Frozen light in a near-zero index metasurface

We discuss the nature of leaky modes with large phase velocity and an effective refractive index and group velocity near zero in planar arrays of subwavelength dielectric or metallic spheres far from their resonances. Large field enhancements with $Q$ factors exceeding $10{}^{10}$ are theoretically predicted for these modes. Those theoretical findings may have important applications in optical switching and memories, quantum information processing, optical storage, and enhancement of optical nonlinearities. Preliminary experimental results at microwave frequencies qualitatively confirm these predictions.