Exploring plasmonic gradient metasurfaces for enhanced optical sensing in the visible spectrum

While conventional optical sensors hold historical significance, they face inherent limitations in sensitivity, operational intricacies, and bulky size. A breakthrough in this realm comes from the advent of metasurface sensors, which leverage nanoscale optical effects, thereby expanding the horizons of optical sensing applications. However, past methods employed in metasurface sensors predominantly rely on wavelength shifts or intensity changes with high-Q resonances, thereby significantly restricting the detection bandwidth. In response to these challenges, this study introduces a plasmonic gradient metasurface-based sensor (PGMS) designed for refractive index detection across a wide wavelength spectrum. Through the utilization of the Pancharatnam-Berry phase method, the PGMS achieves a distinctive 2 pi phase shift, facilitating the simultaneous generation of specular and deflected beams. The introduction of a far-field intensity ratio (I* = I+1/I-0) amplifies the change in optical response by maximizing the deflected beam's intensity while minimizing specular reflection. Experimental validation attests to the PGMS's consistent performance across diverse media and wavelengths, successfully overcoming challenges associated with oxidation issues. Furthermore, the incorporation of a normalization factor enhances the PGMS's sensing performance and versatility for broadband optical sensing, accommodating variations in the refractive index. Particularly sensitive in green wavelengths, the PGMS demonstrates its potential in visible spectrum applications, such as biomedical diagnostics and environmental monitoring. This research not only addresses challenges posed by conventional sensors but also propels optical sensing technologies into a realm of heightened sensitivity and adaptability.