Trigonal symmetric plasmonic array of standing wires for broadband, polarization insensitive molecular sensing

We present a simple and powerful standing nanowires array device developed for applications in surface-enhanced Raman spectroscopy (SERS). Plasmonic active gold wires grown on iron disks, and partially immersed in a supporting... [ view full abstract ]

We present a simple and powerful standing nanowires array device developed for applications in surface-enhanced Raman spectroscopy (SERS). Plasmonic active gold wires grown on iron disks, and partially immersed in a supporting alumina matrix constitute it.

Looking for a cost-effective large area production technique, we designed a nanostructured device realizable by the galvanic bath and etching process. The elected fabrication method let the parallel growth of the nanowires directly in ordered pores of anodized alumina template with the possibility to select their gaps, size, diameter, and spatial arrangement from nano to microscale for the broadest spectral range scalability.

Extensive numerical calculations in the visible and NIR regimes have been used to characterize the spectroscopic properties versus its design, allowing a detailed description of the sensing abilities of the tool. The electromagnetic design analysis demonstrated a broadband plasmonic enhancement, effect useful for many standard excitation wavelengths in the visible and NIR. The trigonal pores arrangement impressed by the porous alumina and the cylindrical shape of the wire result in a plasmonic coupling efficiency weakly dependent on the polarization of the incident field. The devices, tested with 633 and 830 nm laser lines on two standards, physisorbed Rhodamine 6G and chemisorbed Benzenethiol show a significant Raman enhancement factor, up to around 6 × 10⁴, with respect to the flat gold surface, used as a reference for the measurements of the investigated molecules. Optimizing the electrochemical technique allowed us to realize a large active area device, up to the cm² range, for spectroscopic application with broadband spectral responses, weakly sensitive to polarization orientation, and high spatial homogeneity with respect the typical diffraction limited probe spot areas, appreciated properties in a plasmonic sensor for chemical analysis.