Spatially Encoded Polaritonic Ultra‐Strong Coupling in Gradient Metasurfaces with Epsilon‐Near‐Zero Modes

Abstract

Abstract A platform is introduced to achieve ultra‐strong coupling (USC) between light and matter using widely available materials. USC is a light–matter interaction regime characterized by coupling strengths exceeding 10% of the ground state energy. It gives rise to novel physical phenomena, such as efficient single‐photon coupling and quantum gates, with applications in quantum sensing, nonlinear optics, and low‐threshold lasing. Although early demonstrations in plasmonic systems have been realized, achieving USC in dielectric platforms, which offer lower losses and high Q‐factors, remains challenging due to typically low mode overlap between the photonic field and the material resonance. Here, dielectric dual gradient metasurfaces supporting quasi‐bound‐states‐in‐the‐continuum are leveraged to spatially encode both the spectral and coupling parameter space and demonstrate USC to an epsilon‐near‐zero (ENZ) mode in an ultra‐thin SiO 2 layer. The strong out‐of‐plane electric fields in tapered bar structure overlap exceptionally well with those of the ENZ mode, resulting in a normalized coupling strength of η = 0.10 and a mode splitting equivalent to 20% of the ENZ mode energy; a four‐to‐five‐fold increase compared to previous approaches. The strong field confinement of the approach opens new possibilities for compact and scalable polaritonic devices, such as tunable frequency converters and low‐energy optical modulators.