We study the propagation of photons in a homogeneous axion dark matter
background. When the axion decay into two photons is stimulated, the photon
field exhibits a parametric instability in a small bandwidth centered on one
half of the axion mass. We estimate analytically the enhancement for both
coherent and non-coherent axion fields and we find that this effect could be
relevant in the context of miniclusters and galactic halos.Comment: 4 pages, 3 figure
