Hidden black: coherent enhancement of absorption in strongly scattering media.

We show that a weakly absorbing, strongly scattering (white) medium can be made very strongly absorbing at any frequency within its strong-scattering bandwidth by optimizing the input electromagnetic field. For uniform absorption, results from random matrix theory imply that the reflectivity of the medium can be suppressed by a factor ∼(ℓ(a)/ℓ)N(-2), where N is the number of incident channels and ℓ, ℓ(a) are the elastic and absorption mean free paths, respectively. It is thus possible to increase absorption from a few percent to >99%. For a localized weak absorber buried in a nonabsorbing scattering medium, we find a large but bounded enhancement.