Perfect coupling of light to surface plasmons by coherent absorption

We show theoretically that coherent light can be completely absorbed and transferred to surface plasmons in a two-or three-dimensional metallic nanostructure by exciting it with the time-reversed mode of the corresponding surface plasmon laser (spaser''). The narrow-band perfect absorption is a generalization and application of the concept of critical coupling to a nanocavity with surface plasmon resonances. Perfect coupling of light to nanostructures has potential applications to nanoscale probing as well as background-free spectroscopy and ultrasensitive detection or sensing.