is attenuated to reach the level of −60 dBm power incident upon the crystal, and the cavity output signal is amplified by a low noise cryogenic amplifier.…”

Influence of the electron spin resonance saturation on the power sensitivity of cryogenic sapphire resonators

“…The concentration of these residuals should be very low as the turnover temperature imposed by the Mo 3+ ions is not greatly affected. Indeed other measurements show that Cr 3+ and Fe 3+ concentrations are of some tens of ppb [2,26]. The figure 4 shows the temperature dependance of the WGH …”