Plasmonic–cavity hybrid relative humidity sensor based on Ag nanobowl arrays integrated with a poly(vinyl alcohol) film

The growing demand for relative humidity (RH) monitoring in environmental assessment, industrial process control, and localized microenvironment surveillance has stimulated research on highly sensitive optical humidity sensing schemes amenable to spectral demodulation. Here, we present a plasmonic–cavity hybrid optical humidity sensor that integrates a silver (Ag) nanobowl array with a poly(vinyl alcohol) (PVA) film. A self-assembled polystyrene (PS) microsphere monolayer serves as a template, and direct-current (DC) magnetron sputtering is employed to form the Ag nanobowl structure, which is subsequently combined with the PVA film to yield a metal–dielectric microcavity composite. The resulting structure simultaneously supports Fabry–Perot (F–P) cavity-mode-dominated resonances in the near-infrared (NIR) band and surface plasmon polariton (SPP)-related resonances in the visible band within a single device. The hygroscopic swelling of PVA concurrently modulates the effective cavity length and the local refractive index, inducing a complementary redshift of the dual-band reflectance spectrum and thereby enabling reflectance-based RH readout. At 26±1 ∘ C, the sensor exhibits a stable response over the 2.5%–85.5% RH range with low hysteresis. In the high-humidity regime, the maximum wavelength sensitivity of the NIR channel reaches 2.10 nm/%RH, and that of the visible channel reaches 0.66 nm/%RH, with a dynamic response time of approximately 13 s. The device is fabricated via a lithography-free colloidal self-assembly process and employs a PVA film as the supporting substrate, offering a structural design approach for multimode optical humidity sensors.