Bound states in the continuum in anisotropic structures

Bound states in the continuum (BICs) are radiationless localized states embedded in the part of the parameter space that otherwise corresponds to radiative modes. They were predicted in the field of quantum optics [1], first demonstrated in acoustic systems [2] and recently in photonic systems [3]. BICs discovered to date are almost-pure transverse-electric (TE) or transverse-magnetic (TM). They coexists with guided modes in symmetric photonic structures at a fixed propagation direction and frequency [4]. We address a mechanism, based on waveguiding structures containing anisotropic birefringent materials, which affords the existence of BICs with fundamentally-new properties [5]. In particular, anisotropy-induced BICs exists in symmetric and asymmetric geometries; along tunable angular propagation directions, with pure transverse-electric, pure transverse-magnetic or full-vector hybrid polarization, and they may be the only possible bound states of properly designed structures, thus appearing as a discrete, isolated bound state embedded in a whole sea of radiative states. In this presentation we introduce the concept and explore the existence of BICs in more general anisotropic structures.