Enrico Gazzola

Coupled SPP Modes on 1D Plasmonic Gratings in Conical Mounting

Plasmonic nanostructures exhibit a variety of surface plasmon polariton (SPP) modes, with different characteristic properties. While a single metal dielectric interface supports a single-interface SPP mode, a thin metal film can support extended long range SPPs and strongly confined short range SPPs. When the coupling between the incident light and the SPP is provided through a diffraction grating, it is possible to azimuthally rotate the grating with respect to the scattering plane, introducing the possibility to propagate the SPP along an arbitrary direction. We present a theoretical and experimental analysis of the coupling conditions for long range and short range SPPs under this configuration. We also investigate the propagation length of the modes depending on the propagation direction with respect to the grating grooves, showing in particular that the long range SPP propagation length can be sensibly enhanced with respect to the null-azimuth case.

Propagation of grating-coupled surface plasmon polaritons and cosine–Gauss beam generation

When a surface plasmon polariton (SPP) is excited through a metallic grating in the conical mounting configuration, it propagates along a direction nonparallel to the grating Bragg vector. We will derive, under any possible coupling condition, the propagation direction as a function of experimental parameters, with particular attention to its relation with the azimuthal rotation angle. We will identify the conditions to achieve the maximum angular deflection with respect to the grating vector, also in relation to the grating geometry. Moreover, we will investigate the special configuration in which two SPP modes propagating toward different directions are simultaneously excited by the same light beam, suggesting to exploit this configuration to generate nondiffractive plasmonic beams (cosine–Gauss beams). The analytical treatment is supported by simulations with Chandezon’s method.

Grating-Coupled Surface Plasmon Resonance (GC-SPR) Optimization for Phase-Interrogation Biosensing in a Microfluidic Chamber

Surface Plasmon Resonance (SPR)-based sensors have the advantage of being label-free, enzyme-free and real-time. However, their spreading in multidisciplinary research is still mostly limited to prism-coupled devices. Plasmonic gratings, combined with a simple and cost-effective instrumentation, have been poorly developed compared to prism-coupled system mainly due to their lower sensitivity. Here we describe the optimization and signal enhancement of a sensing platform based on phase-interrogation method, which entails the exploitation of a nanostructured sensor. This technique is particularly suitable for integration of the plasmonic sensor in a lab-on-a-chip platform and can be used in a microfluidic chamber to ease the sensing procedures and limit the injected volume. The careful optimization of most suitable experimental parameters by numerical simulations leads to a 30–50% enhancement of SPR response, opening new possibilities for applications in the biomedical research field while maintaining the ease and versatility of the configuration.

Grating-coupled surface plasmon resonance gas sensing based on titania anatase nanoporous films

Nanoporous TiO2 anatase film has been investigated as sensitive layer in Surface Plasmon Resonance sensors for the detection of hydrogen and Volatile Organic Compounds, specifically methanol and isopropanol. The sensors consist of a TiO2 nanoporous matrix deposited above a metallic plasmonic grating, which can support propagating Surface Plasmon Polaritons. The spectral position of the plasmonic resonance dip in the reflectance spectra was monitored and correlated to the interaction with the target gases. Reversible blue-shifts of the resonance frequency, up to more than 2 THz, were recorded in response to the exposure to 10000 ppm of H2 in N2 at 300°C. This shift cannot be explained by the mere refractive index variation due to the target gas filling the pores, that is negligible. Reversible red-shifts were instead recorded in response to the exposure to 3000 ppm of methanol or isopropanol at room temperature, of magnitudes up to 14 THz and 9 THz, respectively. In contrast, if the only sensing mechanism was the mere pores filling, the shifts should have been larger during the isopropanol detection. We therefore suggest that other mechanisms intervene in the analyte/matrix interaction, capable to produce an injection of electrons into the sensitive matrix, which in turn induces a decrease of the refractive index.

Plasmonic Sensors for Aromatic Hydrocarbon Detection

The development of innovative materials for sensitive and selective gas sensing is a very relevant field for the current nanotechnology research. A strong effort is dedicated to the fabrication of low-cost and efficient nanoscale devices capable of a fast detection. Resistive electrical devices are the most adopted solutions for in-situ and real-time detection, but their main drawbacks are the low selectivity, response drift, electromagnetic noise dependence and need of contact measurements. Optical gas sensors allow to overcome such limits, and could moreover exhibit thermal and mechanical stability, operate at room temperature, and be integrated on-chip. Within this framework, plasmon-based optical devices are knowing an increasing development and diffusion. Herein plasmonic sensors for aromatic hydrocarbon detection are presented. These systems are based on aryl-bridged polysilsesquioxanes (aryl-PSQs), obtained either \(\textcircled{1}\) coupling such hybrid films with Au nanoparticles (NPs), aiming to the excitation of localized surface plasmon resonances (LSPRs), or \(\textcircled{2}\) depositing them onto metallic waveguiding layers, to form gratings supporting the propagation of surface plasmon polaritons (SPPs). Aryl-PSQs are sol-gel materials characterized by a native controlled porosity and other functionalities (Loy and Shea, Chem Rev 95:1431–1442, 1995; Dabrowski et al., Appl Surf Sci 253:5747–5751, 2007; Brigo et al., Nanotechnology 23:325302, 2012). Temperature programmed desorption investigations of xylene on phenyl-bridged (ph-PSQ) and diphenyl-bridged (diph-PSQ) PSQ films indicate a specific π −π interaction between the organic component of the films and xylene molecules: the interaction energy is quantified in 38 ± 14 kJ/mol and 115 ± 13 kJ/mol, respectively (Brigo et al., J Mater Chem C 1:4252, 2013). For type \(\textcircled{1}\) sensors, a thin film of aryl-PSQ was deposited on a submonolayer of Au NPs coating a fused silica substrate. These sensors were tested monitoring the variation of the LSPRs under cycles of exposure to N2 and to 30 ppm xylene in N2.