  Optically-active point defects in various host materials, such as diamond and
silicon carbide (SiC), have shown significant promise as local sensors of
magnetic fields, electric fields, strain and temperature. Current sensing
techniques take advantage of the relaxation and coherence times of the spin
state within these defects. Here we show that the defect charge state can also
be used to sense the environment, in particular high frequency (MHz-GHz)
electric fields, complementing established spin-based techniques. This is
enabled by optical charge conversion of the defects between their
photoluminescent and dark charge states, with conversion rate dependent on the
electric field (energy density). The technique provides an all-optical high
frequency electrometer which is tested in 4H-SiC for both ensembles of
divacancies and silicon vacancies, from cryogenic to room temperature, and with
a measured sensitivity of ~41 (V/cm)**2 / $\sqrt{Hz}$. Finally, due to the
piezoelectric character of SiC, we obtain spatial 3D maps of surface acoustic
wave modes in a mechanical resonator.
