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1. Selective excitation and imaging of ultraslow phonon polaritons in thin hexagonal boron nitride crystals

   https://doi.org/10.1038/s41377-018-0039-4  

   Ambrosio, Antonio ; Tamagnone, Michele ; Chaudhary, Kundan ; Jauregui, Luis A. ; Kim, Philip ; Wilson, William L. ; Capasso, Federico ( June 2018 , Light: Science & Applications)

   We selectively excite and study two new types of phonon-polariton guided modes that are found in hexagonal boron nitride thin flakes on a gold substrate. Such modes show substantially improved confinement and a group velocity that is hundreds of times slower than the speed of light, thereby providing a new way to create slow light in the mid-infrared range with a simple structure that does not require nano-patterning. One mode is the fundamental mode in the first Restrahlen band of hexagonal boron nitride thin crystals on a gold substrate; the other mode is equivalent to the second mode of the second Restrahlen band of hexagonal boron nitride flakes that are suspended in vacuum.

   The new modes also couple efficiently with incident light at the hexagonal boron nitride edges, as we demonstrate experimentally using photo-induced force microscopy and scanning near-field optical microscopy. The high confinement of these modes allows for Purcell factors that are on the order of tens of thousands directly above boron nitride and a wide band, with new perspectives for enhanced light-matter interaction. Our findings demonstrate a new approach to engineering the dispersion of polaritons in 2D materials to improve confinement and light-matter interaction, thereby paving themore »way for new applications in mid-infrared nano-optics.

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2. Surface plasmons induce topological transition in graphene/α-MoO3 heterostructures

   https://doi.org/10.1038/s41467-022-31477-z  

   Ruta, Francesco L. ; Kim, Brian S. Y. ; Sun, Zhiyuan ; Rizzo, Daniel J. ; McLeod, Alexander S. ; Rajendran, Anjaly ; Liu, Song ; Millis, Andrew J. ; Hone, James C. ; Basov, D. N. ( June 2022 , Nature Communications)

   Polaritons in hyperbolic van der Waals materials—where principal axes have permittivities of opposite signs—are light-matter modes with unique properties and promising applications. Isofrequency contours of hyperbolic polaritons may undergo topological transitions from open hyperbolas to closed ellipse-like curves, prompting an abrupt change in physical properties. Electronically-tunable topological transitions are especially desirable for future integrated technologies but have yet to be demonstrated. In this work, we present a doping-induced topological transition effected by plasmon-phonon hybridization in graphene/α-MoO₃heterostructures. Scanning near-field optical microscopy was used to image hybrid polaritons in graphene/α-MoO₃. We demonstrate the topological transition and characterize hybrid modes, which can be tuned from surface waves to bulk waveguide modes, traversing an exceptional point arising from the anisotropic plasmon-phonon coupling. Graphene/α-MoO₃heterostructures offer the possibility to explore dynamical topological transitions and directional coupling that could inspire new nanophotonic and quantum devices.
3. Lithography-free IR polarization converters via orthogonal in-plane phonons in α-MoO3 flakes

   https://doi.org/10.1038/s41467-020-19499-x  

   Abedini Dereshgi, Sina ; Folland, Thomas G. ; Murthy, Akshay A. ; Song, Xianglian ; Tanriover, Ibrahim ; Dravid, Vinayak P. ; Caldwell, Joshua D. ; Aydin, Koray ( November 2020 , Nature Communications)

