High-Q cavity coupled to a high permittivity dielectric resonator for sensing applications

The use of high-quality factor resonators is of undoubted interest for high-precision measurements and for applications in quantum technologies. Novel types of microwave sensors can be realized by coupling a first resonator acting as a stable frequency reference with a second resonator that is sensitive to a particular physical quantity. Here we report on a coupled cavity configuration in which a high Q factor elliptical TESLA-shaped superconducting cavity is coupled with a high permittivity $varepsilon_r$ SrTiO₃ puck, whose resonant frequency varies as a function of temperature due to the temperature dependence of the permittivity that reaches values higher than 30000 below 1 K. Extensive electromagnetic simulations are used to test different coupling configurations, showing great versatility in tuning the coupling in the weak or strong regime, depending on the puck's position within the cavity. Moreover, for the coupled system, they allow investigation of the dependence of the zero transmission frequency (notch frequency) on changes in permittivity varepsilon_r, obtaining a maximum value of 2.8 MHz per unit change of varepsilon_r, for varepsilon_r approx 230. Finally, we discuss the use of the coupled system as a sensor that operates in different temperature ranges.