Browsing by Subject "Nanophotonics"

Now showing 1 - 4 of 4
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    Alternative Methods and Materials for use in Plasmonics
    (2019-03) Klemme, Daniel
    Plasmonic devices are extremely useful across a wide variety of fields and have been used for ultra-high-resoulution imaging, drug detection, metamaterials, and single-molecule studies among other things. One major hurdle to achieving useful plasmonic structures is that deeply subwavelength patterns need to be generated, both for coupling the light to the device and to fabricate the device itself. Many plasmonic devices such as optical antennas used for nanofocusing are nonplanar, which vastly increases the difficulty of fabricating subwavelength structures on them. Standard lithographic processes such as photolithography and electron beam lithography are of limited use on three-dimensional substrates, which necessitates the development of novel fabrication techniques. Shadow mask lithography and conformal coating of metallic sidewalls via atomic layer deposition are two techniques that will be used to achieve subwavelength patterning of three-dimensional structures. Additionally, plasmonic materials have typically been dominated by gold and to a lesser extent silver because they exhibit good dielectric properties at optical frequencies and are reasonably robust to ambient conditions. However, these materials do come with their own fabrication limitations that other plasmonically active materials such as titanium nitride and copper do not necessarily have. In particular, atomic layer deposition recipes now exist for titanium nitride that allow sub-10 nm, continuous, and conformal metallic films to be created which opens up the door to novel ultrathin plasmonic structures. In this dissertation, plasmonic structures that were generated using nonstandard nanofabrication techniques and/or metallic materials will be explored, demonstrating the advantages that come with using such techniques and materials.
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    Coupling of far-infrared light to the surface phonon-polaritons in Transition metal dichalcogenides
    (2021-05) Saha, Subhodip
    Enhanced light-matter interactions through a plethora of dipole-typepolaritonic excitations started to emerge in two-dimensional (2D) layered materials in recent years. 2D Van der Waals (vdWs) polar crystals sustaining phonon-polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications within the mid-infrared (mid-IR) to terahertz ranges. One of the fundamental hurdles in polaritonics is the trade-off between electromagnetic field confinement and the coupling efficiency with free-space light, a consequence of the large momentum mismatch between the excitation source and polaritonic modes. Our group recently demonstrated the fundamental problem of momentum mismatch can be overcome with a graphene acoustic plasmon resonator with nearly perfect absorption (94%) of incident mid-infrared light. This high efficiency is achieved by utilizing a two-stage coupling scheme: free-space light coupled to conventional graphene plasmons, which then couple to ultraconfined acoustic plasmons. To date, experimental demonstration of excitation of the surface phonon-polaritons (SPhPs) in transition metal dichalcogenides (TMDs) remains unexplored, so here we demonstrate novel strategies for dynamically controlling of far-infrared light using unique optical properties of TMDs in particular ZrS2. In comparison to other vdW materials, the phonon modes of these TMDs are much softer and exhibits phonon peaks within the far-infrared (far-IR) regime. Here we experimentally demonstrate the excitation of SPhPs in ZrS2 acoustic resonator by the far-IR absorption spectroscopy. The surface optical (SO) phonon modes of these TMDs were tuned and tailored to lie anywhere within the reststrahlen band, by controlling the ribbon width, which brings extreme tunability. In addition, the absorption can be pushed beyond 90% with the integration of gold reflectors and can become promising material for far-IR biosensing. The results demonstrate TMDs as a new platform for studying phonon-polaritons exhibiting good quality factors and excellent tunability which enable far-IR nanophotonics devices. Recently our group demonstrated ultra strong coupling (USC) between polar phonons and mid-IR light in coaxial nanocavities. Here we push the boundary further up to the far-IR range and we propose to demonstrate USC coupling between polar phonons and far-IR light using the coaxial nanocavity platform. Our numerical simulation predicts a level splitting of strongly coupled polaritons of 95% of the resonant frequency, enabled by epsilon-near-zero (ENZ) responses in the far-IR of the ZrS2 filled coaxial nanocavities. The ability to reach the USC regime in mass-produced nanocavity systems can open up new avenues to explore non-perturbatively coupled light-matter systems, multiphoton effects, as well as higher-order nonlinear effects, which may lead to novel applications in sensing, spectroscopy, and nanocavity optomechanics.
