Plasmonic metamaterials for sensing applications
Author Kristof Lodewijks
Publication type Ph.D. thesis
Publication date 01-06-2012
Summary Metamaterials are man-made artificial materials of which the optical properties can be engineered to generate the desired response to an incident electromagnetic wave.They consist of sub-wavelength sized structures which can be thought of as the atoms in conventional materials. The collective response of a randomly or periodically ordered ensemble of such meta-atoms defines the properties of the metamaterial, which can be described in terms of effective material parameters such as the permittivity,permeability, refractive index and impedance. Here we show how these metamaterials can be exploited for sensing experiments in the visible and near-infrared wavelength ranges of the electromagnetic spectrum. The meta-atoms used in this work consist of nanostructures defined in gold and silica, which are both very stable and biocompatible materials. At the interface between nano-sized noble metal particles and dielectric media, collective oscillations of the electron cloud in the metal particles can be resonantly excited, which are known as plasmon resonances. In this work we deal with two types of plasmon resonances: localized surface plasmon resonances (LSPRs) and propagating surface plasmon polaritons (SPPs). The investigated sample structures are manufactured by combining conventional lithography (top-down) and self-assembly based colloidal lithography (bottom-up) protocols with standard microprocessing techniques. In that way, we fabricated a self-assembled version of the widely studied double fishnet negative refractive index metamaterials and benchmarked this structure to e-beam lithography based reference structures. We proved that these self-assembled metamaterials can be produced on large scales with a small number of defects and similar performance as the reference structures. In the second part of this work, we focused on self-assembled randomly distributed nanoparticle arrays on top of a continuous gold layer and a thin silica spacer for refractive index sensing applications. We proved that we can reduce the line widths of intrinsically broad dipole resonances in gold nanoparticle arrays by measuring both the amplitude and phase of the reflected waves in spectroscopic ellipsometrymeasurements. By spectrally detuning the electric dipole LSPR for P- and S-polarized waves we can pick up the transition between in- and out-of-phase oscillation of the free electrons in the metal nanoparticles with respect to the incident wave. As a result the line width of the LSPRs is largely reduced, resulting in a major boost of the Figure-Of-Merit (FOM) for refractive index sensing which could eventually result in much lower detection limits. In the third part of this work we optimized the plasmonic metamaterial substrates for refractive index sensing by changing from random particle distributions towards periodic arrays on top of a continuous gold layer and a thin silica spacer. We clearly observe that the eects of inhomogeneous broadening are largely reduced, giving rise to narrower line widths both in amplitude- and phase sensitive measurements,resulting in even larger values for the FOM. The grating structure allows for very efficient excitation of propagating SPP modes on the gold film below, which interact strongly with the localized modes. As we scan the angle of incidence, we clearly observe anti-crossing of the SPP and LSPR modes resulting in highly asymmetric line shapes and increased phase dierences due to Fano-interference. We show thatthe interaction between the SPP mode and the LSPR mode can be used to increase the refractive index sensitivity of the LSPR mode dramatically, which in combination with the reduced line widths results in extremely high values for the FOM.
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