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Abstract
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. [PUBLICATION ABSTRACT]