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

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 SiO₂ 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.