Nanophotonics (May 2020)

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

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

DOI
https://doi.org/10.1515/nanoph-2020-0048
Journal volume & issue
Vol. 9, no. 6
pp. 1457 – 1467

Abstract

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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.

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