Finite difference time domain (FDTD) method is adapted to investigate near-field enhancement effects on plasmonic structures (patterned in gold film) such as concentric rings with small separation, square, and rectangle. The near-fields effect on surface enhanced Raman scattering (SERS) is typically studied on square and rectangular structures. These metal structures are fabricated by laser interference lithography. Raman active molecules (Rhodamine 6G in PMMA (polymethyl methacrylate)) are spread onto patterned structure by spin coating, and Renishaw inVia Raman spectrometer was used to study SERS. Typical SERS enhancement of the order of 105 is seen for square and rectangular structures. It is observed that the corner points and edges of square and rectangular structures are most sensitive to concentrate near fields. In the case of concentric rings, huge near fields are seen to exist at the gap between the metal rings. Concentric rings are proposed to be very effective structure for SERS sensing applications such as molecular identification and biological mapping. 1. Introduction Near-field assisted surface enhanced Raman scattering (SERS) is an efficient light scattering process which can be used for molecular sensing purpose. In SERS process, huge plasmonic near fields existing due to the localized and propagating surface plasmon resonances around the metal nanostructures have been the most important factors [1–5]. It can assist in achieving even 108–1010 times enhancement of the scattered signal. Thus, engineering the plasmonic structures to achieve densely packed near-field spots or hot spots is a prior must. This optimized structure can be used to explore trace molecular detection, biological sensing, and molecular vibrational studies [1–10]. Near-field localization and studies of SERS on a variety of plasmonic structures have been reported previously by various researchers [11–15]. It is also reported that two-dimensional (2D) structures are superior compared to one-dimensional (1D) structure. It is preferable if the plasmonic structures are insensitive to the input excitation beam. In this work we propose a novel concentric ring structure (Figure 1) for the efficient near-field localization. Here concentric rings will be highly insensitive to the polarization state of the input light beam (unpolarized or transverse magnetic (TM) and transverse electric (TE)); thus SERS templates consisting of concentric rings will be very easy to perform SERS without prior information of the polarization of the input beam. Near-field localization is compared
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