Bound states within the radiation continuum in diffraction gratings and the role of leaky modes

We discuss resonant states with diverging Q factor within the radiation continuum based on the anomalous interaction of leaky guided modes and diffracted waves in suitably designed reflection gratings. We show that these trapped optical states can be understood within the framework of leaky-wave theory, which unveils their generation process and dynamics. Our findings reveal an interesting mechanism to realize embedded eigenstates in periodic structures, shedding light on their electromagnetic properties, and offering the possibility to quantitatively predict their occurrence and systematically design optimal structures that support them. The realization of extraordinary optical trapping in open structures may be important for applications that require strongly confined and enhanced fields and high selectivity in angle and frequency.