Combined Tip-Enhanced Raman Spectroscopy and Scattering-Type Scanning Near-Field Optical Microscopy

P. Kusch, N. M. Azpiazu, N. S. Mueller, S. Mastel, J. I. Pascual, and R. Hillenbrand

ACS Publications – The Journal of Physical Chemistry C, 10, 1021 (2018)
Tip-enhanced Raman spectroscopy (TERS) and scattering-type scanning near-field optical microscopy (s-SNOM) enable optical imaging with a spatial resolution far below the diffraction limit of light. While s-SNOM records the elastically scattered light (yielding information about the local refractive index and absorption), in TERS the Raman scattered light is detected, which provides, for example, chemical information. Here we introduce a combined TERS and s-SNOM setup for correlative studies of tip-enhanced elastically and Raman scattered light. To that end, we equipped a conventional s-SNOM with a grating spectrometer. We validate our setup by characterizing a sample consisting of a self-assembled pNTP monolayer on an Au surface. Comparing s-SNOM and TERS signals, we demonstrate a qualitative correlation between tip-enhanced elastic and tip-enhanced Raman scattered light. Thus, recording the tip-enhanced elastically scattered light enables a fast and reliable TERS alignment. Further, we demonstrate experimentally and by simulations that Pt coated silicon tips can be used for TERS in the gap-mode configuration. In the future, our setup could be employed for correlative analyses of structural, chemical and photonic sample properties.