Direct Laser Writing of Ultrathin and Scalable Optomechanical Membranes in Fiber‐Tip Micro‐Cavities for Multi‐Gas Photoacoustic Spectroscopy

Optical fiber acoustic sensors with high sensitivity and compact footprints have advanced the development of miniaturized photoacoustic spectroscopy, rendering them attractive for in situ and real‐time trace gas detection. However, the limited fabrication accuracy and scalability prevent the rational design of miniature fiber photoacoustic spectroscopy for multi‐gas sensing at the sub‐ppm detection limit. In this study, fiber‐tip Fabry–Pérot microcavities with ultrathin and scalable optomechanical membranes are prepared using high‐precision direct laser writing for miniaturized multigas photoacoustic spectroscopy. This fabrication technique facilitates the flexible and precise preparation of trampoline‐shaped submicron‐thick membranes. By adjusting the size of the membranes, their size‐dependent resonant frequencies could be tuned for multi‐gas photoacoustic sensing based on a frequency‐multiplexing scheme. As a proof of concept, a photoacoustic microprobe (∼300 µm in diameter) is assembled from three microcavities for simultaneous detection of a gas mixture comprising acetylene (C 2 H 2 ), carbon dioxide (CO 2 ), and water vapor (H 2 O). For C 2 H 2 , a high detection sensitivity of 258 ppb and a short response time of ∼28 ms are achieved by amplifying the photoacoustic signals by leveraging the mechanical resonance of the membrane. This compact fiber‐tip structure with the capability of in situ and fast multicomponent gas detection is promising for broad applications such as explosive gas monitoring and medical diagnosis.