Development of Ultra High Sensitivity and High resolution Electron Spin Resonance Imaging Probes for Small Samples

Electron spin resonance (ESR) is a spectroscopic method that addresses electron spins in paramagnetic samples. Although it is very powerful and informative, ESR traditionally suffers from low sensitivity, requiring many billions of spins to get a measurable signal with commercial systems using the Faraday induction-detection principle. Furthermore, imaging the electron spin distribution in a sample can be useful for gaining structural information and characterization of paramagnetic defects, but the resolution available in commercial systems ( >100 μ m ) is too limited to observe fine features. In view of these disadvantages, significant efforts are made to improve ESR sensitivity and develop alternative detection schemes. We present here two such recent advancements: (1) an ultra sensitive experimental induction-detection ESR setup that can detect the signal from just a few tens of spins, and (2) an experimental optical-detection and imaging ESR setup that can detect the signal from just a few thousands of spins and image them at sub-micrometer resolutions. The first setup was made possible thanks to the development of an ultra-miniature micrometer-sized microwave resonator that operates at ~34 GHz at cryogenic temperatures in conjunction with a unique cryogenically cooled low noise amplifier. The second setup is a result of our continuing efforts to produce an ESR imaging probe with a single spin sensitivity and sub-diffraction limited resolutions for optically-detectable paramagnetic samples. The results presented here demonstrate orders of magnitude improvement in ESR sensitivity and imaging resolution over commercially available setups, and so they represent a paradigm shift with respect to the capabilities and possible applications of induction-detection-based and optical-detection based ESR spectroscopy and imaging.