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    Coupling of Surface Plasmons and Semiconductor Nanocrystals for Nanophotonics Applications
    (2015-08) Jayanti, Sriharsha
    The goal of this thesis is to engineer the interaction between surface plasmons and semiconductor nanocrystals for nanophotonic applications. Plasmonic metals support surface plasmon polaritons, hybrid photon and electron waves that propagate along a metal-dielectric interface. Unlike photons, surface plasmons can be confined in sub-diffraction geometries. This has two important consequences: 1) optical devices can be designed at the nanoscale, and 2) the high density of electromagnetic fields allows study of enhanced light-matter interactions. Surface plasmons have been exploited to demonstrate components of optoelectronic circuits, optical antennas, surface enhanced spectroscopy, enhanced fluorescence from fluorophores, and nanolasers. Despite the advances, surface plasmon losses limit their propagation lengths to tens of micrometers in the visible wavelengths, hindering many applications. Recently, the template-stripping approach was shown to fabricate metal films that exhibit larger grains and smoother surface, reducing the grain boundary and roughness scattering. To further improve the plasmonic properties, we investigate the importance of deposition conditions in the template-stripping approach. We provide insight and recipes to enhance the plasmonic performance of the most commonly used metals in the ultraviolet, visible, and near-infrared. We also explore the potential of low temperatures to improve the performance of metal films, where the electron-electron and electron-phonon scattering should be reduced. This sets a limit on the minimum loss metals can exhibit. Using this knowledge, we study the optical properties of quantum-confined semiconductor nanocrystals near metal structures. Semiconductor nanocrystals have many attractive characteristics that make them suitable for solid-state lighting and solar cells among others. Specifically, CdSe nanocrystals have been heavily studied for their large absorption and emission cross-sections, size dependent emission wavelengths, photostability, and high quantum yields. Here, we focus on studying the emission from CdSe nanocrystals near plasmonic structures in the weak and strong coupling regimes. In the weak coupling regime, plasmonic structures can be used to selectively modify the radiative rates at the desired wavelengths. We tailor plasmonic structures to enhance and tune the emission from the surface states of CdSe nanocrystals throughout the visible. Due to their size, a significant fraction of atoms are on the surface; however, electron-hole recombination via surface states is typically dark. We further use electrochemistry to probe the energy levels of the surface states. In the strong coupling regime, the energy levels of the surface plasmons and nanocrystals hybridize to form polariton states. In this regime, we demonstrate polariton emission from CdSe/CdSZnS core/shell/shell nanocrystals on silver hole arrays. Emission from these polariton states should be coherent and has implications for thresholdless lasing. While the above studies focus on the change in nanocrystal behavior near metals, these nanocrystals can also be used to improve plasmonic performance. We study the potential of thin layers of CdSe nanocrystals to amplify surface plasmons and enhance their propagation lengths. When the nanocrystals are excited using an external pump, propagating surface plasmons can stimulate emission from these nanocrystals and amplify. If more surface plasmons are generated than lost, then surface-plasmon signals can propagate over extremely long distances and even amplified. We calculate the gain provided and discuss the importance of key parameters such as the absorption and emission cross section, spacer layer thickness, nanocrystal lifetime, and temperature. Finally, we systematically study the emission properties and exciton decay in Ag-doped CdSe nanocrystals, which were recently shown to exhibit enhanced photoluminescence. Overall, this thesis aims to improve plasmonic performance with and without the presence of a gain medium, and advances the understanding of optical behavior of CdSe nanocrystals near metal structures in the weak and strong coupling regimes.
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    Nanophotonic Assemblies and Light Management for Increased Absorption and Photoluminescence
    (2020-10) Quan, Matthew
    This thesis explores three projects involving the interaction of incident light with nanocrystals: absorption enhancement in patterned quantum dot solids (pQDS), time dynamics of plasmonic nanorod assemblies, and the usage of luminescent solar concentrators (LSCs) to enhance upconversion. First, the effect of nanostructure shape on the absorption in pQDS is studied, and then these structures are integrated with plasmonic nanorings and bullseyes to enhance both absorption and photoluminescence (PL). In the next chapter, the time-dependent optical properties of a gold nanorod assembly is modeled following ultrafast laser illumination, accounting for both changes in the electron density and the nanostructure geometry. Finally, Monte Carlo modeling is used to study the effect of LSCs and mirrors on the absorption and upconverted PL from a hydrogel containing the upconverting donor/acceptor pair PtOEP/DPAS.