Revealing Nanoscale Confinement Effects on Hyperbolic Phonon Polaritons with an Electron Beam

Abstract

Hyperbolic phonon polaritons (HPhPs) in hexagonal boron nitride (hBN) enable the direct manipulation of mid‐infrared light at nanometer scales, many orders of magnitude below the free‐space light wavelength. High‐resolution monochromated electron energy‐loss spectroscopy (EELS) facilitates measurement of excitations with energies extending into the mid‐infrared while maintaining nanoscale spatial resolution, making it ideal for detecting HPhPs. The electron beam is a precise source and probe of HPhPs, which allows the observation of nanoscale confinement in HPhP structures and directly extract hBN polariton dispersions for both modes in the bulk of the flake and modes along the edge. The measurements reveal technologically important nontrivial phenomena, such as localized polaritons induced by environmental heterogeneity, enhanced and suppressed excitation due to 2D interference, and strong modification of high‐momenta excitations such as edge‐confined polaritons by nanoscale heterogeneity on edge boundaries. The work opens exciting prospects for the design of real‐world optical mid‐infrared devices based on hyperbolic polaritons.

Curvature-Induced One-Dimensional Phonon Polaritons at Edges of Folded Boron Nitride Sheets

“…Therefore, the beam−sample geometry, illumination condition, and even the strong anisotropy of h-BN could not be the reason for the blue-shifted HPhPs at the folding edge. Compared to previous vibrational EELS results, 12,15,35 we unraveled the sub-nanometer feature of HPhPs, which plays an essential role in detecting the abnormal HPhP modes at the folding edge. Since we used a circular EELS entrance aperture and on-axis EELS setup, our experiments collected the phonon polariton propagating along all directions.…”

“…Since we used a circular EELS entrance aperture and on-axis EELS setup, our experiments collected the phonon polariton propagating along all directions. 35 Nonetheless, one may still investigate the magnitude modification induced by the edge reflection for the phonon polariton mode propagating perpendicular to the edge of a flake, as demonstrated in recent vibrational EELS studies. 12,35 To understand the origin of the exotic HPhP modes, we modeled realistic atomic structures of the folded 1L h-BN edge by stitching a part of BNNT and a stacked bilayer h-BN together.…”

“…35 Nonetheless, one may still investigate the magnitude modification induced by the edge reflection for the phonon polariton mode propagating perpendicular to the edge of a flake, as demonstrated in recent vibrational EELS studies. 12,35 To understand the origin of the exotic HPhP modes, we modeled realistic atomic structures of the folded 1L h-BN edge by stitching a part of BNNT and a stacked bilayer h-BN together. Figure 3A,B shows the relaxed structure for TA = 0°.…”

Infrared Near-Field Spectroscopy of Gold Nanotriangle Fabry-Pérot Resonances

“…Noble metal nanostructures have emerged as powerful optical tools owing to their ability to efficiently confine long wavelength electromagnetic radiation to nanometer length scales. , This ability to confine electromagnetic radiation has been widely exploited in the visible and near-infrared (NIR) regions to develop efficient solar energy harvesting, sensing, and catalytic applications. − Recently, there has been increased interest in extending plasmonic responses to the mid-IR (∼50–500 meV) region, , as it encompasses molecular vibrations and phonons of solid-state materials. , Careful studies of the near-field responses of IR plasmonic materials will aid the development of new physical and chemical platforms capable of targeted analyte detection schemes, surface-enhanced spectroscopies, harnessing IR Fano interferences, and strategies for efficiently capturing and manipulating IR solar and thermal radiation at the nanoscale.…”

“…3−5 Recently, there has been increased interest in extending plasmonic responses to the mid-IR (∼50−500 meV) region, 6,7 as it encompasses molecular vibrations 8 and phonons of solid-state materials. 9,10 Careful studies of the near-field responses of IR plasmonic materials will aid the development of new physical and chemical platforms capable of targeted analyte detection schemes, 11 surface-enhanced spectroscopies, 12 harnessing IR Fano interferences, 13 and strategies for efficiently capturing and manipulating IR solar 7 and thermal 14 radiation at the nanoscale.…”