Highly Sensitive Refractive Index Sensors Based on Geometric Phase Metasurfaces

We designed a highly sensitive refractive index sensor based on geometric phase metasurfaces. The unit of this sensor consists of eight rotated silver cubes, which are placed on a silicon dioxide substrate. We employed the finite-difference time-domain method to calculate the optical properties of the sensor. The background refractive index changes from 1.33 to 1.34. Experimental results indicate that due to the particularly small change in the background refractive index, the reflectance and transmittance are consistent under both refractive index conditions. This metasurface can produce both anomalous reflection (or anomalous refraction) phenomena and polarization conversion functionality. When detecting anomalous reflection (or anomalous refraction), we found a significant difference in the intensity difference of the two orthogonal polarization states of the reflected light (or transmitted light). The ratio of the intensity difference of these two orthogonal polarizations (ΔIy/ΔIx) can approach 8. This indicates that a change in the background refractive index of less than 1% can be reflected through other optical responses. Utilizing this feature, a refractive index sensor can be realized. The size of the silver cubes is much smaller than the wavelength of the incident light, allowing the silver cubes to be regarded as electric dipoles. By altering the structural dimensions and the polarization state of the incident light, the size of the electric dipole can be adjusted, optimizing Δ Iy/Δ Ix. The excitation of surface plasmons on the silver cubes depends on the polarization state of the incident light, and adjusting the polarization state of the incident light can still change Δ Iy/Δ Ix. Our findings provide new insights for designing efficient optical metasurfaces, further advancing their application potential in refractive index sensing and optical sensing fields.