Wafer-scale fabrication of single-crystalline calcium fluoride thin-film on insulator by ion-cutting

Developing bulk materials into thin-films not only enables multiple physical fields to interact strongly within a smaller volume but also enhances the stability, scalability, and integrability of the system. This transformation has been witnessed to lead to significant improvements in the performance of devices utilizing diverse materials across various fields. Calcium Fluoride (CaF2), with remarkable optical properties such as ultralow optical loss and strong nonlinear features, is a preferable material for building ultra-high-Q resonators and stimulating light-matter interaction with low threshold. However, the integration of CaF2 material has remained elusive due to its low hardness and high thermal expansion, which plagues the large-scale fabrication of CaF2 thin film. Here, an optimized ion-cutting technique is proposed for engineering CaF2 thin films theoretically and experimentally. Taking the thermal expansion into consideration, we employ the Lithium niobate (LN) as substrate, and utilize accessible ion implantation, wafer bonding and precisely designed annealing process to attain the CaF2-LN and the CaF2-on-insulator (CaFOI) with high film thickness uniformity and maintained crystalline quality. The perspectives of our work are also illustrated by numerical simulation for further verifying the potential of integrated photonic applications, which could be extended to monolithic optical network by means of large-scale manufacturing method.