A Metal–Dielectric Photonic Crystal and Its Phase Response Utilized in an Ultra-High-Performance Humidity Sensor

Metal–dielectric photonic crystals (MDPhCs) generating Bloch-like surface waves (BLSWs) are accompanied by narrow resonance dips and sensitivities to gaseous analytes outperforming the BSW-based sensors. In this theoretical paper, we report on a sensing concept based on the phase interrogation and BLSWs supported by an MDPhC in the Kretschmann configuration when the sensing characteristics are substantially improved. For an MDPhC comprising two bilayers of TiO2/Ag with a termination layer of TiO2, we show that the BLSW-based phase response is characterized by a steep and abrupt phase difference between the TM and TE waves. It was revealed that for the refractive index (RI) change in a range of 1.0002727–1.0003751 RI units (RIU) or, equivalently, for a relative humidity (RH) change in air in a range of 0–100 %RH, the sensitivities to the RI and RH are up to 70 μm/RIU and 0.072 nm/%RH, respectively, and the figures of merit (FOMs) are up to 700,000 RIU−1 and 0.717 %RH−1, respectively. In addition, the limits of detection (LODs) reach 1.4 × 10−7 RIU and 0.14 %RH, respectively. We also evaluated the phase difference at specific wavelengths, and the phase sensitivities to the RI and RH at a wavelength of 1637.92 nm are as high as 4.7 × 107 deg/RIU and 47.7 deg/%RH, respectively. The corresponding LODs are as low as 2.1 × 10−10 RIU and 2.1 × 10−4 %RH, respectively. This research demonstrates that the BLSW-based sensors employing MDPhCs and utilizing the phase interrogation exhibit outstanding sensing characteristics, and they can be used in a simple, ultra-high-performance humidity sensing.