  Diffractively coupled nanoparticle arrays are promising candidates for
helping to flatten many photonic devices such as lasers, lenses, and metrology
instruments. Their performance, however, is directly linked with the size of
the metasurfaces, limiting their applicability in nanophotonic applications.
Here, we dramatically reduce array sizes of high-$Q$-factor metasurfaces by
utilizing strongly dispersive media. The effect is demonstrated by
theoretically and numerically studying periodic arrays of plasmonic
nanoparticles embedded inside Bragg-reflector waveguides. We demonstrate array
dimensions reduction up to two orders of magnitude while still achieving
ultra-high $Q$-factors in excess of 10$^4$.
