Enabling infinite Q factors in absorbing optical systems

Resonant optical structures have widespread applications in science and technology. However, their quality (Q) factors can be significantly deteriorated, if some of their parts exhibit optical absorption. Here, we show that by coupling a lossy mode of such a structure to two independent lossless modes, one can create a nonradiating and absorption-free bound state in the continuum (BIC). The Q factor of such a BIC is theoretically unlimited despite interaction with an absorbing structure. We use this mechanism to design a plasmonic metasurface with Q factors that are close to 107 in the visible spectral range. The proposed mechanism is general and can be used to engineer ultrahigh-Q resonances in various systems containing absorbing structures.