   Exploiting polaritons in natural vdW materials has been successful in achieving extreme light confinement and low-loss optical devices and enabling simplified device integration. Recently, α-MoO₃has been reported as a semiconducting biaxial vdW material capable of sustaining naturally orthogonal in-plane phonon polariton modes in IR. In this study, we investigate the polarization-dependent optical characteristics of cavities formed using α-MoO₃to extend the degrees of freedom in the design of IR photonic components exploiting the in-plane anisotropy of this material. Polarization-dependent absorption over 80% in a multilayer Fabry-Perot structure with α-MoO₃is reported without the need for nanoscale fabrication on the α-MoO₃. We observe coupling between the α-MoO₃optical phonons and the Fabry-Perot cavity resonances. Using cross-polarized reflectance spectroscopy we show that the strong birefringence results in 15% of the total power converted into the orthogonal polarization with respect to incident wave. These findings can open new avenues in the quest for polarization filters and low-loss, integrated planar IR photonics and in dictating polarization control.
4. High-Q dark hyperbolic phonon-polaritons in hexagonal boron nitride nanostructures

   https://doi.org/10.1515/nanoph-2020-0048  

   Ramer, Georg ; Tuteja, Mohit ; Matson, Joseph R. ; Davanco, Marcelo ; Folland, Thomas G. ; Kretinin, Andrey ; Taniguchi, Takashi ; Watanabe, Kenji ; Novoselov, Kostya S. ; Caldwell, Joshua D. ; et al ( May 2020 , Nanophotonics)

   Abstract The anisotropy of hexagonal boron nitride (hBN) gives rise to hyperbolic phonon-polaritons (HPhPs), notable for their volumetric frequency-dependent propagation and strong confinement. For frustum (truncated nanocone) structures, theory predicts five, high-order HPhPs, sets, but only one set was observed previously with far-field reflectance and scattering-type scanning near-field optical microscopy. In contrast, the photothermal induced resonance (PTIR) technique has recently permitted sampling of the full HPhP dispersion and observing such elusive predicted modes; however, the mechanism underlying PTIR sensitivity to these weakly-scattering modes, while critical to their understanding, has not yet been clarified. Here, by comparing conventional contact- and newly developed tapping-mode PTIR, we show that the PTIR sensitivity to those weakly-scattering, high-Q (up to ≈280) modes is, contrary to a previous hypothesis, unrelated to the probe operation (contact or tapping) and is instead linked to PTIR ability to detect tip-launched dark, volumetrically-confined polaritons, rather than nanostructure-launched HPhPs modes observed by other techniques. Furthermore, we show that in contrast with plasmons and surface phonon-polaritons, whose Q -factors and optical cross-sections are typically degraded by the proximity of other nanostructures, the high- Q HPhP resonances are preserved even in high-density hBN frustum arrays, which is useful in sensing and quantum emissionmore »applications.« less
5. Hyperbolic shear polaritons in low-symmetry crystals

   https://doi.org/10.1038/s41586-021-04328-y  

   Passler, Nikolai C. ; Ni, Xiang ; Hu, Guangwei ; Matson, Joseph R. ; Carini, Giulia ; Wolf, Martin ; Schubert, Mathias ; Alù, Andrea ; Caldwell, Joshua D. ; Folland, Thomas G. ; et al ( February 2022 , Nature)

   Abstract The lattice symmetry of a crystal is one of the most important factors in determining its physical properties. Particularly, low-symmetry crystals offer powerful opportunities to control light propagation, polarization and phase 1–4 . Materials featuring extreme optical anisotropy can support a hyperbolic response, enabling coupled light–matter interactions, also known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales 5 . Here we show that monoclinic crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid-infrared to far-infrared due to shear phenomena in the dielectric response. This feature emerges in materials in which the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals in which several oscillators with non-orthogonal relative orientations contribute to the optical response 6,7 . Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic 1,3,4 and hexagonal 8,9 crystal systems, unveiling new features, such as the continuous evolution of their propagation direction with frequency, tilted wavefronts and asymmetric responses. The interplay between diagonal loss and off-diagonal shear phenomena in the dielectric response of these materials has implications for new forms of non-Hermitian and topological photonic states. We anticipate that our resultsmore »will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals 10 , many common oxides 11 and organic crystals 12 , greatly expanding the material base and extending design opportunities for compact photonic devices.« less