Design and Room-Temperature ESR Characterization of a Compact Planar Loop-Gap Resonator near 12 GHz for Prospective Cryogenic Applications

We report the design, fabrication, microwave characterization, and room-temperature ESR demonstration of a compact planar loop-gap resonator operating near 12 GHz. The resonator was fabricated by etching a 20-µm-thick copper layer on a glass-composite substrate and consisted of a 4.2-mm-diameter circular conductor with a 0.15-mm gap. Non-contact coupling to a stripline was characterized by varying the relative lateral and vertical positions of the resonator. At the position giving the largest resonance dip, the resonance frequency and loaded Q -factor were 12.2 GHz and 67.8, respectively, corresponding to a bandwidth of approximately 180 MHz. A first-order lumped RLC description was used to interpret the measured resonance characteristics. Room-temperature continuous-wave ESR measurements of DPPH demonstrated spin detection near 12.7 GHz with a signal-to-noise ratio of approximately 20. The present results demonstrate a simple planar fabrication and coupling approach for compact ESR resonators.