In this work, we discussed a novel light coupling mechanism that allows one to obtain huge filed enhancements at the interface of designer photonic structures. The phenomenon is related to so-called bound states in the continuum of radiation modes. The mechanism of a bound state mode formation stabilizes the trapped electromagnetic field controlling its amplification, which increases with increasing mode lifetime. We presented numerical simulations and experimental results demonstrating that a thin-slab photonic crystal in a high refractive index material can support very narrow resonances with a g-factor (~ 105). These resonances are connected with an extremely large field enhancement, as large as 700-fold the amplitude of the incident wave. As a first experimental characterization, we applied our PhC metasurfaces supporting BIC modes to enhance the fluorescence emission of standard fluorophores dispersed over the crystal matrix. We demonstrated state-of-the-art SEF enhancement factor for an all-dielectric structure. Our approach is highly robust, loss-free and easily scalable for real-world applications. In addition, multiplatform applications can be envisaged. We expect our approach might provide new routes to light manipulation at the nanoscale, especially for sensing and nonlinear optics applications.
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