Polar phonon mode selection rules in tip‐enhanced Raman scattering

We discuss the use of the symmetry selectivity of phonon Raman scattering to determine nanocrystallographic information of solids using tip‐enhanced Raman scattering (TERS). The necessary degrees of freedom arise from the combination of the Raman selection rules reflecting crystal symmetry superimposed by the polarization and k ‐vector‐dependent field enhancement and scattering of the scanning probe tip. The resulting phonon TERS selection rules are discussed, including the use of the crystal Raman tensor and momentum conservation for polar phonon modes. We demonstrate the selection rules for both far‐field and tip‐enhanced near‐field Raman scattering from bulk and nanocrystalline LiNbO 3 .