Similar to quantum mechanics, where we use potential wells to confine particles, in optics we use dielectric or metallic cavities to confine light. Confining light without absorbing it is of particular importance. Conventional optical cavities trap light using mirrors, total internal reflection or scattering in periodic microstructures such as photonic crystals. For these systems, the quality factor — a measure of the efficiency of the light storage — is limited due to radiative and Joule losses. An alternative way to confine light is to use exotic optical ‘modes’ called bound states in the continuum (BICs), which can give rise to extremely large quality factors, depending on the systems’ geometry. Such modes can be used, e.g. in compact lasers that operate at telecommunication wavelengths, which could have many applications [Nature 541, 196 (2017)]. BICs were first theorized in the early days of quantum mechanics, when it was discovered that certain potential-energy profiles (potentials) could support spatially localized states of electrons that have energies larger than the maximum energy of the potential. For several decades, these states were considered to be only a mathematical curiosity. However, the advanced theory of resonances revealed that BICs can occur in many systems that support waves and wave propagation. Because of the generality of this effect, BICs are not restricted to quantum mechanical systems — they are general wave phenomena found in electromagnetic waves, water and elastic waves, and in acoustic waves in air, and have been shown to exist as a special type of surface state.
In this project we will study BICs in various geometries to gain deep understanding of this concept and its potential applications in various parts of electromagnetic spectrum.