Electrothermal automodulation in incipient ferroelectric whispering-gallery microwave resonators at 4.2 K

Threshold conditions for electrothermal automodulation instability in high-Q ferroelectric microwave cryogenic resonators operating in the two-mode regime are investigated. The dependence of the electrothermal automodulation frequency on the numbers of interacting modes for different combinations of thermal modes and surface electromagnetic whispering-gallery modes is presented. The threshold power exciting the electrothermal automodulation of the oscillation of the partial mode electromagnetic amplitudes is compared with the threshold power of strictional parametric excitation of acoustic oscillations in the resonator. It is shown that the electrothermal automodulation in the two-mode regime may take place at an excitation power from 10 to 120 μW depending on the combination of interacting thermal and electromagnetic surface modes. Calculated threshold powers are low, which makes it possible to apply the electrothermal automodulation for improving the sensitivity of resonance bolometers and distributed microwave antennas with basic elements built around nonlinear microwave resonators. In addition, the electrothermal automodulation could be applied in developing novel microwave metamaterials. Nonlinear microwave whispering-gallery cryogenic resonators can be used as elements increasing the sensitivity of EPR spectroscopy methods.