High sensitivity refractive index sensing with strong light confinement in high-aspect-ratio U-cavity arrays

U-cavities made of gold high-aspect-ratio nano-fins are proposed as a sensitive refractive index sensing element in the near-infrared region. The sensing mechanism is based on the shift of the resonance wavelength of arrays of the U-cavities upon change in the cavity surrounding medium. The U-cavity arrays support strong light confinement in the cavities as evidenced by the formation of vortices in the optical power flow of the cavities. The bottom gold layer of the cavities enables efficient light trapping with powerful vortex, as evidenced by the low power leakage of the cavities. The strong light confinement gives rise to resonance exhibiting sharp reflectance dips. The values of the dip wavelengths are determined from the geometry of the cavity and therefore are easily predicted. The condition for resonance on the incidence angle of light is not restrictive, so that the structure offers little angular dependency. The resonance dips of the zero-order reflectance shift toward higher values when the refractive index of the surrounding medium is increased. U-cavity arrays were fabricated with fast and cost-effective manufacturing processes such as photolithography, gold sputtering, and plasma etching. Zero-order reflectance spectra were measured in the near-infrared wavelength range under different refractive index media in order to quantify the sensitivity to refractive index change. The U-cavity arrays achieved a sensitivity of 743 nm per refractive index unit in good agreement with simulation results.