  Flat optics has demonstrated great advances in miniaturizing conventional,
bulky optical elements due to the recent developments in metasurface design.
Specific applications of such designs include spatial differentiation and the
compression of free space. However, metasurfaces designed for such applications
are often polarization-dependent and are designed for a single functionality.
In this work, we introduce a polarization-independent metasurface structure by
designing guided resonances with degenerate band curvatures in a photonic
crystal slab. Our device can perform both free-space compression and spatial
differentiation when operated at different frequencies at normal incidence.
This work demonstrates the promise of dispersion engineering in metasurface
design to create ultrathin devices with polarization-independent functionality.
