High frequency single-mode resonators for EPR spectroscopy enabling rotations of the sample about two orthogonal axes

Abstract

A novel single-mode resonant structure which enables the rotation of the sample about two orthogonal axes is investigated in view of electron paramagnetic resonance applications. The proposed solution is based on cylindrical non-radiative resonators laterally loaded by the sample holder. The resulting structure can still operate in non-radiative regime, although no longer rotationally invariant. These theoretical predictions, based on symmetry considerations, are confirmed by means of a finite element numerical modelling. Theoretical and computational results are then substantiated by experimental investigations at millimeter wavelengths. As a result, a single-mode resonator which enables all the relevant rotations of the sample is demonstrated at millimeter wavelengths for the first time. In this resonator the intensity of the microwave field on the sample and its orientation with respect to the static magnetic field can be kept constant during the rotations. Therefore, a complete characterization of anisotropic systems is possible at the highest sensitivity, without the need of split-coil magnets. Possible applications at very high frequencies are discussed.

High-frequency EPR applications of open nonradiative resonators

“…In addition to the reported case of NR resonator enabling the rotation of the sample about two orthogonal axes [26], a NR cavity is currently under development for application in ENDOR spectroscopy. The preliminary results obtained with a slightly modified version of the resonator discussed in this paper show that the sample can be irradiated by a radiofrequency field with high efficiency, without influencing the microwave response of the resonator.…”

“…Consider now a hypothetical TE c 0 mode trapped in the central volume of the cavity. Such a mode cannot transfer energy to the cutoff-less transverse electromagnetic (TEM) modes propagating along the plates, since the TE c 0 mode does not have common field components with the TEM modes [16,26]. The radiative decay of the TE c 0 mode requires a coupling with the TE w 0 modes propagating along the parallel-plate waveguide (indicated by the superscript w), whose cutoff k cutoff is given by…”

“…Such modes can be isolated, namely placed in the frequency spectrum below the cutoff of the modes propagating away from the structure, or embedded in a continuous spectrum of propagating modes. In the latter case, the trapped mode is usually decoupled from the propagating modes by symmetry reasons, such as opposite parity with respect to a symmetry plane [26]. An ante litteram version of a NR cavity was presented early in the 70's within the framework of optically detected magnetic resonance (ODMR) studies at X-band and Q-band [27][28][29].…”

A High-Conversion-Factor, Double-Resonance Structure for High-Field Dynamic Nuclear Polarization

“…Different NR resonators have been developed in the context of high-field EPR studies [43][44][45][46]. The basic design consists of a central resonant region enclosed between two relatively thick metal plates placed at a distance less than half the wavelength.…”

A New Design of Resonant Cavity for the W-band EPR spectrometer

“…Another obstacle for a universally compatible resonant cavity is the design for coupling. When a solenoid is used, the coupling strength is usually adjusted by an iris 12 , or changing the relative position between the cavity and the waveguide 13,14 . A novel method of adjusting the coupling using a metal sphere has been developed for cavities operating in split-pair magnets 15 .…”