Nanoscale light–matter interactions in metal–organic frameworks cladding optical fibers

The utilization of refractive index (RI) change due to guest-host interactions between the guest volatile organic compound vapor and porous metal-organic frameworks (vapor-MOF interactions) is promising in photonic vapor sensors. Therefore, the study of light-matter interactions in nanoporous metal-organic frameworks (MOFs) is fundamental and essential for MOF-based photonic devices. In this work, the manipulation of light in MOFs to investigate the vapor-MOF interactions by using optical fiber devices is demonstrated. The vapor-MOF interactions and the light-vapor interactions (light in MOFs to sense the RI changes resulting from the vapor-MOF interactions) are investigated. The cladding mode is excited by a long-period fiber grating (LPFG) for evanescent field sensing in a ZIF-8 sensitive coating. The experimental results combining quantum chemical calculations and optical simulations reveal the relationships between the microscopic energy of vapor desorption, RI changes and evanescent field enhancement in ZIF-8 during the vapor-MOF interactions. With exceptionally large RI changes, the evanescent field of cladding mode in ZIF-8 is greatly enhanced to sense the vapor-MOF interactions. As a proof-of-concept, a LPFG sensor with ZIF-8 coating showed a high sensitivity of 1.33 pm ppm^-1 in the linear range from 9.8 ppm to 540 ppm for the sensing of ethanol vapor. The investigation of light-matter interactions in ZIF-8 provides a useful guideline for the design and fabrication of MOF-based optical waveguide/fiber sensors.