  Coherent perfect absorber (CPA) was proposed as the time-reversed counterpart
to laser: a resonator containing lossy medium instead of gain medium can absorb
the coherent optical fields completely. Here, we exploit a monolayer graphene
to realize the CPA in a non-resonant manner. It is found that quasi-CPA point
exists in the terahertz regime for suspending monolayer graphene, and the CPA
can be implemented with the assistant of proper phase modulation among two
incident beams at the quasi-CPA frequencies. The graphene based CPA is found of
broadband angular selectivity: CPA point splits into two frequency bands for
the orthogonal $s$ and $p$ polarizations at oblique incidence, and the two
bands cover a wide frequency range starting from zero frequency. Furthermore,
the coherent absorption can be tuned substantially by varying the
gate-controlled Fermi energy. The findings of CPA with non-resonant graphene
sheet can be generalized for potential applications in terahertz/infrared
detections and signal processing with two-dimensional optoelectronic materials.
