Browsing by Subject "Transition metal dichalcogenides"
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Item Coupling of far-infrared light to the surface phonon-polaritons in Transition metal dichalcogenides(2021-05) Saha, SubhodipEnhanced 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.Item Nanoscale Coherent-Acoustic-Phonon Dynamics in Molybdenum Disulfide Using Ultrafast Electron Microscopy(2021-06) Zhang, YichaoIn this dissertation, photoexcited, defect-mediated anisotropic acoustic-phonon dynamics in molybdenum disulfide (MoS2) have been directly imaged on the nanometer-picosecond spatiotemporal scales in real space. MoS2 is a prototypical material system of transition metal dichalcogenides extensively studied due to the exceptional tunability of many properties (e.g., electronic band structure) via layer number and strain, and thus attracts interests in a broad range of device applications. Defects have been demonstrated to impact local properties and dynamics in a spatial range from a single atom to hundreds of nanometers. The combined nanometer spatial resolution and femtosecond temporal resolution of ultrafast electron microscopy (UEM) enables direct visualization of photoinduced anisotropic acoustic-phonon dynamics localized at nanoscale defects in freestanding, multilayer MoS2.Excitation of the phonon dynamics is achieved via uniformly illuminating the specimens with 515-nm, 300-fs laser pulses. Propagation of the waves locally deforms the crystal lattice, leading to a modulation of the local Bragg conditions. Visualization of the waves is thus achieved by monitoring modulation of diffraction contrast features (e.g., bend contours). When viewed along the [0001] direction, photoexcited individual phonon wave trains were observed to be emitted from crystal-crystal interfaces (e.g., step edge and terrace) and propagate along a single wave vector perpendicular to the interfaces at frequencies in the tens of gigahertz (GHz) range and at approximately the in-plane speed of sound (7 nm/ps). When viewed along a high-index zone axis (i.e., large angle between specimen normal and the incident electron wave vector), the observed diffraction-contrast dynamics exhibits no propagations but only oscillations. Such specimen configuration allows for projection of c-axis wave dynamics onto the image plane. The c-axis phonon velocity was extracted from the oscillation frequency and specimen thickness, consistent with c-axis speed of sound (2.9 nm/ps). The onset of the c-axis dynamics occurs a few picoseconds earlier than that of the in-plane dynamics, suggesting photoinduced modulation of interlayer spacing leading to launching of in-plane compression waves. In addition to serving as launching sites of phonon wave trains, step edges can mediate localized dynamics that are distinct from that observed in pristine regions. The c-axis phonon modes exhibit dephased oscillations at an individual step edge owing to different specimen thicknesses, and even one-unit-cell of thickness difference manifests a few GHz of frequency shift. Step edges have also been demonstrated to induce new relaxation states extending to several hundred nanometers. Subsequently, a frequency reduction in the c-axis phonon oscillation (i.e., phonon softening) at an individual, nanoscale step edge was observed, indicative of associated softening of the elastic constant. This is consistent with results obtained with finite element modeling. In the process of preparing ultrathin, pristine specimens for studying photoinduced structural dynamics in mono- and bilayer MoS2, substrate-directed spontaneous strain patterning was observed in the term of twelve-fold zone-axis patterns and six-fold centroidal Voronoi tessellation patterns. Vertical deformation of up to 35 nm for several bilayer MoS2 crystals were measured with atomic force microscopy. The formation mechanism of such pattern was elucidated with atomistic simulations.