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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. 

Abstract Wavelength‐selective absorbers (WS‐absorbers) are of interest for various applications, including chemical sensing and light sources. Lithography‐free fabrication of WS‐absorbers can be realized via Tamm plasmon polaritons (TPPs) supported by distributed Bragg reflectors (DBRs) on plasmonic materials. While multifrequency and nearly arbitrary spectra can be realized with TPPs via inverse design algorithms, demanding and thick DBRs are required for high quality‐factors (Q‐factors) and/or multiband TPP‐absorbers, increasing the cost and reducing fabrication error tolerance. Here, high Q‐factor multiband absorption with limited DBR layers (3 layers) is experimentally demonstrated by Tamm hybrid polaritons (THPs) formed by coupling TPPs and Tamm phonon polaritons when modal frequencies are overlapped. Compared to the TPP component, the Q‐factors of THPs are improved twofold, and the angular broadening is also reduced twofold, facilitating applications where narrow‐band and nondispersive WS‐absorbers are needed. Moreover, an open‐source algorithm is developed to inversely design THP‐absorbers consisting of anisotropic media and exemplify that the modal frequencies can be assigned to desirable positions. Furthermore, it is demonstrated that inversely designed THP‐absorbers can realize same spectral resonances with fewer DBR layers than a TPP‐absorber, thus reducing the fabrication complexity and enabling more cost‐effective, lithography‐free, wafer‐scale WS‐absorberss for applications such as free‐space communications and gas sensing.