Scanning-resonance optical sensing based on a laterally graded plasmonic layer—optical properties of AgxAl1−x

Plasmonic AgAl/Si3N4 sensor layer structure was deposited on a fused silica glass slide by dual DC magnetron sputtering. The composition of the AgxAl1−x layer was laterally graded in which x linearly changes from x=0 to x=1 over a distance of 20 mm. The Si3N4 layer serves both as a resonator and a protective layer providing homogeneous chemical properties on top of the inhomogeneous AgAl layer. The structure was illuminated through a hemi-cylinder for variable-angle measurements in the Kretschmann–Raether configuration using a focused spot that enabled the change of x by moving it along the gradient of the composition. Optical properties of AgxAl1−x in the whole composition range were obtained. The positions of resonant peaks were shifted by changing the angle of incidence and the lateral position of the spot in a flow-cell configuration. The optical sensing performance of the bilayer system was investigated for x=0 and x=1. Based on the capabilities of ellipsometry limit of detection in refractive index unit values of ≈4−8⋅10−6 can be achieved in the resonant positions utilizing both p- and s-polarizations. Due to the flexible tunability of the resonant wavelengths by moving a focused spot, the above sensitivity is available in-situ, over the spectral range of 265-1504 nm.