An angle-selective photonic crystal for multi-physical sensing applications.

This study reports a high-transmission photonic crystal (PC) that was optimized for TM waves at a frequency of 43.8 GHz and engineered using photonic band gap (PBG) principles to achieve angle selection. The structure demonstrated a remarkable transmission from -68° to 0°, consistently exceeding 80% efficiency. Assessing the fragility of the medium within the PC using the critical angle, a multi-physical sensor (MS) comprising both refractive index sensing (RIS) and plasma density sensing (PDS) functions was proposed. The PDS could detect concentrations from 0.4 × 10 m to 0.8 × 10 m with a sensitivity of 10.925° per m. For RIS, with the change in magnetic field intensity, it could detect refractive index in the ranges of 2.45-2.33 at 1.25 T, 2.33-2.21 at 1.15 T, 2.21-2.09 at 1.05 T, and 2.09-1.97 at 0.65 T, with respective sensitivities of -28° per RIU, -16° per RIU, -18.33° per RIU, and -9.38° per RIU, showcasing broad detection ranges and high sensitivities. Notably, the MS could maintain high transmission (greater than 0.8) in the RIS range from -60° to 0°, enabling dynamic angle selection for refractive index and plasma density. Therefore, it holds promising prospects in the real-time monitoring of refractive index and plasma density changes in healthcare- and environment-related applications, such as in early disease diagnosis, air quality monitoring and detecting metabolic activity or harmful substances.