Variable Coupling Scheme for High-Frequency Electron Spin Resonance Resonators Using Asymmetric Meshes

The sensitivity of a high-frequency electron spin resonance (ESR) spectrometer depends strongly on the structure used to couple the incident millimeter wave to the sample that generates the ESR signal. Subsequent coupling of the ESR signal to the detection arm of the spectrometer is also a crucial consideration for achieving high spectrometer sensitivity. In previous work, we found that a means for continuously varying the coupling was necessary for attaining high sensitivity reliably and reproducibly. We report here on a novel asymmetric mesh structure that achieves continuously variable coupling by rotating the mesh in its own plane about the millimeter-wave transmission-line optical axis. We quantify the performance of this device with nitroxide spin label spectra in both a lossy aqueous solution and a low-loss solid-state system. These two systems have very different coupling requirements and are representative of the range of coupling achievable with this technique. Lossy systems, in particular, are a demanding test of the achievable sensitivity and allow us to assess the suitability of this approach for applying high-frequency ESR, e.g., to the study of biological systems at physiological conditions. The variable coupling technique reported on here allows us to readily achieve a factor of ca. 7 improvement in the signal-to-noise ratio at 170 GHz and a factor of ca. 5 at 95 GHz over what has previously been reported for lossy samples.