Fig. 1. Schematic of surface plasmon. (a) Electric field normal to surface required for charge generation; (b) vertical electric field as function of distance; (c) momentum mismatch between SP and incident light

Fig. 2. Grating structure. (a) Optical image of periodic grating structure; (b) schematic of metallic grating with geometric parameters

Fig. 3. Schematic of TAS7400TS THz-TDS system

Fig. 4. Experimental and simulated results of water by sensors

Fig. 5. Experimentally measured THz absorption curves of alcohol-water mixtures with different volume fractions by sensors. (a) At resonant peak 1; (b) at resonant peak 2

Fig. 6. Physical parameters of alcohol-water mixtures with different volume fractions. (a) Dielectric constant; (b) refractive index

Fig. 7. Relationship between resonant frequency of sensor and volume fraction of alcohol. (a) Peak 1; (b) peak 2