  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.
