Whispering gallery mode dielectric resonators in EMR spectroscopy above 150 GHz: Problems and perspectives

At frequencies above 150 GHz conventional microwave techniques become unsuitable for electron magnetic resonance (EMR) spectroscopy applications. Quasi-optical techniques are the general frame into which the different approaches to high-field high-frequency EMR (HF²-EMR) have been developed. Beyond the single-pass approach the existing HF²-EMR spectrometers make use of Fabry-Perot resonators. In this paper the use of an alternative resonating structure on the basis of the use of whispering gallery mode dielectric resonators is shown. The physics of these resonators and their implementation in a HF²-EMR probe head are described. Their use in HF²-EMR experiments performed on standard samples is described and the results of these experiments critically discussed in order to have insight into the open problems of the employed spectrometer; the more promising solutions to these problems, actually under development, are briefly outlined.