Active Ultrasensitive Phase Singularities with Bound States in the Continuum

We demonstrate that phase singularities, akin to the singularities of BICs in reciprocal space, emerge in dielectric metasurfaces with finite losses. In these systems, losses play a crucial role in achieving the phase singularities within the phase spectra. These phase singularities are robust across parameter space, exhibit sharp phase transitions highly sensitive to external disturbances, and lead to infinite group delays, significantly enhancing sensitivity. By introducing an external pump channel, we can regulate nonradiative losses, thereby actively tuning both the operating conditions and sensitivity of the system. The proposed metasurfaces also improve resonance quality factors through BIC-enabled phase singularities, further enhancing the sensing figure of merit via increased local fields. This active approach to dielectric metasurfaces offers a flexible means of accessing phase singularities, supporting the development of ultrasensitive optoelectronic devices for sensing and detection applications, where the radiative loss channel is governed by BIC physics, and the nonradiative loss channel is modulated by an external pump source.