Depolarization effects in tip-enhanced raman spectroscopy

Tip-enhanced Raman spectroscopy has proven to be a promising technique for stress/strain mapping of silicon-based semiconductor devices on the nanometer scale. Field enhancement factors of up to 10⁴ have been reported and a spatial resolution down to 20 nm has been claimed through exploiting far-field suppression techniques based on an appropriate choice of the excitation/detection polarization states. In this paper, we show that depolarization of light due to scattering from the tip plays a key role in the selective enhancement of the one phonon optical mode peak at 520 cm^-1 with respect to the two phononones. The spatial confinement of the selective enhancement has been studied by means of approach curves, and its dependence on the excitation wavelength and power further explored. Conclusions on the physical nature of the enhancement (depolarization- or plasmonic-based) are presented.