Metamaterials are being increasingly used as highly sensitive detection devices. The design of these structures and the ability to effect changes in response through small changes in the geometry of their constituent elements allow for the enhancement of known analysis techniques such as infrared or Raman spectroscopy. High electromagnetic fields have been shown to occur in features such as small gaps and sharp tips and these so called "hot-spots" are the main focus of recent work in Surface Enhanced Raman Spectroscopy (SERS). Previous work has shown dipole pairs with small gaps between them to be suitable for the SERS detection of very small amounts of organic compounds. The main difficulties lie in the small dimensions (≤100 nm) necessary to attain a significant response at the typical Raman pump wavelengths. Also the small size of the gaps is a challenge when it comes to prevent "bridging" between the structures during the fabrication process. In this work we show, through simulations, that carefully controlling the length of dipolar structures as well as the gap between these dipoles a resonant response can be achieved close to the pump Raman wavelengths. Also, we see that increasing the width of the dipole pair shifts the resonant peaks to longer wavelengths. By optimizing their geometry, more efficient and easier to fabricate structures can be used as environmental organic sensors.
Arrays of nanoantennas consisting of plasmonic dipole pairs have been widely used in surface-enhanced Raman spectroscopy (SERS). Fine-tuned structures that can efficiently convert incident electromagnetic energy to excite molecules and provide enhanced detection. However, this tuning mechanism also has its disadvantages. In order to prevent the cross coupling, the distance between each individual element must be increased. This leads to low packing density values which in turn results in a reduction of the overall enhanced Raman signal when these structures are compared to broadly tuned aggregates of particles such as those obtained through metal sputtering or colloidal deposition. In this work we demonstrate through simulations and experimental work that it is possible to increase the reflected signal of an array of nanoantennas by reducing the distance between them in the direction both perpendicular and parallel to the orientation of the incident electric field. It is shown the resonant wavelength shifts in two different spectral directions depending in how the intercell distance was reduced. These resultant shifts can reduce the tuning capabilities of the structures but also can increase the SERS intensity due to close coupling of the dipole pairs. We believe that these results will enable the design and fabrication of structures possessing a greater degree of tunability together with an overall enhanced Raman signal that can rival aggregated SERS substrates.
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