  Reversible, nonvolatile, and pronounced refractive index modulation is an
unprecedented combination of properties enabled by chalcogenide phase-change
materials (PCMs). This combination of properties makes PCMs a fast-growing
platform for active, low-energy nanophotonics, including tunability to
otherwise passive thin-film optical coatings. Here, we integrate the PCM Sb2Se3
into a novel four-layer thin-film optical coating that exploits photonic Fano
resonances to achieve tunable structural colors in both reflection and
transmission. We show, contrary to traditional coatings, that Fano-resonant
optical coatings (FROCs) allow for achieving transmissive and reflective
structures with narrowband peaks at the same resonant wavelength. Moreover, we
demonstrate asymmetric optical response in reflection, where Fano resonance and
narrow-band filtering are observed depending upon the light incidence side.
Finally, we use a multi-objective inverse design via machine learning (ML) to
provide a wide range of solution sets with optimized structures while providing
information on the performance limitations of the PCM-based FROCs. Adding
tunability to the newly introduced Fano-resonant optical coatings opens various
applications in spectral and beam splitting, and simultaneous reflective and
transmissive displays, diffractive objects, and holograms.
