Abstract
Vibrational transitions contain some of the richest fingerprints of molecules and materials, providing considerable physicochemical information. Vibrational transitions can be characterized by different spectroscopies, and alternatively by several imaging techniques enabling to reach sub-microscopic spatial resolution. In a quest to always push forward the detection limit and to lower the number of needed vibrational oscillators to get a reliable signal or imaging contrast, surface plasmon resonances (SPR) are extensively used to increase the local field close to the oscillators. Another approach is based on maximizing the collective response of the excited vibrational oscillators through molecular coherence. Both features are often naturally combined in vibrational nonlinear optical techniques. In this frame, this paper reviews the main achievements of the two most common vibrational nonlinear optical spectroscopies, namely surface-enhanced sum-frequency generation (SE-SFG) and surface-enhanced coherent anti-Stokes Raman scattering (SE-CARS). They can be considered as the nonlinear counterpart and/or combination of the linear surface-enhanced infrared absorption (SEIRA) and surface-enhanced Raman scattering (SERS) techniques, respectively, which are themselves a branching of the conventional IR and spontaneous Raman spectroscopies. Compared to their linear equivalent, those nonlinear vibrational spectroscopies have proved to reach higher sensitivity down to the single molecule level, opening the way to astonishing perspectives for molecular analysis.
Highlights
A widespread approach in molecular analysis relies on the vibrational fingerprint of matter to obtain an intrinsic chemical selectivity, and to identify specific molecules with no added labels
This study demonstrated the possibility of using reproducible plasmonic surfaces able to support strong and repeatable SE-coherent anti-Stokes Raman scattering (CARS) intensity enhancements, and in consequence, fast vibrational imaging
A new record for SE-CARS enhancement has been set by Zhang et al [91], who obtained an amplification of 11 orders of magnitude compared to spontaneous Raman, by exploiting the Fano resonance generated from a nanostructure made of four discs of gold arranged in a diamond-shaped structure
Summary
A widespread approach in molecular analysis relies on the vibrational fingerprint of matter to obtain an intrinsic chemical selectivity, and to identify specific molecules with no added labels. Offering incredible perspectives in various fields, especially in nano-biosciences [2] in which probing tissues at the molecular level has allowed for a deeper understanding of biological properties and behaviours, and has opened the way to fascinating biomedical and biotechnological applications of single molecules and nanomaterials [7,17,18,19,20] This success is mostly due to the electromagnetic near-field enhancement achieved thanks to more and more sophisticated nanostructures and laser sources, coupled with efficient acquisition methods. If this frequency matches a Raman active molecular vibration, the Stokes emission interacts coherently with the pump beam to drive the resonant oscillators at the anti-Stokes frequency ωas = 2ωp − ωs This leads to the emission of a CARS photon at the anti-Stokes frequency.
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