  Photon regeneration experiments searching for signatures of oscillations of
photons into hypothetical very weakly interacting ultra-light particles, such
as axions, axion-like and hidden-sector particles, have improved their
sensitivity considerably in recent years. Important progress in laser and
detector technology as well as recycling of available magnets from accelerators
may allow a big further step in sensitivity such that, for the first time,
laser light shining through a wall experiments will explore territory in
parameter space that has not been excluded yet by astrophysics and cosmology.
We review these challenges and opportunities for the next generation
experiments.
