  Thin film interference is integral to modern photonics and optoelectronics,
e.g. allowing for precise design of high performance optical filters1,
efficiency enhancements in photovoltaics and light-emitting devices, as well as
the realization of microlasers and high-performance photodetectors. However,
interference inevitably leads to a change of spectral characteristics with
angle, which is generally undesired and can limit the usefulness of thin-film
coatings and devices. Here, we introduce a strategy to overcome this
fundamental limit in optics by utilizing and tuning the exciton-polariton modes
arising in ultra-strongly coupled microcavities. We demonstrate optical filters
with narrow pass bands that shift by less than their half width (<15 nm) even
at extreme angles. Our filters cover the entire visible range and surpass
comparable metal-dielectric-metal filters in all relevant metrics. By expanding
this strategy to strong coupling with the photonic sidebands of dielectric
multilayer stacks, we also obtain filters with high extinction ratios. Based on
these findings, we realize ultrathin and flexible narrowband filter films,
monolithically integrate our filters with organic photodiodes, and demonstrate
polarization-sensitive polariton filters. These results illustrate how strong
coupling provides additional degrees of freedom in thin film optics that will
enable a multitude of exciting new applications in micro-optics, sensing, and
biophotonics.
