Overcoming angle dependence in thin-film optical coatings by ultra-strong light matter coupling

Thin-film interference is essential in optoelectronic devices, allowing to manipulate their spectral characteristics such as emission, transmission, and absorption profiles via constructive and destructive interference. However, photonic systems relying on optical thin films face the fundamental challenge of blue shifts at non-normal angles of incidence. Here, we demonstrate how this issue can be eliminated by utilizing ultra-strong coupling between cavity photons and material excitons, thereby transitioning from a curved photonic to a flat exciton-like dispersion. This technique is especially effective when using organic materials with high oscillator strengths and enables angle-independent, narrowband responses. We demonstrate both narrowband and broadband, angle-independent transmission filters in metallic cavity filters and generalize this enhanced performance to distributed Bragg reflector-like dielectric stacks, i.e. dielectric filters, as well as organic LEDs. Our approach is poised to benefit micro-optics, biophotonics, and other fields that face light collimation challenges, and demonstrates a practical utility of strong coupling.