  Coherent perfect absorption (CPA) refers to interferometrically induced
complete absorption of incident light by a partial absorber independently of
its intrinsic absorption (which may be vanishingly small) or its thickness. CPA
is typically realized in a resonant device, and thus cannot be achieved over a
broad continuous spectrum, which thwarts its applicability to photodetectors
and solar cells, for example. Here, we demonstrate broadband omni-resonant CPA
by placing a thin weak absorber in a planar cavity and pre-conditioning the
incident optical field by introducing judicious angular dispersion. We make use
of monolayer graphene embedded in silica as the absorber and boost its optical
absorption from ~1.6% to ~60% over a bandwidth of ~70 nm in the visible.
Crucially, an analytical model demonstrates that placement of the graphene
monolayer at a peak in the cavity standing-wave field is not necessary to
achieve CPA, contrary to conventional wisdom.
