  A simple design paradigm for making broad-band ultra-thin plasmonic absorbers
is introduced. The absorber's unit cell is composed of sub-units of various
sizes, resulting in nearly 100% absorbance at multiple adjacent frequencies and
high absorbance over a broad frequency range. A simple theoretical model for
designing broad-band absorbers is presented. It uses a single-resonance model
to describe the optical response of each sub-unit and employs the series
circuit model to predict the overall response. Validity of the circuit model
relies on short propagation lengths of the surface plasmons.
