Surface-activated bonded Cr:ZnS-on-sapphire: towards active integrated photonic platform for the mid-infrared

Mid-infrared (MIR) photonic integration is pivotal for applications including molecular fingerprinting and remote sensing. We report Cr:ZnS-Al2O3-sapphire heterostructures fabricated via surface-activated bonding. Through the introduction of atomic-level surface activation and the deposition of an Al2O3 buffer layer, the stress caused by lattice mismatch is effectively alleviated while the diffusion of constituent elements is extremely low, maintaining high optical quality of the heterogeneous structure. The heterostructure achieves a continuous-wave output power of 2.34 W, a 77% enhancement over pristine Cr:ZnS. Thermo-optical simulations show a 12 K temperature reduction under an incident pump power of 6 W. Leveraging the intrinsic negative group delay dispersion of sapphire and chirped mirrors, the net intra-cavity dispersion is reduced to -117 fs², enabling direct Kerr-lens mode-locking and generating sub-100 fs pulses at 2362 nm with the average output power of 550 mW. This approach offers a novel composite structure that can be used as integrated photonic platform with on-chip gain for the MIR.