High-Q dark hyperbolic phonon-polaritons in hexagonal boron nitride nanostructures.

Georg Ramer,Andrey Kretinin,Mohit Tuteja,Joseph R Matson,Thomas G Folland,Andrea Centrone,Takashi Taniguchi,Marcelo Davanco,Joshua D Caldwell,Kenji Watanabe,Kostya S Novoselov

doi:10.1515/nanoph-2020-0048

Abstract

The anisotropy of hexagonal boron nitride (hBN) gives rise to hyperbolic phonon-polaritons (HPhPs), notable for their volumetric frequency-dependent propagation and strong confinement. For frustum (truncated nanocone) structures, theory predicts five, high-order HPhPs, sets, but only one set was observed previously with far-field reflectance and scattering-type scanning near-field optical microscopy. In contrast, the photothermal induced resonance (PTIR) technique has recently permitted sampling of the full HPhP dispersion and observing such elusive predicted modes; however, the mechanism underlying PTIR sensitivity to these weakly-scattering modes, while critical to their understanding, has not yet been clarified. Here, by comparing conventional contact- and newly developed tapping-mode PTIR, we show that the PTIR sensitivity to those weakly-scattering, high-Q (up to ≈280) modes is, contrary to a previous hypothesis, unrelated to the probe operation (contact or tapping) and is instead linked to PTIR ability to detect tip-launched dark, volumetrically-confined polaritons, rather than nanostructure-launched HPhPs modes observed by other techniques. Furthermore, we show that in contrast with plasmons and surface phonon-polaritons, whose Q-factors and optical cross-sections are typically degraded by the proximity of other nanostructures, the high-Q HPhP resonances are preserved even in high-density hBN frustum arrays, which is useful in sensing and quantum emission applications.

Highlights

The coupled excitation of light and coherent charge oscillations in materials, known as polaritons, enables squeezing light to deeply sub-diffractional dimensions
The thermal expansion decays rapidly [43, 45]. The dynamics of this process can be captured directly in photothermal induced resonance (PTIR) using fast nanophotonic probes [43]; the expansion is too rapid for the conventional atomic force microscope (AFM) probes [44], which, instead, are shocked by the expansion and kicked into oscillation like to a struck tuning fork
We have conclusively and unambiguously identified that the mode of operation of the AFM probe is not the discriminating factor enabling the observation of dark and weakly scattering higher (n > 0) order hyperbolic phonon-polaritons (HPhPs) modes within nanostructured hyperbolic materials. This conclusion is manifested in the PTIR spectra and maps on hexagonal boron nitride (hBN) frusta that are unchanged in character when measured in contact- or tapping-mode

Summary

Introduction

The coupled excitation of light and coherent charge oscillations in materials, known as polaritons, enables squeezing light to deeply sub-diffractional dimensions. Polaritons in highly anisotropic materials, where the permittivity along one of the axis is of opposite sign to the others, instead propagate within the volume of the material at an angle dictated by the ratio of the permittivity values along two orthogonal directions [2,3,4,5]. Such materials are referred to as hyperbolic.

Methods

Results

Conclusion