  Absorption of electromagnetic energy by a material is a phenomenon that
underlies many applied problems, including molecular sensing, photocurrent
generation and photodetection. Commonly, the incident energy is delivered to
the system through a single channel, for example by a plane wave incident on
one side of an absorber. However, absorption can be made much more efficient by
exploiting wave interference. A coherent perfect absorber is a system in which
complete absorption of electromagnetic radiation is achieved by controlling the
interference of multiple incident waves. Here, we review recent advances in the
design and applications of such devices. We present the theoretical principles
underlying the phenomenon of coherent perfect absorption and give an overview
of the photonic structures in which it can be realized, including planar and
guided-mode structures, graphene-based systems, parity- and time-symmetric
structures, 3D structures and quantum-mechanical systems. We then discuss
possible applications of coherent perfect absorption in nanophotonics and,
finally, we survey the perspectives for the future of this field.
