Utilizing thin-film interference, one can engineer spectrally narrowband emission, transmission, or absorption for a multitude of applications. However, this usually comes at the cost of a strong angular dependence of the spectrum. Here, we showcase how ultra-strong light-matter coupling can be employed to remedy this strong dispersion and achieve a narrowband, angle-independent response. Coupling cavity photons to material excitons and fine-tuning the properties of the resulting exciton-polariton quasiparticle allows us to exchange the parabolic dispersion of the cavity photon for a flat, exciton-like dispersion. We demonstrate how this principle can be implemented in narrowband transmission filters, photodetectors and highly efficient organic light-emitting diodes.
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