Nanoscale mapping and spectroscopy of non-radiative hyperbolic modes in hexagonal boron nitride nanostructures

TL;DR

This paper demonstrates that photothermal induced resonance (PTIR) can directly observe dark hyperbolic phonon polariton modes in hexagonal boron nitride nanostructures, revealing new optical modes for nanophotonics.

Contribution

The study introduces PTIR as a novel technique to detect dark hyperbolic modes in hBN nanostructures, surpassing limitations of previous far-field methods.

Findings

PTIR successfully visualizes dark hyperbolic modes.

Dark modes previously unobserved are now accessible.

Potential for enhanced control in nanophotonic devices.

Abstract

The inherent crystal anisotropy of hexagonal boron nitride (hBN) sustains naturally hyperbolic phonon polaritons, i.e. polaritons that can propagate with very large wavevectors within the material volume, thereby enabling optical confinement to exceedingly small dimensions. Indeed, previous research has shown that nanometer-scale truncated nanocone hBN cavities, with deep subwavelength dimensions, support three-dimensionally confined optical modes in the mid-infrared. Due to optical selection rules, only a few of many such modes predicted theoretically have been observed experimentally via far-field reflection and scattering-type scanning near-field optical microscopy. The Photothermal induced resonance (PTIR) technique probes optical and vibrational resonances overcoming weak far-field emission by leveraging an atomic force microscope (AFM) probe to transduce local sample expansion due to light absorption. Here we show that PTIR enables the direct observation of previously unobserved, dark hyperbolic modes of hBN nanostructures. Leveraging these optical modes could yield a new degree of control over the electromagnetic near-field concentration, polarization and angular momentum in nanophotonic applications.