Kowalski, Ryan A.Nolen, Joshua RyanVarnavides, GeorgiosSilva, Sebastian MikaAllen, Jack E.Ciccarino, Christopher J.Juraschek, Dominik M.Law, StephanieNarang, PrinehaCaldwell, Joshua D.2022-09-292022-09-292022-09-06Kowalski, R. A., Nolen, J. R., Varnavides, G., Silva, S. M., Allen, J. E., Ciccarino, C. J., Juraschek, D. M., Law, S., Narang, P., Caldwell, J. D., Mid- to Far-Infrared Anisotropic Dielectric Function of HfS2 and HfSe2. Adv. Optical Mater. 2022, 2200933. https://doi.org/10.1002/adom.2022009332195-1071https://udspace.udel.edu/handle/19716/31421This is the peer reviewed version of the following article: Kowalski, R. A., Nolen, J. R., Varnavides, G., Silva, S. M., Allen, J. E., Ciccarino, C. J., Juraschek, D. M., Law, S., Narang, P., Caldwell, J. D., Mid- to Far-Infrared Anisotropic Dielectric Function of HfS2 and HfSe2. Adv. Optical Mater. 2022, 2200933. https://doi.org/10.1002/adom.202200933, which has been published in final form at https://doi.org/10.1002/adom.202200933. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited. This article will be embargoed until 09/06/2023.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.en-USdielectric functionfar-infrarednanophotonicspolaritonstransition-metal dichalcogenidesArticle