  The identification of a complete three-dimensional (3D) photonic band gap in
real crystals always employs theoretical or numerical models that invoke
idealized crystal structures. Thus, this approach is prone to false positives
(gap wrongly assigned) or false negatives (gap missed). Therefore, we propose a
purely experimental probe of the 3D photonic band gap that pertains to many
different classes of photonic materials. We study position and
polarization-resolved reflectivity spectra of 3D inverse woodpile structures
that consist of two perpendicular nanopore arrays etched in silicon. We observe
intense reflectivity peaks $(R > 90\%)$ typical of high-quality crystals with
broad stopbands. We track the stopband width versus pore radius, which agrees
much better with the predicted 3D photonic band gap than with a directional
stop gap on account of the large numerical aperture used. A parametric plot of
s-polarized versus p-polarized stopband width agrees very well with the 3D band
gap and is model-free. This practical probe provides fast feedback on the
advanced nanofabrication needed for 3D photonic crystals and stimulates
practical applications of band gaps in 3D silicon nanophotonics and photonic
integrated circuits, photovoltaics, cavity QED, and quantum information
processing.
