Fabry-Perot resonator for high-field multi-frequency ESR at millimetre andsubmillimetre wavelengths

The design and performance of a Fabry-Perot (FP) resonator for high-field m ulti-frequency electron spin resonance (ESR) at microwave frequencies from 110 GHz to 330 GHz and corresponding Zeeman fields between 3.9 T and 12 T ( at g = 2) is described. The tunable semiconfocal FP arrangement includes an inductive copper mesh that can be mechanically adjusted to obtain best per formance in the entire temperature range from room temperature to 4 K. Sens itivity improvements (as compared to transmission mode experiments without a resonator) of about two orders of magnitude are obtained at 330 GHz. The potential of the configuration for investigating spin systems of low concen tration with millimetre and submillimetre ESR is demonstrated on three exam ples.