High quality-factor terahertz phonon-polaritons in layered lead iodide

Area of Science:

• Nanophotonics
• Condensed Matter Physics
• Materials Science

Background:

• Hyperbolic phonon-polaritons in van der Waals materials have advanced mid-infrared nanophotonics.
• Progress at longer terahertz (THz) wavelengths is limited by material properties and experimental challenges.

Purpose of the Study:

• To discover and characterize long-lived hyperbolic phonon-polaritons in the deep THz range.
• To evaluate layered lead iodide (PbI₂) as a material for THz nanophotonics.

Main Methods:

• Room-temperature scattering-type scanning near-field optical microscopy (s-SNOM) for real-space imaging.
• Broadband spectral analysis of PbI₂ 2D crystals on various substrates.
• Comparison of experimental results with theoretical models.

Main Results:

• Discovery of long-lived hyperbolic phonon-polaritons in layered PbI₂ in the deep THz range.
• Experimental figure-of-merit for polariton propagation efficiency exceeding 15, comparable to mid-IR benchmarks.
• Extreme field confinement observed, reaching 264 for a 144 nm-thick flake, with potential to exceed 300.

Conclusions:

• Layered PbI₂ exhibits strong anisotropy, low losses, and extreme mode confinement in the deep THz range.
• PbI₂ is a compelling candidate material for future deep-THz nanophotonic applications.
• The findings overcome limitations in THz nanophotonics, opening new avenues for research and technology.