Manipulation of Light-Matter Interaction in Two-Dimensional Systems via Localized Surface Plasmons
Localized surface plasmons (LSPs), which are collective charge oscillation confined by metallic nanoparticles, gained much interest in the field of optoelectronics due to its ability to confine light down to nanoscale without a diffraction limit. As light-matter interaction in nanoscale is becoming more important due to the demand in scaling down the optoelectronic devices, my thesis describes the work on manipulation of such light-matter interaction enabled by LSPs. First, periodically patterned two-dimensional arrays of bowties were investigated to study the localized surface plasmon (LSP) resonances via reflection measurements and numerical simulations. Due to the grating created by arrays of bowties, a new, lattice-coupled LSP (lattice-LSP) mode emerged. Comparing the calculated E-field enhancement of the bowtie arrays to the reflection spectra showed that the lattice-LSP mode positions are closely related to the dips in the reflectance spectra. After the study of bowtie arrays, we showed photoluminescence (PL) from bulk, planar silicon coupled with metal bowtie nanocavities, which is an indirect bandgap semiconductor with very low emission efficiency. This was due to the E-field concentrated inside the tips of the metal bowtie achieved by LSPR, leading to increased radiative decay rate. The approach of bowtie-coupled emitter was also applied to monolayer MoS2, a transition metal dichalcogenide semiconductor which transforms to a direct bandgap semiconductor in monolayer. Silver bowtie array coupled with monolayer of MoS2 showed a high enhancement in emission (Raman and PL) due to surface-enhanced fluorescence (SEF) from weak-coupling of MoS2 excitons and bowtie's LSPR. By tailoring the design of bowtie arrays, we controlled the location of surface plasmon resonances which, coupled with MoS2 excitons, led to spectral modification of PL spectra. Furthermore, at low temperature, we achieved stronger coupling between the two systems in some designs of the bowtie array and observed Fano resonances in reflection measurements. The approach was extended to photocurrent studies in MoS2. Utilizing the helicity of monolayer MoS2 is suggested as future work to investigate the circular photocurrent in MoS2 induced by selective linear polarizations. Lastly, by fabricating nanoribbon arrays of fluorographene, evolution of localized surface plasmon mode of graphene in near-infrared wavelength range was studied via Fourier transform infrared spectroscopy (FTIR). The initial result showed possibility of tunable graphene IR plasmon resonance depending on the array design due to the localized surface plasmon mode created by the grating of alternating fluoro-graphene and graphene nanoribbons, confining E-field to excite the plasmon modes in IR range.
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