Mechanical detection and imaging of hyperbolic phonon polaritons in hexagonal Boron Nitride

TL;DR

This paper demonstrates a novel, photon-less method for imaging hyperbolic phonon polaritons in hexagonal Boron Nitride using AFM cantilever sensitivity, achieving high resolution without background light interference.

Contribution

The study introduces a mechanical detection technique for hyperbolic phonon polaritons in hBN, surpassing previous optical methods in resolution and background noise reduction.

Findings

Achieved imaging resolution better than λ/20

First photon-less detection of phonon polaritons

Comparable resolution to s-NSOM techniques

Abstract

Mid-infrared nano-imaging and spectroscopy of two-dimensional (2D) materials have been limited so far to scattering-type Scanning Near-field Optical Microscopy (s-NSOM) experiments where light from the sample is scattered by a metallic-coated Atomic Force Microscope (AFM) tip interacting with the material at the nanoscale. These experiments have recently allowed imaging of plasmon polaritons in graphene as well as hyperbolic phonon polaritons (HP2) in hexagonal Boron Nitride (hBN). Here we show that the high mechanical sensitivity of an AFM cantilever can be exploited for imaging hyperbolic phonon polaritons in hBN. In our imaging process, the lattice vibrations of hBN micrometer-sized flakes are locally enhanced by the launched phonon polaritons. These enhanced vibrations are coupled to the AFM tip in contact to the sample surface and recorded during scanning. Imaging resolution better than {\lambda}/20 is showed, comparable to the best resolution in s-NSOM. Importantly, this detection mechanism is free from light background and it is in fact the first photon-less detection of phonon polaritons.