We provide an overview of Fano resonance and plasmon induced transparency (PIT) as well as on plasmons coupling in planar structures, and we discuss their application in sensing and enhanced spectroscopy. Metal-insulator-metal (MIM) structures, which are known to support symmetric and anti-symmetric surface plasmon polaritons (SPPs) arising from the coupling between two SPPs at the metal-insulator interfaces, exhibit anticrossing behavior of the dispersion relations arising from the coupling of the symmetric SPP and the metal/air SPP. Multilayer structures, formed by a metal film and a high-index dielectric waveguide (WG), separated by a low-index dielectric spacer layer, give narrow resonances of PIT and Fano line shapes. An optimized Fano structure shows a giant field intensity enhancement value of 106 in air at the surface of the high-index dielectric WG. The calculated field enhancement factor and the figure of merit for the sensitivity of the Fano structure in air can be 104 times as large as those of the conventional surface plasmon resonance and WG sensors.
The anticrossing behavior of dispersion curves of the surface plasmon polaritons supported by metalinsulator-metal structures are studied experimentally and theoretically. Samples consisting of a poly(methyl methacrylate) layer sandwiched by Ag films are prepared and their angle-and wavelength-scan attenuated total reflection spectra are measured. From an analysis of the angle-scan spectrum, the coupled-mode nature of the surface plasmon polariton modes is suggested. The dispersion relations obtained from the wavelength-scan spectra exhibit clearly the anticrossing behavior that arises from the coupling of the modes. The experimental dispersion relations are in good agreement with theoretical ones.
We study the steady-state and dynamics of photo-orientation of photoisomerizable azo dyes in films of polymers by using the Kerr gate optical setup, and we show to what extent such experiments can be used to quantify the dyes photoisomerization and photo-orientation. We report on results at different film thicknesses and absorbances and irradiation light intensities. We rationalize the double exponential growth used to describe empirically the onset of photo-induced birefringence in azo-polymers by using the theory of photo-orientation, and we give analytical expressions for the reaction rates, demonstrating a good agreement with experiments, and we show how to extract the trans → cis isomerization quantum yield from birefringence data. We believe that this work will lead to broader studies of the effect of the environment of complex materials on optically reactive molecules.
Optical sensors based on surface plasmons have attracted much attention over the past decades owing to the wealth of applications in bio‐ and chemical and gas sensing. In surface plasmon resonance sensors, a single metal layer is commonly used, but its resolution is limited because of broad resonances. In this context, we have developed a sensor chip based on a stack of metals and a dielectric, e.g. a metal‐insulator‐metal structure, consisting of a thick insulator layer sandwiched by metal layers, that exhibits a sharp resonance due to the excitation of surface plasmon polaritons hybrid modes. We have performed both experiments and theoretical simulations to estimate the enhancement of the sensitivity of such a structure. By changing the refractive index of an aqueous solution of glucose on top of the sensor chip, we found that the use of a metal‐insulator‐metal structure improves the figure of merit of the sensor 7.5 times compared to that of a conventional surface plasmon resonance sensor chip.
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