Low-cost scalable fabrication of functionalized optical waveguide arrays for gas sensing application

Abstract Thermal imprinting, a technique proposed more than four decades ago, is known for its cost-efficiency and high throughput in microstructuring capability. The process involves the structuring of a substrate with a patterned working stamp under certain conditions. The parameters applied in the process effectively determine the replication quality and accuracy. In many cases, there is still no comprehensive study on the effect of system parameters or technical aspects on replication quality. To address this gap, we demonstrated a systematic study of the process for fabricating functional optical structures using a home-built thermal imprinting setup. On this basis, optimization strategies (i.e., reformation of the PDMS mixing time, tuning of the curing process, and optimization of imprinting temperature) were proposed and developed in both the working stamping and the thermal imprinting processes to enhance the structuring accuracy and quality as well as reproducibility in this work. With our optimizations, improved replication accuracies with a dimension difference down to 0.14% for the pattern transfer from silicon master mold onto polydimethylsiloxane (PDMS) working stamp and 0.04% (compared to silicon master mold) for the pattern imprinted onto polymethyl methacrylate (PMMA) substrate were achieved, respectively. The replication patterns on PDMS working stamp and the imprinted structures on PMMA exhibit a roughness of 24 ± 3 nm and 101 ± 8 nm, respectively, with high structuring quality. In this work, we also demonstrated the gas sensing functionality of thermal-imprinted waveguides by integrating them with a thin metal–organic framework film, which features porous structures enabling the adsorption of gas molecules and serves as the sensing layer. In our experiments, a micro-Watt power was used for the sensing performance characterization. The integrated optical sensor device exhibits high sensibility to CO 2 , with a relative output power decrease of 2.8 µW when it was exposed to CO 2 . In addition, an adsorption time of 28 s and desorption time of 61 s were demonstrated, respectively. This work opens an attractive path for the development of low-cost, scalable, and flexible on-chip optical sensors for gas detection and industrial monitoring.