High Sensitivity of Plasmonic Microstructures near the Transition from Short-Range to Propagating Surface Plasmon

Abstract

The optical properties of Au triangles with 700 and 950 nm and 1.8 μm edge length and of microhole arrays with a 3 μm periodicity show a strong absorption peak highly sensitive to refractive index with excitation in the Kretschmann configuration of surface plasmon resonance (SPR). This absorption peak is due to a SPR phenomenon only present with microstructures of micrometer length, as demonstrated with SPR active materials, Au and Ag; but absent in SPR inactive materials, TiOxNy and Ag triangles covered with a thin film of silver oxide. The Au triangle arrays were exhibiting: 1) two transmission minima at λ = 525 and 800 nm with low refractive index sensitivity and 2) a transmission maximum near λ = 600 nm exhibiting a high sensitivity to bulk refractive index for the triangles with an edge length of 950 nm (1993 nm/RI) and 1.8 μm (1038 nm/RI). This is a significant improvement of the sensitivity compared to nanoparticles and is near the sensitivity of SPR on a smooth film. The transmission minima at λ = 800 nm was also present in microhole arrays in a Au film, but of reduced intensity as the size of the hole decreases. Moreover, the microhole arrays with smaller holes also presented a strong transmission minimum around λ = 600 nm, with a similar wavelength, bandwidth and sensitivity as SPR on a smooth gold film. Lastly, the response measured for the formation of a monolayer on the triangle arrays results in a large shift of the SPR response of 13 ± 3 nm, which correlates to a short penetration depth of 24 nm. Triangle arrays investigated in the present configuration could significantly improve SPR biosensing by increasing the measured response from a binding event and could decrease the temperature drift due to a lower bulk refractive index sensitivity.