  Owing to their ability to concentrate light on nanometer scales, plasmonic
surface structures are ideally suited for on-chip functionalization with
nonlinear or gain materials. However, achieving a high effective quantum yield
across a surface not only requires strong light localization but also control
over losses. Here, we report on a particular class of tunable low-loss
metasurfaces featuring dense arrangements of nanometer sized focal points on a
photonic chip with an underlying waveguide channel. Guided within the plane,
the photonic wave evanescently couples to the nano-gaps, concentrating light in
a lattice of hot-spots. In studying the energy transfer between photonic and
plasmonic channels of single trimer molecules and triangular nano-gap tilings
in dependence on element size, we identify different regimes of operation. We
show that the product of field enhancement, propagation length and element size
is close-to-constant in both the radiative and subwavelength regimes, opening
pathways for device designs that combine high field enhancements with large
propagation lengths.
