  The maximal absorption rate of ultra-thin films is 50% under the condition
that the tangential electric (or magnetic) field is almost constant across the
film in symmetrical environment. However, with certain reflectors, the
absorption rate can be greatly increased, to even perfect absorption (100%). In
this work, we explicitly derive the general conditions of the ultra-thin
absorptive film parameters to achieve perfect absorption with general types of
reflectors under the condition that the tangential electric (or magnetic) field
is almost constant across the film. We find that the parameters of the film can
be classified into three groups, exhibiting: 1) a large permittivity
(permeability), 2) a near-zero permeability (permittivity), or 3) a suitable
combination of the permittivity and the permeability, respectively.
Interestingly, the latter two cases demonstrate extraordinary absorption in
ultra-thin films with almost vanishing losses. Our work serves as a guide for
designing ultra-thin perfect absorbers with general types of reflectors.
