Highly Sensitive Free-Standing Waveguide-Integrated Bolometer on Germanium-on-Insulator Platform for Mid-Infrared On-Chip Spectroscopy

The mid-infrared (MIR) on-chip spectroscopy has emerged as a promising solution to overcome the limitations of traditional bulky optical gas sensors. However, achieving the stringent limit of detection (LoD) required for various applications remains a significant challenge. The LoD of on-chip spectrometers is closely tied to the performance of key optical components, particularly the detectors. Here, we demonstrate a highly sensitive free-standing waveguide-integrated bolometer on a germanium-on-insulator platform. The device leverages an air trench structure for enhanced thermal isolation, significantly boosting thermal efficiency and ensuring thermal stability. As a result, we achieved a remarkable noise equivalent power (NEP) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.7\times 10^{-{8}}$ </tex-math></inline-formula> W/Hz<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text {1/2}}$ </tex-math></inline-formula> at room temperature, outperforming previously reported MIR waveguide-integrated bolometers. The design leverages free-carrier absorption (FCA) for heat generation and employs Ti/TiOx as the bolometric material, ensuring that only CMOS-compatible materials are utilized throughout the fabrication process. As a result, this work paves the way for the realization of high-performance mid-IR on-chip spectrometers, capable of operating across a wide wavelength range of up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$13~\mu $ </tex-math></inline-formula>m.