Using full-wafer processing, we demonstrate a sophisticated nanotechnology for the realization of an ultrahigh sensitive cavity-coupled plasmonic device that combines the advantages of Fabry-Perot microcavities with those of metallic nanostructures. Coupling the plasmonic nanostructures to a Fabry-Perot microcavity creates compound modes, which have the characteristics of both Fabry-Perot and localized surface plasmon resonance (LSPR) modes, boosting the sensitivity and figure-of-merit of the structure. The significant trait of the proposed device is that the sample to be measured is located in the substrate region and is probed by the compound modes. It is demonstrated that the sensitivity of the compound modes is much higher than that of LSPR of plasmonic nanostructures or the pure Fabry-Perot modes of the optical microcavity. The response of the device is also investigated numerically and the agreement between measurements and calculations is excellent. The key features of the device introduced in this work are applicable for the realization of ultrahigh sensitive plasmonic devices for biosensing, optoelectronics, and related technologies.

The far-field polarization of the optical response of a plasmonic antenna can be tuned by subtly engineering of its geometry. In this paper, we develop design rules for nano antennas which enable the generation of circular polarized light via the excitation of circular plasmonic modes in the structure. Two initially orthogonal plasmonic modes are coupled in such a way that a rotational current is excited in the structure. Modifying this coupling strength from a weak to a strong regime controls the helicity of the scattered field. Finally, we introduce an original sensing approach that relies on the rotation of the incident polarization and demonstrates a sensitivity of 0.23 deg·nm −1 or 33 deg·RIU −1 , related to changes of mechanical dimensions and the refractive index, respectively. KEYWORDS: Plasmonics, antennas, coupling strength, polarization, sensors I n the past few decades, localized plasmon resonances supported by metallic nanostructures have attracted significant attention thanks to their applications in a variety of fields, such as biosensing, 1 photovoltaics, 2 and optoelectronics.3 It is now well-known that, with appropriate tuning of a plasmonic nanostructure, it is possible to engineer both its near and far-field response.4−8 In the optical regime, plasmonic nanostructures react similarly to antennas in the sense that incident light can be collected and stored in the near-field, and conversely, energy stored in the near-field can be radiated into the far-field. The far-field emission pattern of planar plasmonic structures is determined by the near-field distribution. As a consequence, it is possible by engineering the near-field of a plasmonic nanostructure to design its response in a way such that a linearly polarized excitation results in a circular polarized (CP) response.9−18 Recently, nanostructures supporting not only one single plasmon mode but also multiple interacting plasmonic modes have been developed. 19−23 The interaction between such plasmonic modes allows energy transfer between the particular modes within the corresponding wavelength range, and consequently, different modes can indirectly be excited through their near-field. In line with the observed asymmetric scattering spectra, these modes are named Fano resonances. 24 Potential applications of such structures exhibiting Fano line shapes include sensing, energy storage, and spectroscopy enhancement.25−28 Most of those applications rely on the modulation of the intensity of the scattered light induced by the interaction of the plasmonic modes over different spectral regions. It has already been shown that light polarization can be altered using plasmonic nano antennas 29−31 or plasmonic metamaterials. 32−34 In this paper, we show that the polarization of the scattered light can be controlled by introducing a perturbation into a system with initially orthogonal plasmonic modes. Furthermore, we demonstrate that such structures can be used for the generation of CP light from an incident linearly polarized plane wave. ...

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