CPM Seminar

Tip-enhanced optical spectroscopy — breaking the diffraction limit towards the nanoworld

Andreas Rüdiger

Nanoelectronics-Nanophotonics
INRS-EMT

Optical spectroscopy is among the most versatile tool in physics, chemistry, material and life science covering a large spectral range and therefore a variety of light-matter interaction, both linear and non-linear. But regardless of whether the technique is absorption, fluorescence, elastic and inelastic scattering, second harmonic generation only to mention a few and regardless of the use of lasers for scanning techniques: the diffraction limit as quantified in the 19th century limits the spatial resolution to the order of the excitation wavelength. This corresponds to several hundred nanometers in the visible range and used to be a roadblock for the application in nanoscience. Complementary techniques including electron microscopy and scanning probe microscopy pushed the resolution limit to the atomic level, however at the price of severe limitations to the chemical and structural information as compared to optical techniques. In very recent years, the combination of scanning probe techniques such as scanning tunneling and atomic force microscopy (STM and AFM respectively) with confocal laser scanning microscopy has pushed the limits of optical spectroscopy to new horizons. The concept relies on the use of a scanning probe tip to act as a local near-field amplifier (‘nano-antenna’) in the focus of a confocal laser-scanning microscope. The enhancement and confinement of the electromagnetic field is typically achieved through the resonant excitation of surface plasmons in noble-metal tips.

The proof of concept for this technique was delivered about a decade ago [1] and provided evidence for single-molecule sensitivity in resonant-Raman spectroscopy. With dedicated experimental setups, tip-enhanced Raman spectroscopy has achieved spatial resolution of 10 nm and less while preserving most of the spectral information of conventional Raman spectroscopy. The experimental key challenge remains in the reproducible fabrication of resonating tips and their mid-term stability.

This presentation will briefly revisit some merits of conventional spectroscopy in physics and material science before introducing the basics of tip-enhanced spectroscopy. We report on a tip-enhanced Raman experiment for non-transparent, isolating samples as frequently encountered in oxide nanoelectronics, one of the research focuses of our team. We achieved a spatial resolution of 14 nm FWHM on carbon nanotubes [2]. Our latest results on lead titanate (PbTiO₃) nanoislands grown by a template approach show that we achieve tip-enhanced Rayleigh and Raman scattering, the latter with an unprecedented spatial resolution of about 6 nm. Tip-enhanced optical spectroscopy clearly is a promising contender for future generations of surface characterization techniques and the new limits for optical resolution are yet to be determined.

[1] B. Pettinger, B. Ren, G. Picardi, R. Schuster, G. Ertl, Nanoscale Probing of Adsorbed Species by tip-enhanced Raman spectroscopy, Physical Review Letters, 92 (2004) 096101
[2] M. Nicklaus, C. Nauenheim, A. Krayev, V. Gavrilyuk, A. Belyaev, A. Ruediger, Tip-enhanced Raman spectroscopy with objective scanner on opaque samples, Review of Scientific Instruments, 83 (2012) 066102