Ultrasensitive Dual‐Mode Humidity Detection Using a Plasmonic‐Photonic Hybrid Waveguide

Abstract Reliable and ultrasensitive humidity detection is essential in industrial process controls, meteorology, and medical diagnostics. In this study, a high‐performance relative humidity (RH) sensor driven by a plasmonic‐photonic hybrid system (PPHS), which supports waveguide modes and surface plasmon polariton mode simultaneously, is proposed. Due to the distinct sensitivities of these two modes to refractive index variation and volume expansion, the dual‐mode RH detection platform offers a unique tool for isolating the individual contribution of each effect and further revealing the mechanism of moisture absorption in a crosslinked PVA film. As an RH sensor, PPHS delivers a wavelength sensitivity of 1.16 nm %RH⁻¹ (40–70 %RH) and an intensity sensitivity of 1.1 dB %RH⁻¹ (55–90 %RH), respectively. Benefited from such an ultrahigh sensitivity, together with excellent recyclability and a quick response time of a few seconds, the PPHS shows an almost identical real‐time RH curve with an even higher resolution (<0.1 %RH) compared to a commercial electrical hygrometer in the practical ambient RH monitoring. The PPHS is further demonstrated as a human breathing sensor and a skin‐humidity‐based noncontact switch, suggesting that the PPHS provides a great potential in medical diagnostics and optical modulation.