Plasmonic gradient structures of nanoparticle arrays for optical sensing applications

Driven by a demand for integrated optical sensors in structural health and environmental monitoring we present the application of plasmonic gradient structures as sensor substrate. Therefore, nanoparticle arrays of gold are fabricated by interference lithography, which exhibit localized surface plasmons (LSPs). The plasmonic properties of such nanoparticles can be tuned by altering their size. In our approach, an additional photochemical growth by exposure to HAuCl4 and light is used to manufacture gradients of nanoparticle sizes within the array. These gradients in turn induce different spectral responses depending on the illuminated region of the array gradient. To enable sensing applications, such plasmonic gradient structures are placed as a filter in front of a photodetector to allow detection of transmitted optical signals from different locations of the array. Different applications can be envisioned in this configuration: On the one hand, sensing of wavelength shifts of the illuminating light source can be enabled by comparing the photocurrents generated in adjacent sensor elements. Additionally the application of refractive index measurements is demonstrated with the same detector configuration. The change in extinction of the illuminating light at different wavelengths can be used to obtain an intensity shift at the detector elements. This shift correlates to the change of the spectral resonance conditions in the array gradient upon change of refractive index.