  We study light absorption by a dipolar absorber in a given environment, which
can be a nanoantenna or any complex inhomogeneous medium. From first-principle
calculations, we derive an upper bound for the absorption, which decouples the
impact of the environment from the one of the absorber. Since it is an
intrinsic characteristic of the environment regardless of the absorber, it
provides a good figure of merit to compare the ability of different systems to
enhance absorption. We show that, in the scalar approximation, the relevant
parameter is not the field enhancement but the ratio between the field
enhancement and the local density of states. Consequently, a plasmonic
structure supporting hot spots is not necessarily the best choice to enhance
absorption. We also show that our theoretical results can be applied beyond the
scalar approximation and the plane-wave illumination.
