High-Frequency Microstrip Cross Resonators for Circular Polarization EPR Spectroscopy

TL;DR

This paper introduces a novel microstrip cross resonator capable of precisely controlling microwave polarization up to 30 GHz, enhancing EPR spectroscopy at low temperatures with high polarization fidelity.

Contribution

The paper presents a new microstrip resonator design that allows absolute control of microwave polarization, enabling improved EPR measurements at high frequencies and low temperatures.

Findings

Achieved 82% fidelity in circular polarization at low temperatures.

Designed a microstrip cross resonator with tunable polarization control.

Demonstrated effective polarization control up to 30 GHz.

Abstract

In this article we discuss the design and implementation of a novel microstrip resonator which allows for the absolute control of the microwaves polarization degree for frequencies up to 30 GHz. The sensor is composed of two half-wavelength microstrip line resonators, designed to match the 50 Ohms impedance of the lines on a high dielectric constant GaAs substrate. The line resonators cross each other perpendicularly through their centers, forming a cross. Microstrip feed lines are coupled through small gaps to three arms of the cross to connect the resonator to the excitation ports. The control of the relative magnitude and phase between the two microwave stimuli at the input ports of each line allows for tuning the degree and type of polarization of the microwave excitation at the center of the cross resonator. The third (output) port is used to measure the transmitted signal, which is crucial to work at low temperatures, where reflections along lengthy coaxial lines mask the signal reflected by the resonator. EPR spectra recorded at low temperature in an S= 5/2 molecular magnet system show that 82%-fidelity circular polarization of the microwaves is achieved over the central area of the resonator.