  Vibrational optical spectroscopies can be enhanced by surface plasmons to
reach molecular-sized limits of detection and characterization. The level of
enhancement strongly depends on microscopic details of the sample that are
generally missed by macroscopic techniques. Here we investigate phonons in h-BN
by coupling them to silver-nanowire plasmons, whose energy is tuned by
modifying the nanowire length. Specifically, we use electron beam milling to
accurately and iteratively change the nanowire length, followed by electron
energy-loss spectroscopy to reveal the plasmon-enhanced vibrational features of
h-BN. This allows us to investigate otherwise hidden bulk phonons and observe
strong plasmon-phonon coupling. The new milling-and-spectroscopy technique
holds great potential for resolving vibrational features in material
nanostructures.
