Efficient silicon-erbium photonic hybrids with flexible spatial control of light via bound states in the continuum

The hybrid of silicon and erbium-doped crystals provides a promising photonic platform for the advancement of quantum technologies, combining scalability, telecom compatibility, and complex quantum functionalities. However, the inherent high refractive index of silicon leads to optical field confinement within the silicon, limiting the number of optically active erbium ions and the achievable light absorption, which are essential for various quantum applications. To fully utilize the potential of erbium-doped crystals, we implemented bound states in the continuum within the hybrid silicon structures, which achieve substantial optical energy confinement within the lower-index crystal. Our structure, which is applicable to both transverse magnetic and transverse electric polarization modes, effectively increases the number of optically active erbium ions and demonstrates simultaneous minimal radiative losses and enhanced erbium absorption. Moreover, the core design principle is extensible to other low-refractive-index materials.