Tip-Enhanced Second Harmonic Generation: an Approach for Hyper-Raman Spectroscopy

Abstract

The near-field optical response generated at the apex of a scanning probe in tip-enhanced Raman spectroscopy (TERS) enhances and confines the interaction of light and matter to a few nanometers, thus breaking the diffraction limit of light while preserving the wealth of chemical and structural information. With the way paved to deploy Raman spectroscopy to surface nanostructures, we now investigate the most favorable experimental conditions and possible limitations for tip-enhanced hyper-Raman spectroscopy (TEHRS). In this two-photon-one phonon process, detected around the second harmonic of the excitation wavelength, selection rules different from those of the conventional Raman effect apply, thus holding promise for an almost comprehensive vibrational signature of the sample. TEHRS signal intensity critically depends on the second harmonic generation (SHG) efficiency. The aim of this work is to outline the experimental conditions that are best suited to obtain TEHRS signals in the challenging context of an extremely low scattering cross section. Using a silver tip and non-centrosymmetric PbTiO3 film on platinum substrate as a sample system in our simulations, we show a strong dependence of the intensity of SHG on the polarization of the incident light, the tip radius, the presence of a non-centrosymmetric sample, and the excitation power. We further discuss the effect of tip-sample distance for early-stage (proof of concept) TEHRS investigations and found 40% of total power dissipation for 1 nm distance.