Far-Infrared Hyperbolic Phonon-Polaritons in Zirconium Disulfide.
basic_science · Level V
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- Also identified by DOI 10.1002/adma.74154.
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Abstract
Group-IVB transition-metal dichalcogenides (TMDs) have recently emerged as a promising material platform for extreme light confinement, with Hf-based compounds demonstrating confinement factors exceeding two orders of magnitude in the far-infrared. As a complementary Zr-based member of this material family, zirconium disulfide ( <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>ZrS</mi> <mn>2</mn></msub> </math> ) combines a comparably broad first Reststrahlen band with semiconducting electronic character, providing a wide spectral window for phonon-dominated far-infrared hyperbolic polaritonics. Here, we report the first experimental demonstration of far-infrared hyperbolic phonon polaritons in the group-IVB TMD <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>ZrS</mi> <mn>2</mn></msub> </math> using a resonator-assisted far-field spectroscopy platform. An unpatterned <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>ZrS</mi> <mn>2</mn></msub> </math> flake integrated with a metallic ribbon array forms a phonon polariton resonator, enabling efficient far-field excitation of phonon polaritons while suppressing extrinsic scattering losses. This high coupling efficiency enables far-field observation of multiple polaritonic resonances beyond the fundamental branch. The large normalized light-matter coupling strength of <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>ZrS</mi> <mn>2</mn></msub> </math> enables ultrahigh in-plane momenta, with effective refractive indices as high as 223. Despite this extreme confinement, linewidth analysis indicates that the measured damping is primarily governed by intrinsic propagation loss, corresponding to a sub-picosecond polariton lifetime. These results establish <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>ZrS</mi> <mn>2</mn></msub> </math> as a van der Waals hyperbolic material platform for ultraconfined far-infrared phonon polaritons and highlight the potential of group-IVB TMDs for compact far-infrared nanophotonic and thermal photonic applications.