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Phonon‐polaritons, mixed excitations of light coupled to lattice vibrations (phonons), are emerging as a powerful platform for nanophotonic applications. This is because of their ability to concentrate light into extreme sub‐wavelength scales and because of their longer phonon lifetimes compared to their plasmonic counterparts. In this work, the infrared properties of phonon‐polaritonic nanoresonators made of monoisotopic¹⁰B hexagonal boron nitride (h‐BN) are explored, a material with increased phonon‐polariton lifetimes compared to naturally abundant h‐BN due to reduced photon scattering from randomly distributed isotopes. An average relative improvement of 50% of the quality factor of monoisotopic h‐BN nanoresonators is obtained with respect to nanoresonators made of naturally abundant h‐BN, allowing for the sensing of nanometric‐thick films of molecules through both surface‐enhanced absorption spectroscopy and refractive index sensing. Further, even strong coupling between molecular vibrations and the phonon‐polariton resonance in monoisotopic h‐BN ribbons can be achieved.

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.