Electron spin resonance (ESR) spectroscopy measures paramagnetic free radicals, or electron spins, in a variety of biological, chemical, and physical systems. Detection of diverse paramagnetic species is important in applications ranging from quantum computation to biomedical research. Countless efforts have been made to improve the sensitivity of ESR detection. However, the improvement comes at the cost of experimental accessibility. Thus, most ESR spectrometers are limited to specific sample geometries and compositions. Here, we present a non-resonant transmission line ESR probe (microstrip geometry) that effectively couples high frequency microwave magnetic field into a wide range of sample geometries and compositions. Importantly the universality of the non-resonant transmission line probe does not sacrifice detection sensitivity. We demonstrate a further extension of the universality by adapting the microstrip signal line geometry to best suit the sample geometry and composition. The high frequency magnetic field homogeneity is greatly increased by positioning the sample between the microstrip signal line and the ground plane. Sample interfacing occurs via a universal sample holder which is compatible with both sold and liquid samples. The unavoidable loss in sensitivity due to the non-resonant nature of the transmission line probe (low Q) is recuperated by using a highly sensitive microwave bridge detection circuit. The combination of our sensitive bridge and non-resonant transmission line provides similar sensitivity (less than 10 % difference) to a commercially available ESR spectrometer equipped with a high-Q resonator. The non-resonant probe allows for transmission, reflection or dual-mode detection (transmission and reflection) where the dual-mode results in a √2 signal enhancement.
Electron Spin Resonance, transmission, reflection, high sensitivity