All-Optical Generation of Quasi-Bound States in the Continuum Resonances in a Homogeneous Dielectric Thin Film

The concept of Bound States in the Continuum (BICs) was introduced in the context of localization of electronic wavefunctions [1], being demonstrated in various wave systems, including optics [2]. In such phenomena, a spatial localization of wave states occurs within a continuum of (non-localized) states. True BICs can only exist in lossless and infinite media, at least in one dimension [3]. Some kinds of BICs present symmetry properties which prevent their coupling to propagating optical modes, thus being called symmetry-protected BICs [4]. Finite photonic systems like metasurfaces (MSs) may present, however, a far-field-accessible counterpart of the BICs, the so-called quasi-BICs (qBICs), which start to be optically detectable after some degree of asymmetry is introduced in the MS unit cell [4]. This asymmetry can be of geometrical nature [5] or be introduced in geometrically symmetric unit cells composed by meta-atoms with materials possessing different dielectric permittivities [6]. In all the cases up to now, MSs, obtained via complex nanofabrication processes with normally unchangeable properties, were necessary to optically demonstrate the existence of qBIC states and investigate their optical characteristics, like near-field enhancement, frequency conversion or incident polarization dependence.