We report a device that allows inductive-detection of electron spins in picoliter volumes. The invention is a ‘planar inverse anapole’ microresonator with a spin detection limit as low as 6 x 106 (mT√Hz)-1 (i.e. 6 x 105 (G√Hz)-1) at 10 GHz and room temperature, which is an improvement of an order of magnitude compared to previously reported microresonator designs.
We report a device that allows inductive-detection of electron spins in picoliter volumes with increased sensitivity. Microresonators used for spin detection in volume-limited samples suffer from poor quality-factors, which adversely affects sensitivity. We have exploited anapole structures with toroidal moment to diminish radiative losses from planar microresonators while simultaneously confining microwave magnetic fields to picoliter volumes. Using the Babinet principle to create a magnetic field hotspot, we have created a planar inverse anapole microresonator for inductive-detection electron magnetic resonance. High quality-factors enable facile coupling with a microwave feedline, allowing continuous wave (CW) EPR experiments that are not possible with other microresonator designs. The size of the hotspot (the active volume of the resonator) is much smaller than the overall dimensions of the microresonator. This provides scalability to microwave frequencies of > 100 GHz. Thus, the resonators can be scaled for use with multiple frequencies without sacrificing device functionality, overcoming a major limitation of other microresonator designs.
High quality-factors of the planar inverse anapole microresonators allow easy coupling to microwave feedlines. This enables easy deployment of the microresonators in existing spectrometers for routine measurements of picoliter-volume samples. The scalability of the design to frequencies > 100 GHz open the route to multifrequency EPR measurements on volume-limited samples. Lastly, the absolute sensitivity we obtain is an order of magnitude higher than other microresonator designs.