Paradigms for Realizing Symmetry-Guaranteed Paris of Bound States in the Continuum and Identifying Super-Mossian Materials

Over the last decade, all-dielectric metasurfaces have emerged as a powerful platform for enhancing light-matter interactions, enabling device performance improvements across a range of applications. To increase light-matter interactions, photonic states with long lifetimes (high-quality factors) are desirable, requiring improvements in photonic designs, material quality, and fabrication tolerances. In this talk, I will present one structural design paradigm and one material search heuristic for improving all-dielectric metasurfaces.<br/><br/>Bound states in the continuum are states that are completely decoupled from a continuum leading to infinitely long lifetimes. In practice, while states with infinitely long lifetimes are undesirable, they enable a design paradigm that solves the photonic design component leaving material quality and fabrication tolerances as the primary means to achieve arbitrary lifetimes. Demonstrations using bound states in the continuum have enabled improvements for lasers, sensors, and frequency mixers. However, many applications require degenerate or nearly degenerate high-quality factor (Q) states, such as spontaneous parametric down conversion, non-linear four-wave mixing, and intra-cavity difference frequency mixing for terahertz generation. I will present a group theory approach based on lattices with six-fold rotational symmetry for creating symmetry-guaranteed pairs of bound states in the continuum. Using this design paradigm, the splitting and Q-factors can be independently and robustly controlled creating new pathways for improving metasurfaces for frequency conversion, heralded single-photon production, and sensing.<br/><br/>Finally, I will discuss a search heuristic for materials with refractive indices that surpass the common form of the Moss rule (n⁴E_g ≈ 95 eV) leading to the creation of super-Mossian materials. With this heuristic, I will present an experimental realization of a super-Mossian metasurface using iron pyrite (FeS₂), with refractive index greater than 4.37 and a bandgap of 1.03 eV, surpassing the common for of the Moss rule by nearly 40%.<br/><br/>Work supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (grant BES 20-017574). The work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the US Department of Energy, Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the US Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the US Department of Energy or the United States government.