Optical Properties and Sensing Performance of Au/SiO₂ Triangles Arrays on Reflection Au Layer

In order to enhance the refractive index sensing performance of simple particle arrays, a structure, consisting of Au/SiO₂ triangle arrays layers and reflection Au substrate, with increasing size and lengthening tips of triangles, is studied. The triangle arrays are modeled after an experimentally realizable "imprint" of microsphere lithography. Numerical calculation was carried out to study its optical properties and spectral sensitivity. The calculation results show that a large local enhancement of electric field (61 times) and simultaneously high absorption is due to combination of the resonance absorption of Au triangle disks, plasmonic couplings between the Au triangle disks and the Au film, and the high-density packing of triangle disks. The absorption peaks were not detuned when the gap between neighboring tips of the triangles varied from 10 to 50 nm. When the thickness of SiO₂ layer increased from 10 to 50 nm, the absorption peak shifted to longer wavelengths and the amplitude rises quickly signaling the dominance of the gap mode resonance between the two Au layers. As the thickness of the top Au layer varies from 10 to 50 nm, the absorption peak is also red shifted and the peak amplitude increases. The full width at half maximum of the peaks for high absorption (> 90%) is about 5 nm. When fixing the gap, the thicknesses of Au/SiO₂ triangle layer, and increasing the surrounding refractive index from 1.33 to 1.36, the absorption peaks shifted quickly, with a refractive index sensitivity and figure of merit as high as 660 nm per refractive index unit and 132, respectively. Such arrays can be easily fabricated by using microsphere array as projection masks and find application in refractive index monitoring of liquid and identification of gas and liquid phases.