Towards high performance dielectric microwave resonators for X-band EPR spectroscopy

Haakon T A Wiedemann; Stefan Ruloff; Rudolf Richter; Christoph W Zollitsch; Christopher W M Kay

doi:10.1016/j.jmr.2023.107519

Towards high performance dielectric microwave resonators for X-band EPR spectroscopy

J Magn Reson. 2023 Sep:354:107519. doi: 10.1016/j.jmr.2023.107519. Epub 2023 Jul 14.

Authors

Haakon T A Wiedemann¹, Stefan Ruloff², Rudolf Richter², Christoph W Zollitsch³, Christopher W M Kay⁴

Affiliations

¹ Department of Chemistry, Saarland University, Saarbrücken 66123, Saarland, Germany. Electronic address: haakon.wiedemann@uni-saarland.de.
² Department of Chemistry, Saarland University, Saarbrücken 66123, Saarland, Germany.
³ Department of Chemistry, Saarland University, Saarbrücken 66123, Saarland, Germany; London Centre of Nanotechnology, University College London, London WC1H 0AH, United Kingdom; Department of Physics & Astronomy, University College London, London WC1E 6BT, United Kingdom.
⁴ Department of Chemistry, Saarland University, Saarbrücken 66123, Saarland, Germany; London Centre of Nanotechnology, University College London, London WC1H 0AH, United Kingdom.

PMID: 37541024
DOI: 10.1016/j.jmr.2023.107519

Abstract

Microwave (MW) resonators in Electron Paramagnetic Resonance (EPR) spectroscopy concentrate the MW magnetic field (B₁) at the sample and separate the MW electric field from the sample. There are numerous experimental methods in EPR spectroscopy which all impose different requirements on MW resonators (e.g. high or low quality factor, MW conversion, and B₁-field homogeneity). Although commercial spectrometers offer standardized MW resonators for a broad application range, newly emerging and highly-specialized research fields push these spectrometers to or beyond their sensitivity limits. Optimizing the MW resonator offers one direct approach to improve the sensitivity. Here we present three low-cost optimization approaches for a commercially available X-band (9-10 GHz) MW resonator for three experimental purposes (continuous-wave (CW), transient and pulse EPR). We obtain enhanced MW conversion factors for all three optimized resonators and higher quality factors for two optimized resonators. The latter is important for CW and transient EPR. Furthermore, we fabricated a resonator which features an extended area of homogeneous B₁-field and, hence, improved pulse EPR performance. Our results demonstrate that small changes to a commercial MW resonator can enhance its performance in general or for specific applications.

Keywords: B(1)– field homogeneity; CW EPR; EPR resonators; Microwave resonators; Pulse EPR; Spin relaxation; Transient EPR; Transient nutations; X-band.