We show that hexagonal boron nitride (hBN), a two-dimensional insulator supports tunable excitons in the near and middle ultraviolet if subjected to an external superlattice potential. Our calculations predict that as we increase the strength of the potential, the gap reduces, and the anisotropy of the dispersion is enhanced. Consequently, the binding-energies of the excitons decrease, leading to a red-shift of the excitonic levels. We also observe that the absorption is reduced when we change the polarization from along the periodicity of the potential to perpendicular to it, with the system acting as an optical polarizer. As we reduce the gap, the characteristic frequency range for which we can excite exciton-polaritons red-shifts as well. These modes behave quite differently from pristine hBN in extreme cases where the anisotropy of the system grows indefinitely. In this way, by tuning the potential, we can manipulate the excitonic and sub-gap optical properties of hBN.
When graphene is placed in the near vicinity of a metal substrate, graphene plasmons are screened by the metal thereby giving rise to acoustic graphene plasmons. These exhibit record-high field confinement, squeezing light down to nanometer dimensions. Hence, plasmons in such heterostructures make ideal candidates to probe the quantum nonlocal electrodynamic response of the nearby metal. We treat graphene at the level of the RPA and describe the nonclassical optical response of the metal using a framework of mesoscopic electrodynamics based on microscopic surface-response functions, known as Feibelman d-parameters, which embody quantum corrections in the metal’s response. We show that the graphene plasmons’ resonances exhibit quantum shifts due to the quantum surface-response of the metal, and show how these spectral shifts can be used to interrogate the quantum surface-response of metals, thus provide a theoretical basis for experimentally inferring the said quantum surface-response.
Acoustic-graphene-plasmons (AGPs) are highly confined electromagnetic modes, which carry extreme momentum and low loss in the Mid-infrared (MIR) to Terahertz (THz) spectra. They are therefore enablers of extremely strong light-matter interactions at these long wavelengths. However, owing to their large momentum they are also challenging to excite and detect. Here, we demonstrate a new way to excite AGPs that are confined to nanometric-scale cavities directly from the far-field, via localized graphene-plasmon-magnetic-resonators (GPMRs). This approach enables the efficient excitation of single AGP cavities, which are able to confine MIR light to record-breaking ultra-small mode-volumes, which are over a billion times smaller than their free-space volume.
Acoustic-graphene-plasmons (AGPs) are highly confined electromagnetic modes, which carry extreme momentum and low loss in the Mid-infrared (MIR) to Terahertz (THz) spectra. They are therefore enablers of extremely strong light-matter interactions at these long wavelengths. However, owing to their large momentum they are also challenging to excite and detect. Here, we demonstrate a new way to excite AGPs that are confined to nanometric-scale cavities directly from the far-field, via localized graphene-plasmon-magnetic-resonators (GPMRs). This approach enables the efficient excitation of single AGP cavities, which are able to confine MIR light to record-breaking ultra-small mode-volumes, which are over a billion times smaller than their free-space volume.
Interesting plasmonic effects can arise from the combination of small polarisable particles with graphene, related to surface plasmon-polaritons (SPPs) supported by the latter in the terahertz (THz) spectral range. The electromagnetic coupling between graphene SPPs and dipole moments of nanoparticles (NPs) deposited on top of it gives rise to optical properties that aren’t present in the individual components of this system. The NPs’ polarisability is renormalized due to the electromagnetic back action of the SPPs which are excited in graphene when an external propagating electromagnetic wave impinges on the particle. Moreover, beyond the usual dipole-dipole interaction, an indirect particle-particle coupling arises via polarisation charges induced on the graphene-covered interface by each particle – this indirect coupling oscillates with the interparticle distance. We derived coupled-dipole equations taking into account all these effects, allowing us to calculate an effective optical conductivity of the particles’ monolayer. One of the G+NPs system’s unique properties is a collective polariton mode, causing a considerable enhancement of the THz radiation absorption in graphene, while the reflection drops to nearly zero for a broad range of angles of incidence. The frequency of this resonant mode can be adjusted by changing the Fermi energy in graphene via electrostatic gating and therefore it can be used for electrically controlled reflection and transmission of THz radiation
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