Fiber-optic meta-tip with multi-sensitivity resonance dips for humidity sensing

In response to the requirements of small effective sensing volume for simultaneous interpreting of resonant peaks with different sensing sensitivities, a surface plasmon resonance (SPR) fiber-optic probe based on non-uniform Au groove array is proposed, which realizes relative humidity (RH) sensing by coating silk fibroin film. The RH meta-tip is mainly composed of multi-mode fiber (MMF), a metasurface, and silk fibroin film. The metasurface is transferred to the end face of the MMF by the “contact and separate” transfer method, and then a layer of silk fibroin is coated on the metasurface by dipping method. Because of the interaction of interference and plasma excitation, the proposed SPR probe has many resonance dips with different refractive index (RI) sensitivities, which has the potential to monitor multiple sensing parameters. Because the RI of the silk fibroin film is directly affected by the external environment RH, the phase matching conditions of the propagating surface plasmon polaritons (SPPs) change accordingly, leading to the shifts in the resonance wavelength of those dips. By further improving the functional integration of the fiber-optic SPR probe, the single-ended sensor paves the way for chemical and biological sensing.