  Nitrogen-Vacancy (NV) centers in diamond are attractive tools for sensing and
quantum information. Realization of this potential requires effective tools for
controlling the spin degree of freedom by microwave (mw) magnetic fields. In
this work we present a planar microwave resonator optimized for
microwave-optical double resonance experiments on single nitrogen-vacancy (NV)
centers in diamond. It consists of a piece of wide microstrip line which is
symmetrically connected to two 50 $\Omega$ microstrip feed lines. In the center
of the resonator, an $\Omega$-shaped loop focuses the current and the mw
magnetic field. It generates a relatively homogeneous magnetic field over a
volume of 0.07mm$^2\times$0.1mm. It can be operated at 2.9 GHz in both
transmission and reflection modes with bandwidths of 1000 MHz and 400 MHz,
respectively. The high power-to-magnetic field conversion efficiency allows to
produce $\pi$-pulses with a duration of 50 ns with only about 200 mW and 50 mW
microwave power in transmission and reflection, respectively. The transmission
mode also offers capability for efficient radio frequency excitation. The
resonance frequency can be tuned between 1.3 GHz and 6 GHz by adjusting the
length of the resonator. This will be useful for experiments on NV-centers at
higher external magnetic fields and on different types of optically active spin
centers.
