  The observation of cosmic sub-PeV gamma-rays from the Crab Nebula opens up
the possibility of testing cosmic ray photon transparency at the multi-hundred
TeV scale. Assuming no deviation from a source gamma-ray emission due to
accelerated electron inverse-Compton scattering, higher event energies can
extend constraints on the effects of new physics; We consider oscillation
between gamma-rays and axions/dark photons, plus attenuation effects from
gamma-ray absorption in the case of dark photon dark matter. Combining the
recent AS$\gamma$ and HAWC sub-PeV data with earlier MAGIC and HEGRA data,
axion-like particles are most constrained in the $10^{-7}-10^{-6}$ eV mass
range, where the coupling $g_{a\gamma}$ is constrained to be below ${1.8}\times
10^{-10}$ GeV$^{-1}$. In comparison, gamma ray flux attenuation due to
oscillation with a dark photon leads to a very weak constraint on the mixing
parameter; $\epsilon\lesssim$ 0.2 for dark photon mass between $10^{-7}$ and
$10^{-6}$ eV. Direct scattering from dark photon dark matter limits
$\epsilon\lesssim$ 0.01 for masses between $6$ and $400$ eV.
