Polarization- and frequency-tunable microwave circuit for selective   excitation of nitrogen-vacancy spins in diamond
  We report on a planar microwave resonator providing arbitrarily polarized oscillating magnetic fields that enable selective excitation of the electronic spins of nitrogen-vacancy (NV) centers in diamond. The polarization plane is parallel to the surface of diamond, which makes the resonator fully compatible with (111)-oriented diamond. The field distribution is spatially uniform in a circular area with a diameter of 4 mm, and a near-perfect circular polarization is achieved. We also demonstrate that the original resonance frequency of 2.8 GHz can be varied in the range of 2-3.2 GHz by introducing varactor diodes that serve as variable capacitors. 
Johannes Herrmann, Marc A. Appleton, Kento Sasaki, Yasuaki Monnai,   Tokuyuki Teraji, Kohei M. Itoh, Eisuke Abe
cond-mat.mes-hall quant-ph