Phonon imaging in the sub-diffraction limit

Abstract

Abbe’s limit of optical spatial resolution restricts an accurate spectroscopic characterization of an object at the nanoscale. Plasmonic assisted nanoscopy, however, can overcome the limit of resolution utilizing the localized electric field in the proximity of noble metal nanoclusters. We report the phonon imaging of GaN nanowire (NW) of ~225 nm and studies of Mg doping in GaN with the help of Au nanoparticle assisted tip-enhanced Raman spectroscopy (TERS) in the near-field well below the diffraction limit, for the first time. We also image InN nanorods in the sub-diffraction limit using surface optical phonon mode to differentiate the orientation of the one-dimensional nanostructures. The smallest size of 25 nm Si NW is imaged using TERS and is analyzed for the deviation of the phonon behavior with respect to that for NWs with a larger diameter as well as in the ensemble for far-field measurements. The challenge in phonon imaging of nanostructures having varying polarizability with different fractional ionic component (FIC) is also discussed while analyzing one order larger sized, at least in one dimension, AlN (FIC ≈0.72) nanotip as compared to the Si NW (FIC ≈0). In this regard, imaging GaN NW in the sub-diffraction limit with FIC ≈0.5 for GaN may be appreciated.The original version of this article supplied to AIP Publishing excluded the citations for each figure. The author has since added this information and an updated version of this article was published on 19 November 2020.