Monolithically Integrated Si-on-AlN Mid-Infrared Photonic Chips for Real-Time and Label-Free Chemical Sensing.

Chip-scale chemical sensors were demonstrated using optical waveguides consisting of amorphous silicon (a-Si) and aluminum nitride (AlN). A mid-infrared (mid-IR) transparent AlN thin film was prepared by room-temperature sputtering, which exhibited high Al/N elemental homogeneity. The Si-on-AlN waveguides were fabricated by a complementary metal-oxide-semiconductor process. A sharp fundamental mode and low optical loss of 2.21 dB/cm were obtained. Label-free chemical identification and real-time monitoring were performed by scanning the mode spectrum while the waveguide was exposed to various chemicals. Continuous tracing of heptane and methanol was accomplished by measuring the waveguide intensity attenuation at λ = 2.5-3.0 μm, which included the characteristic -CH and -OH absorptions. The monolithically integrated Si-on-AlN waveguides established a new sensor platform that can operate over a broad mid-IR regime, thus enabling photonic chips for label-free chemical detection.