Unveiling the Mechanism of Phonon-Polariton Damping in \alpha-MoO_3

TL;DR

This paper investigates the microscopic damping mechanisms of phonon polaritons in lpha-MoO_3, combining theoretical calculations and experimental techniques to identify phonon-phonon scattering as the dominant loss process, informing future nanophotonic applications.

Contribution

It provides the first comprehensive analysis linking ab initio calculations with experimental data to identify phonon-phonon scattering as the main damping mechanism in lpha-MoO_3 PhPs.

Findings

Third-order anharmonic phonon-phonon scattering is the primary damping mechanism.

Excellent agreement between theory and experiment across temperature range.

Fundamental limits of low-loss PhPs are characterized for nanophotonic device design.

Abstract

Phonon polaritons (PhPs) (light coupled to lattice vibrations) in the highly anisotropic polar layered material molybdenum trioxide (\alpha-MoO_3) are currently the focus of intense research efforts due to their extreme subwavelength field confinement, directional propagation and unprecedented low losses. Nevertheless, prior research has primarily concentrated on exploiting the squeezing and steering capabilities of \alpha-MoO_3 PhPs, without inquiring much into the dominant microscopic mechanism that determines their long lifetimes, key for their implementation in nanophotonic applications. This study delves into the fundamental processes that govern PhP damping in \alpha-MoO_3 by combining ab initio calculations with scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier-transform infrared (FTIR) spectroscopy measurements across a broad temperature range (from 8 to 300 K). The remarkable agreement between our theoretical predictions and experimental observations allows us to identify third-order anharmonic phonon-phonon scattering as the main damping mechanism of \alpha-MoO_3 PhPs. These findings shed light on the fundamental limits of low-loss PhPs, a crucial factor for assessing their implementation into nanophotonic devices.