  The possibility to detect gravitational waves (GW) from planetary-mass
primordial black hole (PBH) binaries with electromagnetic (EM) detectors of
high-frequency GWs is investigated. We consider two patented experimental
designs, based on the inverse Gertsenshtein effect, in which incoming GWs
passing through a static magnetic field induce EM excitations inside either a
TM cavity or a TEM waveguide. The frequency response of the detectors is
computed for post-newtonian GW waveforms. We find that such EM detectors based
on current technology may achieve a strain sensitivity down to $h \sim
10^{-30}$, which generates an EM power variation of $10^{-10}$ W. This allows
the detection of PBH binary mergers of mass around $10^{-5} M_\odot$ if they
constitute more than $0.01$ percent of the dark matter, as suggested by recent
microlensing observations. We envision that this class of detectors could also
be used to detect cosmological GW backgrounds and probe sources in the early
Universe at energies up to the GUT scale.
