Mechanism and Performance of a Reflective Plasmonic Humidity Sensor Based on an Au–PVA–Au Nanohole Sandwich Structure

A reflective plasmonic humidity sensor based on an Au–PVA–Au nanohole sandwich structure is investigated. The device consists of a periodic gold nanohole array, a poly(vinyl alcohol) (PVA) spacer, and a continuous gold film. A humidity-dependent model considering both the refractive-index decrease and thickness swelling of PVA is established to analyze the optical response and resonance-modulation mechanism. Within the relative humidity range of 20–98%RH, the reflection resonance dip exhibits a continuous blueshift with a total wavelength shift of approximately 135 nm. Piecewise linear fitting shows sensitivities of 1.3857 nm/%RH in the 20–74%RH range and 2.5000 nm/%RH in the 74–98%RH range. At approximately 74%RH, the resonance wavelength, full width at half maximum, and quality factor are about 830 nm, 19 nm, and 43.7, respectively. Decoupling analysis confirms that both PVA refractive-index reduction and thickness swelling contribute to the blueshift, while their combined effect produces the largest response. These results demonstrate that the proposed structure converts humidity-induced optical and geometric variations in PVA into a pronounced wavelength response, providing a mechanism-guided design route for reflective nanoplasmonic humidity sensors based on polymer-assisted cavity modulation.