Infoscience

Thesis

Multifunctional Plasmonic Metasurfaces for Novel Spectral and Spatial Applications

Intensive developments of plasmonic nanomaterials over recent decades have inspired appealing applications in biosensing, optical trapping, fluorescence enhancement and light harvesting in solar cells. These nanostructures supporting unique light-matter interactions are also the important ingredients for constructing two-dimensional metasurfaces and three-dimensional metamaterials that exhibit artificial optical properties. The main focus of this thesis is to investigate the complex resonances supported by nanostructures and to apply them in the framework of metasurfaces for improving performances and creating new functionalities. Here, we manipulate light in two aspects: by using low-loss high order plasmonic modes, namely Fano resonances and by generating strong magnetic enhancement in nanophotonic systems. Optical Fano resonance is one of the pivotal concepts in plasmonic metamaterials that benefits from the strong excitation of a bright mode and the slow decay of a dark mode. We first study the near-field interaction in a Fano resonant system by exploring the transition of the Fano lineshape from a dense dolmen array as a function of periodicities and coupling directions. We experimentally observe different regimes of Fano resonance responses from the strong hybridization to the weak coupling regimes. Additionally, the spectral response of the system can be influenced when the system is placed on substrates or in more general stratified media. To quantify this behavior, we propose a general formalism that disentangles the interferences using the transfer matrix method and the coupled oscillator model. We show that the phase of reflected light can be tuned at will and achieve a phase singularity with zero reflection in the case of a dolmen array on a glass substrate. Another application of Fano resonant surfaces is to arbitrarily control the propagation direction of light using a phase gradient metasurface composed of dolmen unit cells on a metallic backplane. According to Huygens' principle, the signal we perceive from a metasurface is determined by the phases of the different nanostructures. We demonstrate experimentally a narrow band metasurface that benefits from the phase variations of dark modes within a defined spectral range. The flexibility of the design is further illustrated by demonstrating a highly directional color routing effect between two channels in green and red light. Then, we isolate the dark mode from a Fano system and investigate the realization of pure magnetic resonances in the visible spectral range. In nature, magnetic effects on materials are weaker than their electric counterparts and are especially difficult to achieve at optical frequencies. We engineer a novel plasmonic meta-atom which supports a pure magnetic mode without any electric components under plane wave illumination. The magnetic dipole is strongly enhanced and can be distinguished from the electric quadrupole by detuning the phase difference between two sub-parts along with oblique incidence. In addition to the previously mentioned systems, the effect of the diffraction can be incorporated to further tailor the magnetic response in a two-dimensional surface. To this end, we employ a three-disk compound, of which the magnetic dipole mode is forbidden under normally incident light. By engineering the radiative coupling, a collective magnetic resonance is achieved with extremely narrow linewidth (  5 nm). The critical condition for the scattering of the individual unit cell and in-plane diffraction grants the excitation of plasmonic mode with a high-quality factor. Finally, one specific class of metamaterials, extrinsic chiral surfaces, is investigated. We show that the circular dichroism at oblique incidence can be optimized using a T-shaped antenna by combining electric dipole and an orthogonal quadrupole. Furthermore, the emission from incoherent quantum sources can be modified thanks to the intricate interplay between the near-field absorption and the far-field scattering of plasmonic nanostructures. Fluorescence emission is experimentally twisted into two directional channels with opposite circular polarizations by coupling into a guided mode in a high index dielectric slab. These findings provide versatile controls over the angle, color as well as polarization of light using two-dimensional metasurfaces based on multipole resonances. We believe that it can potentially lead to the development of polarization-, spectro- and angleresolved ultra-compact optical devices.

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