Effects of electron spin resonance in a cryogenic sapphire whispering gallery mode maser

Daniel Creedon

    Research output: ThesisDoctoral Thesis

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    A novel type of solid-state frequency standard is presented based on maser oscillation in a cylindrical sapphire monocrystal. A dilute concentration of residual Fe3+ ions in the crystal have an Electron Spin Resonance (ESR) associated with them at zero applied magnetic eld. A 3-level system is observed corresponding to the electron spin 1/2>, 3/2>, and 5/2> states of the active ion. When a pump frequency of 31.3 GHz is applied at su cient power, population inversion is achieved and maser oscillation is measured at the `signal' frequency of 12.037 GHz, corresponding to a radiative transition between the lower two states. The pump and signal modes of the maser are enhanced by extremely high Q-factor Whispering Gallery (WG) modes (several billion at liquid helium temperature), and frequency instability of only parts in 1014 is easily achieved. A number of important experimental results are presented. The rst observation of bimodal maser oscillation and the measurement of the Schawlow-Townes noise limit in the thermal regime is described, as well as the determination that annealing in air converts Fe2+ ions to Fe3+ ions in sapphire. This conversion enabled improvement in the output power of the maser by 20 dB. A flicker noise cancelling readout system was developed for ultra low power microwave signals in order to counteract performance-limiting behaviour of the necessary ampli cation chain. The rst cooling of a high-Q factor WG mode resonator to millikelvin temperatures was performed, where for high power we characterise a thermal instability brought on by the material properties of sapphire, and in the low limit of single photon power, the Q-factor degraded by a factor of 2 to 10, and was measured to be a few times 108. Finally, a strong Kerr-type nonlinearity in the sapphire due to the Fe3+ impurity is observed and modelled, and the potential for quantum applications is discussed.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2011


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