Fabry–Pérot Microtube Cavity Structure for Optical Sensing at Mid-infrared Spectrum

In this manuscript, we propose and design two different Fabry-Pérot microtube cavity refractometer configurations operating at mid-infrared (mid-IR) wavelengths. The designed refractometers' topologies are based on single-walled and double-walled cylindrical Si-microtubes. Here, by infiltrating the target analyte into the cavity region, the light is localized and high-depth light-matter interaction is achieved to reveal sensing characteristic of the structure. Numerical analyses and performance evaluations of the designed refractometers are carried out by using the three-dimensional finite difference time domain (3D FDTD) method. In this regard, for the single-walled microtube configuration, overall sensitivity is calculated to be around 2340 nm/RIU. Moreover, to reduce the cross talks and to sharpen resonance peaks, the double-walled microtube design is introduced and overall sensitivity is found to be around 1820 nm/RIU. In addition, the possible fabrication and optical characterization approaches of the designed structure are also discussed. The proposed configurations of the refractometer are compact, feasible for manufacture, simple for optical measurements and, importantly, provide label free sensing characteristics. We think that the presented study may broaden further the research on the mid-IR label-free refractometer-based sensing devices for detection of various liquid and gaseous materials.