Development of an ultra-compact microwave resonator for electron-spin-resonance measurement

Abstract

In a typical inductively detected electron spin resonance (ESR) measurement system, the characteristics of the resonator are known to have a significant effect on sensitivity. The advantage of using a resonator is that it makes it possible to obtain a weak resonance signal by increasing the millimeter wave intensity irradiating the sample by a standing wave. In a typical X-band (~10 GHz) ESR system, a high Q-value (~2×10⁴) is obtained by using a cavity resonator. It is known that the oscillating magnetic field generated in the resonator and the measurement sensitivity are approximately proportional to [V/(ω₀Q_u)]^1/2 where Q_u is the Q value at no load, V is the effective volume of the resonator, and ω₀ is the resonant frequency.

Therefore, it is expected that high-sensitivity ESR measurement will be possible by providing the sample with an oscillating magnetic field component of high intensity. As a method to increase the oscillating magnetic field to the sample, it is possible to generate a localized oscillating magnetic field at the sample area by narrowing the volume V of the resonator. A loop-gap resonator (LGR) has been proposed by Twigg as a candidate for such an ultra-compact microwave resonator.

We have developed an ultra-small LGR for X-band ESR and investigated its resonance characteristics using calculations with COMSOL Multiphysics and an actual LGR fabricated by etching. An LGR with a notch of about 1 mm in width was fabricated on a disk of about 4 mm in diameter and placed on a stripline of about 1 mm in thickness, and its positional relationship with the stripline where the LGR has the largest resonance was investigated in the x-axis and z-axis directions. Simulations and measurements of the frequency response revealed that the vibration field generated at the center of the LGR is stronger at a distance of about 1.3 mm on the x-axis from directly above the stripline. From these resonance characteristics results, it is clear that the Q value is around 70. Based on the above results, ESR measurements of DPPH, a standard sample for ESR, were performed at room temperature. The center frequency was found to be 12.7 GHz, and changing its oscillation frequency by ±700 MHz resulted in a decrease in signal intensity. The details will be presented in this talk.