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‐MoO3 heterostructure is reported where leveraging both the low‐dimensionality and low‐loss nature of the polar NWs, higher‐order HPhPs modes within 2D α‐MoO3 crystal 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‐MoO3 crystal, 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.

spectral windows due to the lack of an established optical infrastructure. Spectroscopy in this range can be a difficult task, due to the lack of efficient radiation sources within the so-called "terahertz gap" that extends beyond the range where blackbody sources can provide sufficient power. Despite recent developments to create powerful radiation within this region, accessing solid-state properties at these frequencies remains challenging. [4][5][6] Passive optical components in the mid-to far-IR also suffer from undesirable dispersion due to the optic phonon resonances of the material. This dispersion is associated with high absorption losses and optical performance that is strongly frequency dependent, resulting in a non-uniform transmission intensity over the spectral band of interest. [7] Materials with suitable spectral properties exist, but they all suffer from other challenges. For example, cesium iodide (CsI) has extremely lowenergy vibrational modes outside of this spectral range giving rise to suitable transmission, [8][9][10] but it is hygroscopic and therefore must be operated under vacuum or kept within other inert atmospheres. While thin polymer coatings can be applied over CsI optics as a protective layer, these induce additional vibrational resonances,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 (HfS 2 ) and hafnium diselenide (HfSe 2 ) flakes are measured and their corresponding dielectric functions are extracted. These exceptionally broad Reststrahlen bands (HfS 2 : 165 cm −1 ; HfSe 2 : 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.