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Abstract One of the main bottlenecks in the development of terahertz (THz) and long-wave infrared (LWIR) technologies is the limited intrinsic response of traditional materials. Hyperbolic phonon polaritons (HPhPs) of van der Waals semiconductors couple strongly with THz and LWIR radiation. However, the mismatch of photon − polariton momentum makes far-field excitation of HPhPs challenging. Here, we propose an In-Plane Hyperbolic Polariton Tuner that is based on patterning van der Waals semiconductors, here α-MoO₃, into ribbon arrays. We demonstrate that such tuners respond directly to far-field excitation and give rise to LWIR and THz resonances with high quality factors up to 300, which are strongly dependent on in-plane hyperbolic polariton of the patterned α-MoO₃. We further show that with this tuner, intensity regulation of reflected and transmitted electromagnetic waves, as well as their wavelength and polarization selection can be achieved. Our results can help the development of THz and LWIR miniaturized devices. 

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.