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Abstract Hyperbolic phonon polaritons (HPhPs) are stimulated by coupling infrared (IR) photons with the polar lattice vibrations. Such HPhPs offer low‐loss, highly confined light propagation at subwavelength scales with out‐of‐plane or in‐plane hyperbolic wavefronts. For HPhPs, while a hyperbolic dispersion implies multiple propagating modes with a distribution of wavevectors at a given frequency, so far it has been challenging to experimentally launch and probe the higher‐order modes that offer stronger wavelength compression, especially for in‐plane HPhPs. In this work, the experimental observation of higher‐order in‐plane HPhP modes stimulated on a 3C‐SiC nanowire (NW)/α‐MoO₃heterostructure is reported where leveraging both the low‐dimensionality and low‐loss nature of the polar NWs, higher‐order HPhPs modes within 2D α‐MoO₃crystal are launched by the 1D 3C‐SiC NW. The launching mechanism is further studied and the requirements for efficiently launching of such higher‐order modes are determined. In addition, by altering the geometric orientation between the 3C‐SiC NW and α‐MoO₃crystal, the manipulation of higher‐order HPhP dispersions as a method of tuning is demonstrated. This work illustrates an extremely anisotropic low dimensional heterostructure platform to confine and configure electromagnetic waves at the deep‐subwavelength scales for a range of IR applications including sensing, nano‐imaging, and on‐chip photonics. 

Abstract The far‐infrared (far‐IR) remains a relatively underexplored region of the electromagnetic spectrum extending roughly from 20 to 100 µm in free‐space wavelength. Research within this range has been restricted due to a lack of optical materials that can be optimized to reduce losses and increase sensitivity, as well as by the long free‐space wavelengths associated with this spectral region. Here the exceptionally broad Reststrahlen bands of two Hf‐based transition metal dichalcogenides (TMDs) that can support surface phonon polaritons (SPhPs) within the mid‐infrared (mid‐IR) into the terahertz (THz) are reported. In this vein, the IR transmission and reflectance spectra of hafnium disulfide (HfS₂) and hafnium diselenide (HfSe₂) flakes are measured and their corresponding dielectric functions are extracted. These exceptionally broad Reststrahlen bands (HfS₂: 165 cm⁻¹; HfSe₂: 95 cm⁻¹) dramatically exceed that of the more commonly explored molybdenum‐ (Mo) and tungsten‐ (W) based TMDs (≈5–10 cm⁻¹), which results from the over sevenfold increase in the Born effective charge of the Hf‐containing compounds. This work therefore identifies a class of materials for nanophotonic and sensing applications in the mid‐ to far‐IR, such as deeply sub‐diffractional hyperbolic and polaritonic optical antennas, as is predicted via electromagnetic simulations using the extracted dielectric function.