Observation of trapped light within the radiation continuum

Theoretical and experimental studies reveal that light can be confined within a planar dielectric photonic crystal slab even though the frequency of this optical bound state is inside the continuous spectrum of extended states from the same symmetry group. The confinement of light has many practical applications — in optical fibres, waveguides and photonics for instance — but current methods of confinement all use materials or systems such as metallic mirrors that forbid outgoing waves. Here Chia Wei Hsu et al. demonstrate a new approach to confinement that does not rely on mirrors. The authors calculate and go on to show experimentally that for a patterned dielectric slab certain stable optical 'bound states' exist that trap light even though outgoing waves are allowed in the surrounding medium. This new method of trapping electromagnetic waves has wavevector and wavelength selectivity, so is suitable for optical filters, modulators and waveguides. In addition, it can be used with electronic and mechanical waves as well as light. 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 outgoing 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 proposals5,6,7,8. 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 incompatibility5,6,7,8,10,11,12,13,14,15,16. 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.