Development of planar microstrip resonators for electron spin resonance spectroscopy

This work focuses on the development of planar microwave resonators for use in electron spin resonance spectroscopic studies. Two half-wavelength microstrip resonators of different geometrical shapes, namely straight ribbon and omega, are fabricated on commercially available microwave laminates. Both resonators have a characteristic impedance of 50 Ω. We have performed electromagnetic field simulations for the resonators and have extracted design parameters that were used for fabrication. The effect of the geometry on the quasi-transverse electromagnetic (quasi-TEM) modes of the resonators is noted from simulation results. The fabrication is done using optical lithography in which laser printed photomasks are used. This prototyping technique allows us to fabricate resonators in a few hours with accuracy up to 6 mils. The resonators are characterized using a Vector Network Analyzer. The fabricated resonators are used in a home built low-temperature continuous wave electron spin resonance (CW-ESR) spectrometer which operates in S-band. It captures the absorption spectrum of the free radical DPPH, at both room temperature and 77 K. The measured g-factor using our resonators is consistent with the values reported in literature. Spin sensitivity of 1015 spins/gauss is achieved at 77 K. The designed resonators will be used in setting up a pulsed electron spin resonance spectrometer by suitably modifying the CW-ESR spectrometer.