Perfect coupling of light to surface plasmons by coherent absorption

We show theoretically that coherent light can be completely absorbed in a two-dimensional or three-dimensional metallic nanostructure by matching the frequency and field pattern of an incident wave to that of a localized surface plasmon resonance. This can be regarded as critical coupling to a nano-plasmonic cavity, or as an extension of the concept of time-reversed laser to the spaser. Light scattering is completely suppressed via impedance matching to the nano-objects, and the energy of incoming wave is fully transferred to surface plasmon oscillations and evanescent electromagnetic fields. Perfect coupling of light to nanostructures has potential applications to nanoscale probing as well as background-free spectroscopy and ultrasensitive detection of environmental changes.