Tailoring plasmons in van der Waals nano-materials by design

Plasmonics in two-dimensional (2D) materials and their van der Waals (vdW) heterostructures, as a key component of modern nanophotonics, have significant advancements with the advent of 2D materials and their vdW heterostructures. This dissertation explores various aspects of plasmons in 2D materials and their vdW heterostructures, their interactions with anisotropic dielectric environments, their potential for asymmetric chiral propagation, and their coupling with phonons in vdW heterostructures. We first investigate Dirac plasmons in graphene surrounded by anisotropic dielectric environments, demonstrating that dielectric anisotropy can strongly influence their spatial propagation and polarization properties. Our findings show that anisotropic dielectric environments not only introduce new plasmon modes and damping pathways but also enable the realization of hyperbolic plasmons, thereby offering a promising route for precise plasmonic control. Building upon this understanding, we propose a mechanism for chiral plasmon polariton propagation, showing that chiral plasmon transport is both more efficient and tunable compared to recently observed chiral shear phonon polaritons. By leveraging anisotropic electron-electron interactions and twist engineering in anisotropic 2D materials, we demonstrate that gate voltage and twist angle can be used to achieve precise control over chiral plasmon propagation. This provides a feasible and scalable approach for on-chip, high-performance optical devices. In addition, we explore plasmon-phonon coupling in vdW heterostructures, particularly focusing on acoustic plasmons. While the coupling between optical plasmons and phonons has been extensively studied, the hybridization of acoustic plasmons with phonons remains largely unexplored. Here, we show that double-layer graphene intercalated with a transition-metal dichalcogenide (TMD) can exhibit ultrastrong acoustic plasmon-phonon coupling, which we quantify using the quantum-electrostatic heterostructure method. Our analysis reveals the optimal conditions for achieving enhanced plasmon-phonon interactions, opening pathways for highly sensitive nanoscale thermal and optical applications. Finally, we address the enhancement of acoustic graphene screened plasmon-phonon polaritons (AG-SPPPs), a class of acoustic modes in graphene-TMD heterostructures. While AG-SPPPs are highly sensitive to their dielectric surroundings, achieving strong coupling in realistic heterostructures remains a challenge. We show that using a metal substrate (i) significantly enhances the coupling strength between acoustic plasmons and TMD phonons and (ii) improves the sensitivity of the plasmon wavelength to the structural details of the vdW heterostructure, thereby enabling monolayer-resolution probing of material composition. In summary, these findings demonstrate the crucial role of dielectric environmental engineering and vdW engineering in controlling plasmons, highlight new routes for chiral and hyperbolic plasmonic devices, and provide a foundation for harnessing plasmon-phonon interactions in vdW heterostructures. These insights contribute to the advancement of next-generation nanophotonic platforms and their applications in compact, tunable optical devices.