  Surface polaritons display short wavelengths compared to propagating light,
thus enabling large spatial concentration and enhancement of electromagnetic
energy. However, this wavelength mismatch is generally accompanied by poor
light-to-polaritons coupling that limits potential applications in areas such
as optical sensing and optoelectronics. Here, we address this problem by
demonstrating that a small scatterer placed at a suitable distance from a
planar surface can produce complete coupling of a focused light beam to surface
polaritons. We present rigorous theoretical results for light coupling to
plasmons in silver films and graphene, as well as phonon polaritons in
hexagonal boron nitride films. We further formulate detailed general
prescriptions on the beam profile and particle response that are required to
achieve maximum coupling, which we supplement by analytical calculations for
dipolar scatterers and finite-size particles. Our results open a practical
route to circumvent the long-standing photon-polariton wavelength mismatch
problem in nanophotonics.
