Selective excitation and imaging of ultraslow phonon polaritons in thin hexagonal boron nitride crystals

TL;DR

This paper demonstrates selective excitation and hyperspectral nano-imaging of ultraslow phonon polaritons in thin hexagonal boron nitride, revealing new opportunities for strong light-matter interactions in 2D materials.

Contribution

It introduces methods for selective excitation of confined phonon polaritons and compares imaging techniques, advancing understanding of ultraslow polaritonic modes in h-BN.

Findings

Selective excitation of RS1 and RS2 polariton modes achieved.

Full hyperspectral nano-imaging of RS1 modes demonstrated.

Comparison of PiFM and s-SNOM imaging techniques on the same platform.

Abstract

Polaritons in 2D and van der Waals (vdW) materials have been investigated in several recent works as an innovative platform for light-matter interaction, rich of new physical phenomena.Hexagonal Boron Nitride (h-BN), in particular, is an out of plane anisotropic material (while it is in-plane isotropic) with two very strong phonon polaritons bands where the permittivity becomes negative. In the first restrahlen band (RS1, 780-830 cm-1) the relative out of plane permittivity is negative, while in the second restrahlen band (RS2, 1370-1610 cm-1) the relative in-plane permittivity is negative. Due to these optical properties, thin h-BN flakes support guided modes which have been observed experimentally both via far field and near field methods. In this work, we show how selectively excite the more confined modes in the RS1 and RS2 bands. The supported guided modes have phase and group velocities respectively tens and hundreds of times slower than the speed of light. We also show the possibility of full hyperspectral nano-imaging of modes in RS1 band by means of photo-induced force microscopy (PiFM). Moreover, a direct comparison of (PiFM) and scattering-type near-field microscopy (s-SNOM) is obtained by imaging the modes of the RS2 band with both techniques implemented on the same platform. The possibility of addressing ultraslow (ultraconfined) polaritonic modes of h-BN crystal flakes together with the possibility of optical nano-imaging in both the restrahlen bands have many innovative aspects that can lead to unprecedented schemes for strong light-matter interaction, slow and confined light.