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arXiv · 2501.01319

Launching Focused and Spatially Confined Phonon-Polaritons in Hexagonal Boron Nitride

Bogdan Borodin·Sergey Lepeshov·Kenji Watanabe·Takashi Taniguchi·Petr Stepanov
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Abstract

Phonon-polaritons offer significant opportunities for low-loss, subdiffractional light guiding at the nanoscale. Despite extensive efforts to enhance control in polaritonic media, focused and spatially confined phonon-polariton waves have only been realized in in-plane-anisotropic crystals (e.g., MoO$_3$) and remain elusive in in-plane-isotropic materials (e.g., hexagonal boron nitride, hBN). In this study, we introduce a novel approach to launching phonon-polaritons by leveraging hBN subwavelength cavities at the Au/SiO$_2$ interface, enabling efficient coupling of cavities to the far-field component of mid-infrared light. Utilizing standard lithographic techniques, we fabricated subwavelength cavities of various shapes and sizes, demonstrating strong field enhancement, resonant mode localization, and generation of propagating phonon-polaritons with well-defined spatial structure. The cavity geometry governs wavefront curvature, spatial confinement, and polariton focusing, providing control over their propagation and achieving record-high in-plane confinement up to $\lambda/70$. Scattering-type scanning near-field optical microscopy reveals the real-space optical contrast of these cavity-launched modes, allowing for detailed characterization. We believe that our cavity-based approach to phonon-polariton focusing in isotropic media will pave the way for advanced nanophotonic applications.

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Bogdan Borodin, Sergey Lepeshov, Kenji Watanabe, Takashi Taniguchi, Petr Stepanov. 2025-01-02. Launching Focused and Spatially Confined Phonon-Polaritons in Hexagonal Boron Nitride. https://arxiv.org/abs/2501.01319

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