Millimeter wave dielectric resonators

In this paper the most common millimeter wave dielectric resonators have been reviewed. Their basic theoretical aspects are discussed starting from open configurations, in which the confinement of the radiation is ensured by the total internal reflection mechanism. In particular, the dielectric resonators operating in the so-called whispering gallery modes are extensively discussed, with specific attention to their allowed dimensions and working frequencies. The analysis is then generalized to partially open structures, which exploit a partial metallic confinement. Among these, the fractional geometry and the nonradiative resonators are analyzed in some details. The use of partially open structures ensures specific advantages in comparison to fully open resonators, as a simplified spectrum and a reduced active volume. The excitation setup is then included in the theoretical model, together with the effects due to a perturbing sample, in order to calculate the overall response of a typical spectrometer based on dielectric resonators. The predictions of the theoretical analysis are compared with meaningful experimental results, which demonstrate the performances and the capabilities of the proposed devices. The analysis of the employed experimental setups allows the discussion of the practical aspects of these resonators, which appear relevant for millimeter wavelengths applications. The final part of the paper is devoted to the illustration of practical employments of the proposed devices. In particular, the realization of a very wideband resonator and applications to dielectric and paramagnetic spectroscopy are reported.