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

• Main Authors: 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
• Format: Article
• Language: English
• Published: De Gruyter 2020-05-01
• Series: Nanophotonics
• Subjects: dark modes; hexagonal boron nitride; high-q; hyperbolic phonon polariton; ptir; s-snom
• Online Access: https://doi.org/10.1515/nanoph-2020-0048
• ISSN: 2192-8606; 2192-8614

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.