  Engineering the dispersion of light in a metasurface allows for controlling
the light-matter interaction strength between light confined in the metasurface
and materials placed within its near-field. Specifically, engineering a
flatband dispersion increases the photonic density of states thereby enhancing
the light-matter interaction. Here, we experimentally demonstrate a metasurface
with a flat dispersion at visible wavelengths. We designed and fabricated a
suspended one-dimensional gallium phosphide metasurface and measured the
photonic band structure via energy-momentum spectroscopy, observing a photonic
band that is flat over $10^o$ of half-angle at $\sim 580$nm. We integrated
cadmium selenide nanoplatelets with the metasurface, and measured coupled
photoluminescence into the flatband. Our demonstration of a photonic flatband
will enable the possibility of integrating emerging quantum emitters to the
metasurface with possible applications in nonlinear image processing, and
topological photonics.
