  The spin readout of an ensemble of nitrogen-vacancy (NV) centers in diamond
can be realized by a photoconductive detection that is a complementary method
to the optical detection of the NV electron spin magnetic resonance. Here, we
implement the photoconductive detection through graphitic planar electrodes
that collect the photocurrent. Graphitic electrodes are patterned using a xenon
Focused-Ion Beam on an Optical-Grade quality diamond crystal containing a
nitrogen concentration of ~1 ppm and a NV concentration of a few ppb.
Resistance and current-voltage characteristics of the NV-doped diamond junction
are investigated tuning the 532 nm pump beam intensity. The junction has an
ohmic behavior and under a strong bias field, we observe velocity saturation of
the optically-induced carriers in the diamond junction. We perform the
photoconductive detection in continuous-wave regime of the magnetic resonance
of the NV centers ensemble for a magnetic field applied along the <100> and the
<111> direction with a magnitude above 100 mT. This technique enables the
realization of all-carbon diamond quantum sensors integrating graphitic
microstructures for the electrical readout.
