20 March 2017 BICs in Nature Photonics

ht cone (grey surface) for a structure composed of a negative birefringent core on a positive birefringent substrate

ICFO researchers discover a new way to generate bound states in the continuum. Controlling the propagation of waves is crucial for technologies that implement electromagnetic waves, acoustic waves in air, water waves and elastic waves in solids. The confinement of propagating waves allows scientists to control and manipulate at will the wave-based applications at use.

A wave is confined or “bounded” within a system when its frequency is below the cone of light. As one would expect, a wave with frequency within the continuum spectrum of radiation, known as resonance, it is only partially confined, and radiates energy outwards. However, there are bound states in the continuum (BICs) in which waves are confined within a continuum spectrum of radiation but do not radiate at all, their existence defying conventional wisdom.

Although BICs were first proposed in quantum mechanics, they are a general wave phenomenon and have since been used in a wide variety of material systems such as photonic crystals, optical waveguides and fibers, quantum dots, and acoustic materials.

In a recent paper published in Nature Photonics, ICFO researchers Dr. Jordi Gomis, Dr. David Artigas, led by Prof. Lluis Torner, developed a mechanism, based on wave-guiding structures that contain anisotropic birefringent materials, that affords the existence of BICs with fundamentally 
new properties.

They have observed that full-vector BICs exhibit tunable angular propagation direction and tunable polarization hybridity, as well as highly directional, ultrasharp transitions from radiationless to radiative states, properties that may find applications in photonic filters, spatial-light modulators and sensors based on angular selectivity. They also foresee these similar phenomena in off-plane geometries, multilayer and multimaterial structures, biaxial crystals and generalized anisotropic media, such as chiral and hyperbolic materials, as well as in engineered natural and artificial materials crafted in geometries, including high-contrast ultrathin structures and metasurfaces, with form anisotropy.

The results of this study open a new pathway toward the development of new trapping and confinement techniques with BICs, which could be of significant use for the advancement of wave-based technologies.