Embodiments of the disclosure provide a waveguide-confining layer, a photonic integrated circuit (PIC) die with embodiments of a waveguide-confining layer, and methods to form the same. The waveguide-confining layer may include an oxide layer over a buried insulator layer, a silicon-based optical confinement structure embedded within or positioned on the oxide layer, and first and second blocking layers over the oxide layer and separated from each other by a horizontal slot. The first and second blocking layers include a metal or an oxide. A gain medium is positioned on the oxide layer and within the horizontal slot between the first and second blocking layers, and has a lower refractive index than each of the first and second blocking layers. The gain medium is vertically aligned with the silicon-based optical confinement structure, and a portion of the oxide layer separates the gain medium from the silicon-based optical confinement structure.