Mid- to Far-Infrared Anisotropic Dielectric Function of HfS2 and HfSe2

Ryan A. Kowalski, Joshua Ryan Nolen, Georgios Varnavides, Sebastian Mika Silva, Jack E. Allen, Christopher J. Ciccarino, Dominik M. Juraschek, Stephanie Law*, Prineha Narang*, Joshua D. Caldwell*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


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.

Original languageEnglish
Article number2200933
JournalAdvanced Optical Materials
Issue number23
StatePublished - 5 Dec 2022


FundersFunder number
NSF DMR1905295
School for Science and Math at Vanderbilt
National Science Foundation2128240
National Science Foundation
Office of Naval ResearchN00014‐18‐12107
Office of Naval Research
U.S. Department of EnergyDE‐AC02‐05CH11231, DE‐SC0019140
U.S. Department of Energy
Division of Materials Research1904760
Division of Materials Research
National Aeronautics and Space Administration80NSSC 21K0766
National Aeronautics and Space Administration
Defense Advanced Research Projects AgencyD18AC00014
Defense Advanced Research Projects Agency
Gordon and Betty Moore FoundationGBMF8048
Gordon and Betty Moore Foundation
University of Delaware
Office of Science
Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung184259
Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung


    • dielectric function
    • far-infrared
    • nanophotonics
    • polaritons
    • transition-metal dichalcogenides


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