Tip-enhanced near-field Raman microscopy spectroscopy is a scanning probe technique that is capable of providing vibrational spectroscopic information on single nanoobjects and surfaces at (sub-) nanometer spatial resolution and high detection sensitivity. In this review, we first illustrate the physical principle of optical nanoantennas used in tip-enhanced near-field Raman microscopy and tip-enhanced Raman scattering (TERS) to efficiently couple light to Raman excitations on nanometer length scales. Although the antennas' electric near-field distributions are commonly understood to determine the spatial resolution, recent experiments showing subnanometer-resolved optical images put this understanding into question. This is because such images enter a regime in which classical electrodynamical descriptions might no longer be applicable and quantum plasmonic and atomistic effects could become relevant. After summarizing the current understanding of plasmonic phenomena at extremely short length scales, we discuss the different mechanisms contributing to the signal enhancement. In addition to the known contributions from electric-field and chemical enhancement, several new models have been proposed very recently that could provide important guidelines for the optimization of TERS experiments. We then review recent developments in the areas of antenna design, fabrication, and characterization. Finally, we briefly highlight recent applications to illustrate future directions of tip-enhanced near-field Raman microscopy and TERS.

We report the angular distribution of the G and 2D Raman scattering from graphene on glass by detecting back focal plane patterns. The G Raman emission can be described by a superposition of two incoherent orthogonal point dipoles oriented in the graphene plane. Due to double resonant Raman scattering, the 2D emission can be represented by the sum of either three incoherent dipoles oriented 120° with respect to each other, or two orthogonal incoherent ones with a 3:1 weight ratio. Parameter-free calculations of the G and 2D intensities are in excellent agreement with the experimental radiation patterns. We show that the 2D polarization ratio and the 2D/G intensity ratio depend on the numerical aperture of the microscope objective. This is due to the depolarization of the emission and excitation light when graphene is on a dielectric substrate, as well as to tight focusing. The polarization contrast decreases substantially for increasing collection angle, due to polarization mixing caused by the air-dielectric interface. This also influences the intensity ratio I(2D)/I(G), a crucial quantity for estimating the doping in graphene. Our results are thus important for the quantitative analysis of the Raman intensities in confocal microscopy. In addition, they are relevant for understanding the influence of signal enhancing plasmonic antenna structures, which typically modify the sample’s radiation pattern.

We demonstrate the remote excitation and detection of surface-enhanced Raman scattering (SERS) from graphene using a silver nanowire as a plasmonic waveguide. By investigating a nanowire touching a graphene sheet at only one terminal, we first show the remote excitation of SERS from graphene by propagating surface plasmon polaritons (SPPs) launched by a focused laser over distances on the order of 10 μm. Remote detection of SERS is then demonstrated for the same nanowire by detecting light emission at the distal end of the nanowire that was launched by graphene Raman scattering and carried to the end of the nanowire by SPPs. We then show that the transfer of the excitation and Raman scattered light along the nanowire can also be visualized through spectrally selective back focal plane imaging. Back focal plane images detected upon focused laser excitation at one of the nanowire's tips reveal propagating surface plasmon polaritons at the laser energy and at the energies of the most prominent Raman bands of graphene. With this approach the identification of remote excitation and detection of SERS for nanowires completely covering the Raman scatterer is achieved, which is typically not possible by direct imaging.

We developed a new method for retrieving the group delay dispersion of a laser from Multiphoton Intra-pulse Interference Phase Scan (MIIPS) data. The method takes into account the spectral amplitude of the laser pulse and provides a direct feedback on the accuracy of the retrieval. The main advantage of the method derives from providing sufficiently high accuracy to avoid the need for multiple experimental iterations. Another advantage is that the new method can discriminate among pulses with different spectral phase and amplitude profiles, in which MIIPS traces occupy the same position in the phase-frequency MIIPS map.