Easily Tunable Crossed-Loop (Bimodal) EPR Resonator

Abstract

A new crossed-loop resonator (CLR) structure is described that uses two orthogonal resonators to isolate the EPR signal from the microwave source. Resonators of this type are usually referred to as bimodal, which is descriptive of cavity or distributed element resonators. However, it is more useful to think of the new resonator as two virtually independent lumped-element resonators that have a common sample volume where the loops of the two resonators meet orthogonally. The first resonator excites the spins, and the second resonator (or resonant section of the CLR) acts like an antenna that detects only the signal caused by the spin system. In this manner, the very phenomenom that is being studied is used to separate the desired EPR signal from the microwave source and performs the function of the circulator. The signal coupled into the second resonator is due to the spin system. The phase noise of the source is reflected from the first resonator back toward the source and is efficiently isolated from the second resonator and the EPR signal. This resonator structure eliminates the need for the circulator, simplifies the spectrometer circuit, virtually eliminates source phase noise in the detected signal, and allows dispersion spectra to be measured with the same high signal-to-noise (S/N) as absorption spectra. In addition, for pulse experiments, this resonator greatly decreases the dead time of the instrument and allows measurement of a portion of the signal previously inaccessible with other resonators. The CLR is rugged, easy to tune, does not have to be critically coupled, and is easily adjusted to maintain source isolation when the sample is changed.