Nanophotonic nonlinear and laser devices exploiting bound states in the continuum

The quality factor (Q), describing the rate of energy loss from a resonator, is a defining performance metric for nanophotonic devices. Suppressing cavity radiative losses enables strong nonlinear optical responses or low-power operation to be achieved. Exploiting long-lived, spatially-confined bound states in the continuum (BICs) has emerged from the numerous approaches considered as a promising route to boost nanophotonic Q factors. Initial research explored the formation mechanisms of various types of BICs, drawing parallels to topological physics. With these fundamentals now established, we review the recent application of BICs in passive and active nanophotonic devices.