Amplitude- and phase-resolved nano-spectral imaging of phonon polaritons in hexagonal boron nitride

TL;DR

This study uses broadband mid-IR synchrotron radiation to image and analyze phonon polaritons in hexagonal boron nitride, revealing spectral and spatial properties crucial for nanoscale light manipulation.

Contribution

It introduces amplitude- and phase-resolved near-field interferometry for spectral imaging of phonon polaritons in 2D materials, providing new insights into their spectral response and dependence on sample thickness.

Findings

Out-of-plane phonon polariton response is weak compared to in-plane.

Phonon polariton wavelength scales proportionally with sample thickness.

Simultaneous measurement of multiple phonon modes enhances understanding of polariton behavior.

Abstract

Phonon polaritons are quasiparticles resulting from strong coupling of photons with optical phonons. Excitation and control of these quasiparticles in 2D materials offer the opportunity to confine and transport light at the nanoscale. Here, we image the phonon polariton (PhP) spectral response in thin hexagonal boron nitride (hBN) crystals as a representative 2D material using amplitude- and phase-resolved near-field interferometry with broadband mid-IR synchrotron radiation. The large spectral bandwidth enables the simultaneous measurement of both out-of-plane (780 cm-1) and in-plane (1370 cm-1) hBN phonon modes. In contrast to the strong and dispersive in-plane mode, the out-of-plane mode PhP response is weak. Measurements of the PhP wavelength reveal a proportional dependence on sample thickness for thin hBN flakes, which can be understood by a general model describing two-dimensional polariton excitation in ultrathin materials.