Different longitudinal optical—transverse optical mode amplification in tip enhanced Raman spectroscopy of GaAs(001)

We report on tip enhanced Raman spectroscopy of GaAs(100) crystals taking advantage of the fields polarization to maximize the near-field contrast. Excitation parallel to either the [100] or the [110] crystallographic axis provide minimum far-field background provided that cross- or parallel-component of the Raman field are detected. Experiments highlight a tip-induced selective enhancement of the one-phonon longitudinal optical mode with respect to the transverse optical mode. We interpret this effect in terms of depolarization of the excitation field and beam deflection which breaks the polarization-based selection rules of the Raman scattering.