Abstract
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.
Highlights
Several techniques are available for the fabrication of nanostructured plasmonic materials supported on substrates, such as self-assembly methods[17,18,19,20,21], lithography[22,23], or physical vapour deposition (PVD) methods
The near-field enhancement has been measured by means of scattering-type scanning near-field optical microscopy (s-SNOM), which allowed us to directly visualize the electromagnetic hot spots between the nanostructures[44,45,46,47,48] and characterize their spatial distribution
The formation of nanocolumnar structures, elongated along the particle flux direction, arises from the self– shadowing effect[26,27,28]: the first islands, formed at the early stages of the deposition process, project a shadow behind them that obstructs the deposition of further material within these regions
Summary
Several techniques are available for the fabrication of nanostructured plasmonic materials supported on substrates, such as self-assembly methods[17,18,19,20,21], lithography[22,23], or physical vapour deposition (PVD) methods Among the latter, magnetron sputtering[24,25] has several advantages with respect to other approaches due to its simplicity, low cost and amenability to large-scale fabrication in relatively short times. In this work we have fabricated two different types of gold nanostructured thin films by GLAD magnetron sputtering[41,42,43] and characterized their optical behaviour, their spatial and spectral near-field response, with a combination of finite-differences time-domain (FDTD) simulations and experimental measurements. The spectral response of the near-field enhancement has been determined measuring their SERS response for 532, 633 and 785 nm excitation wavelengths, which is useful to prove their applicability as broad-band sensors
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