A Coherent perfect absorber is:
An optical device has been designed that performs a function exactly opposite to that of a laser. It perfectly absorbs incoming coherent radiation and turns it into thermal or electrical energy.
Backward lasing yields a perfect absorber
Just as a laser can emit coherent light from an amplifying medium, an absorbing medium can perfectly capture incoming radiation under the right conditions
CPA-based interferometers could thus have potential applications in the realization of transducers, modulators, or optical switches, for example, in on-chip integrated optical circuits based on Si waveguide/resonator technology.