On the large near-field enhancement on nanocolumnar gold substrates

One of the most important and distinctive features of plasmonic nanostructures is their ability to confine large electromagnetic fields on nanometric volumes; i.e., the so-called hot spots. The generation, control and characterization of the hot spots are fundamental for several applications, like surface-enhanced spectroscopies. In this work, we characterize the near-field distribution and enhancement of nanostructured gold thin films fabricated by glancing angle deposition magnetron sputtering. These films are composed of columnar nanostructures with high roughness and high density of inter-columnar gaps, where the electromagnetic radiation can be confined, generating hot spots. As expected, the hot spots are localized in the gaps between adjacent nanocolumns and we use scattering-type scanning near-field optical microscopy to image their distribution over the surface of the samples. The experimental results are compared with finite-difference time-domain simulations, finding an excellent agreement between them. The spectral dependence of the field-enhancement is also studied with the simulations, together with surface-enhanced Raman spectroscopy at different excitation wavelengths in the visible-NIR range, proving a broad-band response of the substrates. These findings may result in interesting applications in the field of surface-enhanced optical spectroscopies or sensing.P.D.N. acknowledges DIMMAT project funded by Madrid region under programme S2013/MIT-2775, the technical assistance provided by Dr. Judith Langer and CIC biomaGUNE for an internship to perform the SERS measurements. Funding from Universidad Politécnica de Madrid (VJIDOCUPM18OYPR) and MINECO (MAT2014-59772-C2-1-P and ENE2015-70300-C3-3-R) is also acknowledged, as well as the computer resources and technical assistance provided by CESVIMA (Centro de Supercomputación y Visualización de Madrid, Universidad Politécnica de Madrid) and the service from the MiNa Laboratory at IMN funded by CM (S2013/ICE2822), MINECO (CSIC13-4E-1794), the EU (FEDER, FSE) and funding by EUROfusion Consortium through project AWP15-ENR-01/CEA-02.Peer reviewe