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The ability to confine light is important both scientifically and technologically. Many light confinement methods exist, but they all achieve confinement with materials or systems that forbid out- going waves. These systems can be implemented by metallic mirrors, by photonic band-gap materials1, by highly disordered media (Anderson localization2) and, for a subset of outgoing waves, by translational symmetry (total internal reflection1) or by rotational or reflection symmetry3,4. Exceptions to these examples exist only in theoretical proposals5-8. Here we predict and show experimentally that light can be perfectly confined in a patterned dielectric slab, even though outgoing waves are allowed in the surrounding medium. Technically, this is an observation of an 'embedded eigenvalue'9-namely, a bound state in a continuum of radiation modes-that is not due to symmetry incompatibility5"8,10"16. Such a bound state can exist stably in a general class of geometries in which all of its radiation amplitudes vanish simultaneously as a result of destructive interference. This method to trap electromagnetic waves is also applicable to electronic12 and mechanical waves14,15.
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The propagation of waves can be easily understood from the wave equation, but the localization of waves (creation of bound states) is more complex. Typically, wave localization can be achieved only when suitable outgoing waves either do not exist or are forbidden owing to symmetry incompatibility. For electromagnetic waves this is commonly imple- mented with metals, photonic bandgaps or total internal reflections; for electron waves this is commonly achieved with potential barriers. In 1929 von Neumann and Wigner proposed the first counterexample10, in which they designed a quantum potential to trap an electron whose energy would normally allow coupling to outgoing waves. However, this artificially designed potential does not exist in reality. Further- more, the trapping is destroyed by any generic perturbation to the potential. More recently, other counterexamples have been proposed theoretically in quantum systems11"13, photonics5"8, acoustic and water waves1415 and mathematics16; the proposed systems in refs 6 and 14 are most closely related to what is demonstrated here. Although no general explanation exists, some cases have been interpreted as two interfering resonances that leave one resonance with zero width61 U2. Among these proposals, most cannot be readily realized because of their inherent fragility. A different form of embedded...