Ultra-high-Q chalcogenide glass on-chip resonators attaining internal impurity-limited loss in the mid-infrared

High-Q on-chip resonators are essential for elevating the efficiency and performance of on-chip functional devices. However, achieving their loss performance that approaches the ultra-low loss of optical fiber in the mid-infrared region has remained a challenge. Here, we demonstrate ultra-high-Q mid-infrared on-chip microresonators based on multiple chalcogenide glasses, exhibiting optical loss characteristics comparable to those of state-of-the-art optical fibers. By suppressing optical losses from scattering and molecular absorption, Q-factors exceeding 10 million were achieved across the 3.2 to 4.6 µm range, peaking at 67 million (0.29 dB/m) at 3.91 µm. The loss performance of these resonators is fundamentally limited by the intrinsic material losses, as good agreement with the optical loss spectrum of ChG fibers. In addition, the absence of absorption peaks induced by Se-H in the As₂Se₃ core layer is consistent with the spectral characteristics of Se-H-free fibers, indicating that the deposited film retained the purity of its evaporation source. This result underscores the importance of mitigating internal impurities at the chip scale to achieve low optical loss in the mid-infrared.