Long-Lived Phonon Polaritons in Hyperbolic Materials.

Area of Science:

• Condensed matter physics
• Nanophotonics
• Materials science

Background:

• Hyperbolic materials exhibit unique optical properties due to anisotropic dielectric permittivity.
• Hyperbolic phonon polaritons (HPPs) are confined electromagnetic waves coupled to phonons in hyperbolic dielectrics.
• Significant dissipation of HPPs at ambient conditions limits their applications.

Purpose of the Study:

• To investigate the fundamental loss limits of propagating HPPs.
• To explore the temperature dependence of HPP dissipation in hexagonal boron nitride (hBN) and α-MoO₃.
• To identify mechanisms responsible for HPP damping.

Main Methods:

• Cryogenic nano-infrared imaging was employed to study HPP propagation.
• Experiments were conducted on isotopically pure hBN and naturally abundant α-MoO₃ crystals.
• Temperature-dependent measurements were performed near liquid-nitrogen temperatures.

Main Results:

• HPP losses in isotopic hBN dropped significantly near liquid-nitrogen temperatures.
• Propagation lengths for HPPs in hBN exceeded 8 μm, with lifetimes over 5 ps.
• Acoustic phonons were identified as a key factor in HPP damping.

Conclusions:

• Cryogenic temperatures drastically reduce HPP dissipation in hBN, surpassing previous reports.
• Understanding acoustic phonon damping is crucial for mitigating HPP losses.
• Findings are instrumental for miniaturized mid-infrared technologies operating at liquid-nitrogen temperatures.