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1. Van der Waals isotope heterostructures for engineering phonon polariton dispersions

   https://doi.org/10.1038/s41467-023-40449-w  

   Chen, M. ; Zhong, Y. ; Harris, E. ; Li, J. ; Zheng, Z. ; Chen, H. ; Wu, J. -S. ; Jarillo-Herrero, P. ; Ma, Q. ; Edgar, J. H. ; et al ( August 2023 , Nature Communications)

   Element isotopes are characterized by distinct atomic masses and nuclear spins, which can significantly influence material properties. Notably, however, isotopes in natural materials are homogenously distributed in space. Here, we propose a method to configure material properties by repositioning isotopes in engineered van der Waals (vdW) isotopic heterostructures. We showcase the properties of hexagonal boron nitride (hBN) isotopic heterostructures in engineering confined photon-lattice waves—hyperbolic phonon polaritons. By varying the composition, stacking order, and thicknesses of h¹⁰BN and h¹¹BN building blocks, hyperbolic phonon polaritons can be engineered into a variety of energy-momentum dispersions. These confined and tailored polaritons are promising for various nanophotonic and thermal functionalities. Due to the universality and importance of isotopes, our vdW isotope heterostructuring method can be applied to engineer the properties of a broad range of materials.

    

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2. Nanoscale Guiding of Infrared Light with Hyperbolic Volume and Surface Polaritons in van der Waals Material Ribbons

   https://doi.org/10.1002/adma.201906530  

   Dolado, Irene ; Alfaro‐Mozaz, Francisco Javier ; Li, Peining ; Nikulina, Elizaveta ; Bylinkin, Andrei ; Liu, Song ; Edgar, James H. ; Casanova, Felix ; Hueso, Luis E. ; Alonso‐González, Pablo ; et al ( January 2020 , Advanced Materials)

   Van der Waals (vdW) materials host a variety of polaritons, which make them an emerging material platform for manipulating light at the nanoscale. Due to the layered structure of vdW materials, the polaritons can exhibit a hyperbolic dispersion and propagate as nanoscale‐confined volume modes in thin flakes. On the other hand, surface‐confined modes can be found at the flake edges. Surprisingly, the guiding of these modes in ribbons—representing typical linear waveguide structures—is widely unexplored. Here, a detailed study of hyperbolic phonon polaritons propagating in hexagonal boron nitride ribbons is reported. Employing infrared nanoimaging, a variety of modes are observed. Particularly, the fundamental volume waveguide mode that exhibits a cutoff width is identified, which, interestingly, can be lowered by reducing the waveguide thickness. Further, hybridization of the surface modes and their evolution with varying frequency and waveguide width are observed. Most importantly, it is demonstrated that the symmetrically hybridized surface mode does not exhibit a cutoff width, and thus enables linear waveguiding of the polaritons in arbitrarily narrow ribbons. The experimental data, supported by simulations, establish a solid basis for the understanding of hyperbolic polaritons in linear waveguides, which is of critical importance for their application in future photonic devices.

    

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3. Image polaritons in boron nitride for extreme polariton confinement with low losses

   https://doi.org/10.1038/s41467-020-17424-w  

   Lee, In-Ho ; He, Mingze ; Zhang, Xi ; Luo, Yujie ; Liu, Song ; Edgar, James H. ; Wang, Ke ; Avouris, Phaedon ; Low, Tony ; Caldwell, Joshua D. ; et al ( July 2020 , Nature Communications)

   Polaritons in two-dimensional materials provide extreme light confinement that is difficult to achieve with metal plasmonics. However, such tight confinement inevitably increases optical losses through various damping channels. Here we demonstrate that hyperbolic phonon polaritons in hexagonal boron nitride can overcome this fundamental trade-off. Among two observed polariton modes, featuring a symmetric and antisymmetric charge distribution, the latter exhibits lower optical losses and tighter polariton confinement. Far-field excitation and detection of this high-momenta mode become possible with our resonator design that can boost the coupling efficiency via virtual polariton modes with image charges that we dub ‘image polaritons’. Using these image polaritons, we experimentally observe a record-high effective index of up to 132 and quality factors as high as 501. Further, our phenomenological theory suggests an important role of hyperbolic surface scattering in the damping process of hyperbolic phonon polaritons.

    

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4. Reconfigurable infrared hyperbolic metasurfaces using phase change materials

   https://doi.org/10.1038/s41467-018-06858-y  

   Folland, T. G. ; Fali, A. ; White, S. T. ; Matson, J. R. ; Liu, S. ; Aghamiri, N. A. ; Edgar, J. H. ; Haglund, Jr., R. F. ; Abate, Y. ; Caldwell, J. D. ( October 2018 , Nature Communications)

   Metasurfaces control light propagation at the nanoscale for applications in both free-space and surface-confined geometries. However, dynamically changing the properties of metasurfaces can be a major challenge. Here we demonstrate a reconfigurable hyperbolic metasurface comprised of a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with the phase-change material (PCM) single-crystal vanadium dioxide (VO₂). Metallic and dielectric domains in VO₂provide spatially localized changes in the local dielectric environment, enabling launching, reflection, and transmission of hyperbolic phonon polaritons (HPhPs) at the PCM domain boundaries, and tuning the wavelength of HPhPs propagating in hBN over these domains by a factor of 1.6. We show that this system supports in-plane HPhP refraction, thus providing a prototype for a class of planar refractive optics. This approach offers reconfigurable control of in-plane HPhP propagation and exemplifies a generalizable framework based on combining hyperbolic media and PCMs to design optical functionality.

    

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5. Charge-transfer hyperbolic polaritons in α-MoO3/graphene heterostructures

   https://doi.org/10.1063/5.0173562  

   Shen, J. ; Chen, M. ; Korostelev, V. ; Kim, H. ; Fathi-Hafshejani, P. ; Mahjouri-Samani, M. ; Klyukin, K. ; Lee, G.-H. ; Dai, S. ( April 2024 , Applied Physics Reviews)

   Charge transfer is a fundamental interface process that can be harnessed for light detection, photovoltaics, and photosynthesis. Recently, charge transfer was exploited in nanophotonics to alter plasmon polaritons by involving additional non-polaritonic materials to activate the charge transfer. Yet, direct charge transfer between polaritonic materials has not been demonstrated. We report the direct charge transfer in pure polaritonic van der Waals (vdW) heterostructures of α-MoO3/graphene. We extracted the Fermi energy of 0.6 eV for graphene by infrared nano-imaging of charge transfer hyperbolic polaritons in the vdW heterostructure. This unusually high Fermi energy is attributed to the charge transfer between graphene and α-MoO3. Moreover, we have observed charge transfer hyperbolic polaritons in multiple energy–momentum dispersion branches with a wavelength elongation of up to 150%. With the support from the density functional theory calculation, we find that the charge transfer between graphene and α-MoO3, absent in mechanically assembled vdW heterostructures, is attributed to the relatively pristine heterointerface preserved in the epitaxially grown vdW heterostructure. The direct charge transfer and charge transfer hyperbolic polaritons demonstrated in our work hold great promise for developing nano-optical circuits, computational devices, communication systems, and light and energy manipulation devices. 

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