When could dielectric gratings indefinitely trap light with infinite lifetime, or Q factor? Recently, MIT researchers discovered a new phenomenon known as optical bounded states in the continuum (BICs): light is perfectly confined even though there do exist continuum outgoing waves that can carry energy away. This phenomenon is unexpected since conventional methods, such as resonators, photonic bandgap, and Anderson localization, confine light but do not support outgoing waves. Taking advantage of the infinite Q factor, optical BICs can greatly enhance light-matter interactions and have already found promising applications in lasing, nonlinear optics, optical filtering, and sensing. For a planar BIC supporting structure, specifically a dielectric grating, Bulgakov et al. discuss the importance of geometric symmetry for supporting Bloch BICs that propagate along the grating. Their interesting findings will enable the design of novel optical elements, which steer the flow of light harvested from the ambient medium and which can find exciting applications, especially in solar cells, photodetectors, and biomedical sensors.
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