A simple and large-scale fabrication technique for three dimensional structure arrays using a photolithography process was applied to realize an array of high-aspect-ratio metallic fins. The fin array enables light confinement between the high-aspect-ratio fins, thus generating optical vortices. The light confinement between the fins produces sharp dips in the reflection spectrum of the array. We show that the position of the dip wavelength is sensitive to change in the refractive index of the surrounding medium. Sensitivity to change in the refractive index was quantified by optical simulation and experimental measurements.