Anisotropic Optical Response of Silver Nanorod Arrays: Surface Enhanced Raman Scattering Polarization and Angular Dependences Confronted with Ellipsometric Parameters

Abstract

Silver nanorod arrays prepared by oblique angle deposition (AgOADs) represent versatile, simple and inexpensive substrates for high sensitivity surface enhanced Raman scattering (SERS) applications. Their anisotropic nature suggests that their optical responses such as the SERS signal, the depolarization ratio, reflectivity and ellipsometric parameters critically depend on the states of polarization, nanorod angular arrangement and specific illumination-observation geometry. SERS polarization and angular dependences of AgOADs were measured using methylene blue (MB) molecule. Our study constitutes, to our knowledge, the most detailed investigation of such characteristics of plasmonic nanostructures to date. This is due to the 90°-scattering geometry used in which two out of three Euler angles determining the nanorod spatial orientation and four polarization combinations can be varied simultaneously. We attributed the anisotropic optical response to anisotropic (pseudo)refractive index caused by different periodicity of our structures in different directions since the plasmonic properties were found rather isotropic. For the first time we demonstrate very good correspondence between SERS intensities and ellipsometric parameters for all measured configurations as compared on the basis of the surface selection rules. Obtained results enable quantitative analysis of MB Raman tensor elements, indicating that the molecules adsorb predominantly with the symmetry axis perpendicular to the surface.

Polarization- and Angular-Resolved Optical Response of Molecules on Anisotropic Plasmonic Nanostructures

“…For example, island-like Ag films in [ 122 ] exhibited uniaxial optical properties and NRs in [ 121 ] exhibited biaxial properties with the complex refractive index different for different orientations of the incident angle with respect to the NRs, similarly to the case of crystals [ 123 ]. A very similar conclusion was drawn from our work [ 112 ] where we showed that the optical constants of silver NR arrays are sensitive to both the incident angle as well as rotation of the substrate by 90°. On the other hand, they were fairly insensitive to flipping the substrate by 180°.…”

“…Published results indicate that the optical response for light polarized parallel/perpendicular to the long axis of the nanoobjects (related to excitation of longitudinal plasmon modes (LM) or transverse plasmon modes (TM), respectively) is indeed different. However, the SERS intensity profile with varying angle/polarization is a function of a wide range of parameters, such as the dimensions of the metallic objects, their aspect ratios and spatial arrangement, material (Ag or Au), the excitation wavelength or orientation of the probe molecules on the surface [ 85 , 112 , 113 ], which resulted in seemingly contradictory accounts appearing in literature. Last but not least, what hampers more precise analyses of the angular and polarization effects is sample bleaching and photodecomposition induced by the incident laser, which causes poorer reproducibility and brings further uncertainty to the set of experimentally measured points.…”

“…However, possible optimization of plasmon-based sensors for maximum signal enhancement relies, among other things, on the right choice of the excitation angle and/or the scattering angle. Geometric reasons, such as change in the size of the illuminated area and possibly different efficiency in collection of the scattered radiation must be also taken into account when aiming to retrieve angular dependences [ 112 , 113 , 117 ]. This was often neglected.…”

“…Another approach used for elucidation of polarization and angular dependences in SERS was employed in our recent work using MB adsorbed on silver NR arrays [ 112 ] ( Figure 18 ). Although the nanostructures were morphologically anisotropic, the plasmonic properties around the excitation wavelength used (532 nm) were found rather isotropic.…”

Microfluidics-Driven Fabrication of a Low Cost and Ultrasensitive SERS-Based Paper Biosensor

“…The lower homogeneity of the Au@AgNRs substrates can also be explained on the basis of the self-assembly distribution of Au@AgNRs. The application of NRs in SERS spectroscopy strongly relies on: (i) the orientation of the longitudinal and transverse bands of the NRs to the incident polarized light, (ii) to the arrangement of the nanorods with respect to each other (tip to tip, side to side, or side to tip), and (iii) to the adsorption of the molecules either on the tips or on the sides of the nanorod [12,47]. When the longitudinal band of the nanorods is parallel to the incident light, there is a higher enhancement than when the incident light is parallel to the longitudinal band [48].…”