We provide an effective method to investigate the field gradient effect in nanoconfined plasmon-matter interaction.Aligned ultralong SWNTs without defects were grown on marked substrates,followed by assembling gold nanoparticle clusters around individual nanotubes.The Raman scattering behavior of a nanotube placed in an atomic scale nanogap between adjacent nanoparticles was studied.In addition to the expected plasmon-induced Raman enhancement up to 103,the defect-free D-mode of an individual SWNT induced by gradient field is found for the first time.When the light is confined at atomic scale,gradient field Raman scattering becomes significant and dipole-forbidden phonon modes can be activated by quadrupole Raman tensor variation,indicating breakdown of the Raman selection rules.