Circularly polarized microwave antenna for nitrogen vacancy centers in diamond

The sensing applications of nitrogen-vacancy color centers in a diamond require an efficient manipulation of the color center ground state over the whole volume of an ensemble. Thus, it is necessary to produce strong uniform magnetic fields of a well-defined circular polarization at microwave frequencies. In this paper, we develop a circularly polarized microwave antenna based on the excitation of hybrid electromagnetic modes in a high-permittivity dielectric resonator. The influence of the geometrical parameters of the antenna on the reflection coefficient and magnetic field magnitude is studied numerically and discussed. The Rabi frequencies and their inhomogeneity over the volume of a commercially available diamond sample are calculated. With respect to the numerical predictions, a Rabi frequency as high as 34 MHz with an inhomogeneity of 4% over a 1.2 mm × ∅2.5 mm (5.9 mm³ in volume) diamond sample can be achieved for 10 W of input power at room temperature. The antenna prototype is fabricated, and experimental investigations of its characteristics are performed in microwave and optical frequency domains. The circular polarization of the microwave magnetic field with an ellipticity of 0.94 is demonstrated experimentally. The Rabi oscillation frequency and its inhomogeneity are measured, and the results demonstrate a good agreement with the numerically predicted results.