Observation of trapped light within the radiation continuum

Abstract

⇤ These authors contributed equally to this work.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. Such systems can be implemented by metallic mirrors, by photonic band-gap materials 1 , by highly disordered media (Anderson localization 2 ) and, for a subset of outgoing waves, by translational symmetry (total internal reflection 1 ) or rotation/reflection symmetry 3, 4 . Exceptions to these examples exist only in theoretical proposals [5][6][7][8] . Here we predict and experimentally demonstrate 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 incompatibility [5][6][7][8][10][11][12][13][14][15][16] . Such a bound 1 state can exist stably in a general class of geometries where all of its radiation amplitudes vanish simultaneously due to destructive interference. This method to trap electromagnetic waves is also applicable to electronic 12 and mechanical waves 14,15 .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 only be achieved when suitable outgoing waves either do not exist or are forbidden due to symmetry incompatibility. For electromagnetic waves, this is commonly implemented 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 counterexample 10 , in which they designed a quantum potential to trap an electron whose energy would normally allow coupling to outgoing waves. However, such artificially designed potential does not exist in reality. Furthermore, the trapping is destroyed by any generic perturbation to the potential. More recently, other counterexamples have been proposed theoretically in quantum systems 11-13 , photonics 5-8 , acoustic and water waves 14,15 , and mathematics 16 ; the proposed systems in refs. 6 and 14 are most closely related to what is demonstrated here. While no general explanation exists, some cases have been interpreted as two interfering resonances that leaves one resonance with zero width 6,11,12 . Among these many proposals, most cannot be readily realized due to their inherent fragility. A different form of embedded eigenvalue has been realized in symmetry-protected systems 3, 4 , where no outgoing wave exists for modes of a particular symmetry.To show that an optical bound state is feasible even when it is surrounded by symmetrycompatible radiation modes, we consider a practical structure: a dielectric slab with a square array 2 of cylindrical holes (Fig. 1a), an example of photonic crystal (PhC) slab 1 . The periodic geomet...

Fiber guiding at the Dirac frequency beyond photonic bandgaps

“…The fact that there is not a band of true localized modes around the Dirac frequency gives the mode special properties such as an algebraic profile and sharp frequency response. This light-trapping phenomenon occurs at a single frequency and should be distinguished from a recent observation of an 'embedded eigenvalue' 22,23 that also occurs at a single frequency. The existence of the Dirac mode is due to disappearance of radiation states at the Dirac frequency, whereas the trapped light in Refs.…”

Spectrally selective chiral silicon metasurfaces based on infrared Fano resonances

“…Such angular sensitivity prevents miniaturization of photonic crystal devices 22,26,32 and imposes severe restrictions on the angular divergence of the incident light beams. While highly collimated laser beams have been used for interrogating high-Q photonic crystal structures in the visible and telecommunication spectral ranges 25,26,31,33 , the angular divergence of incoherent beams used for mid-infrared spectroscopy is typically prohibitively high for utilizing GRMs supported by photonic crystals. Note that high angular sensitivity is also typical for the frequency-selective surfaces 34 that can be thought of as microwave predecessors 1 of metasurfaces.…”

“…28) of such photonic resonances are often caused by the so-called guided resonance modes (GRMs) 22,23,29 . The spectrally narrow linewidth of these modes originates from the suppression of their radiative losses through the long-range destructive interference between multiple unit cells of a photonic crystal and, therefore, is extremely sensitive to the light's incidence angle 22,24,25,28,30,31 . Such angular sensitivity prevents miniaturization of photonic crystal devices 22,26,32 and imposes severe restrictions on the angular divergence of the incident light beams.…”

Anisotropy-induced photonic bound states in the continuum

“…Many decades after their original prediction 1,2 and early observations in acoustic systems 3 , such states have been recently demonstrated in photonic structures with engineered geometries [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] . In this paper we put forward a mechanism, based on waveguiding structures containing anisotropic birefringent materials, which affords the existence of BICs that exhibit fundamentally-new properties.…”

“…BICs discovered to date are almost-pure transverse-electric (TE) or transversemagnetic (TM) waves 4,[7][8][9][19][20][21] , namely with a negligible fraction of energy being carried by the respective orthogonal polarization. The corresponding trapping mechanism can thus be intuitively viewed as a scalar or spinor potential.…”