Magnetic Field Enhancement from a Single Split Ring Resonator using Optically Detected Magnetic Resonance of the Nitrogen Vacancy Center in Diamond at 4.2 T and 115 GHz

Electron Paramagnetic Resonance (EPR) is an invaluable spectroscopic technique for characterizing materials with applications in biology, chemistry, physics, material science and medicine. EPR uses microwaves (MW) to resonantly excite transitions between different spin states and detects the oscillating magnetic fields generated by the evolution of these states. In principle, EPR at high magnetic fields is very advantageous because of high spectral resolution and high sensitivity. However, the sensitivity is often limited by the technical difficulties in generating strong resonant microwave excitation in the sub-THz and THz bandgap regimes, prohibiting the careful study of many interesting samples like 2-dimensional materials, thin films, or spin-limited biological samples. Cavity resonators are often used in HF-EPR to improve the sensitivity, but as the resonant frequency increases, the size of these resonant cavities becomes prohibitively small for fabrication and sample accommodation. The use of surface resonators for HF-EPR is relatively unexplored.[1]

In this work, we report on the development and characterization of a planar metamaterial, the edge-coupled split-ring resonator (SRR), used to enhance the local magnetic field at 115 GHz on a microscopic scale. Electromagnetic field simulation show that a single resonator structure improves the transverse magnetic field strength by a factor of 11 with -15.11 dB absorptive coupling at 113.2 GHz and an unloaded Q factor of 43. Initial experiments agree with this very well. As a subsequent demonstration, we aim to show this magnetic field enhancement from a single SRR through Optically Detected Magnetic Resonance (ODMR) experiments of the Nitrogen Vacancy (NV) center in diamond at 4.2 T. We expect the MW pi pulse to be between 5-10 ns at 4.2 T based on the simulation results. Combining the MW enhancement of a planar resonator with the advantages ODMR of the NV-center at high magnetic fields[2] may offer further improvements in sensitivity and resolution and help realize highly interesting studies of 2-dimensional materials and spin-limited samples on the micro- and nano-scale.