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Surface phonon polaritons (SPhPs) are potentially very attractive for subwavelength control and manipulation of light at the infrared to terahertz wavelengths. Probing their propagation behavior in nanostructures is crucial to guide rational device design. Here, aided by monochromatic scanning transmission electron microscopy-electron energy loss spectroscopy technique, we measure the dispersion relation of SPhPs in individual SiC nanorods and reveal the effects of size and shape. We find that the SPhPs can be modulated by the geometric shape and size of SiC nanorods. The energy of SPhPs shows redshift with decreasing radius and the surface optical phonon is mainly concentrated on the surface with large radius. Therefore, the fields can be precisely confined in specific positions by varying the size of the nanorod, allowing effective tuning at nanometer scale. The findings of this work are in agreement with dielectric response theory and numerical simulation, and provide novel strategies for manipulating light in polar dielectrics through shape and size control, enabling the design of novel nanoscale phononphotonic devices.

Directly mapping local phonon dispersion in individual nanostructures can advance our understanding of their thermal, optical, and mechanical properties. However, this requires high detection sensitivity and combined spatial, energy and momentum resolutions, thus has been elusive. Here, we demonstrate a four-dimensional electron energy loss spectroscopy technique, and present position-dependent phonon dispersion measurements in individual boron nitride nanotubes. By scanning the electron beam in real space while monitoring both the energy loss and the momentum transfer, we are able to reveal position- and momentum-dependent lattice vibrations at nanometer scale. Our measurements show that the phonon dispersion of multi-walled nanotubes is locally close to hexagonal-boron nitride crystals. Interestingly, acoustic phonons are sensitive to defect scattering, while optical modes are insensitive to small voids. This work not only provides insights into vibrational properties of boron nitride nanotubes, but also demonstrates potential of the developed technique in nanoscale phonon dispersion measurements.

Interface phonon modes that are generated by several atomic layers at the heterointerface play a major role in the interface thermal conductance for nanoscale high-power devices such as nitride-based high-electron-mobility transistors and light-emitting diodes. Here we measure the local phonon spectra across AlN/Si and AlN/Al interfaces using atomically resolved vibrational electron energy-loss spectroscopy in a scanning transmission electron microscope. At the AlN/Si interface, we observe various interface phonon modes, of which the extended and localized modes act as bridges to connect the bulk AlN modes and bulk Si modes and are expected to boost the phonon transport, thus substantially contributing to interface thermal conductance. In comparison, no such phonon bridge is observed at the AlN/Al interface, for which partially extended modes dominate the interface thermal conductivity. This work provides valuable insights into understanding the interfacial thermal transport in nitride semiconductors and useful guidance for thermal management via interface engineering.