Using surface-plasmon polariton at the GaP-Au interface in order to detect chemical species in high-refractive-index media

We exploit surface-plasmon polaritons (SPPs) at the GaP-Au interface to detect chemical species in high refractive index media and verify the device function by detecting 2,4-dichlorotoluene (C7H6Cl2, n=1.55) and 1-bromonaphthalene (C10H7Br, n=1.65). The calculated dispersion relationship shows that the SPP excited in GaP by 635 nm light incident at 31.6° is effective for detecting C7H6Cl2. Moreover, we analyze the reflectance as a function of the Au layer thickness and find that the illumination angle at which SPPs are excited is consistent with simulation results. When illuminated at 31.6°, the optical electric field is concentrated and transmitted along the Au-C7H6Cl2 interface. Furthermore, we use the device to detect solvent concentrations of the solutions C2H5OH-C7H6Cl2 and C2H5OH-C10H7Br. The SPP-excitation angle increases with increasing concentration of C7H6Cl2 and C10H7Br. We conclude that chemical species in solutions with an index of refraction less than 1.62 can be detected with this approach. These results suggest that SPPs excited at the GaP-Au interface can be exploited to detect chemical species in high-refractive-index media.