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Near field microscopy is a long known technique but the last decade was a big boom of IR techniques.

Beating the diffracion limit?

Yes, it is possible. In the far-field, the resolution of a microscope is limited by the wavelength of the light used. However, with the clever tricks of near-field microscopy scientists can use long wavelength light to study few tens of nanometer samples or large samples with few tens of nanometers resolution.

OK, but how to do it?

There are several techniques to realize near-field microscopes. One could place a sub-wavelength aperture close to the sample and raster scan it through the region of interest (apertured Scanning Near-field Optical Microscopy / SNOM). Another solution is to exploit the scattering of a very small apex needle / sample ensemble (scattering SNOM).

Atomic Force Microscopy and Optical Spectroscopy - an unexpected romance

Scattering of light by an AFM tip allows the near field detection of the interaction of long wavelength photons with materials. Recently, the hottest studies are using infrared light to probe two-dimensional materials, such as graphene, MoS<sub>2</sub>, etc. The information revealed by these studies is truly unique and plenty of new physics is being discovered.

As infrared light probes the carrier concentration of materials it was possible to learn about the electronic nanostructure of different semiconductors that are used to build a variety of electronics.

The first nanoscale imaging of phonon polaritons was also a big discovery when the IR sSNOM imaging revealed a curious reflection pattern on thin BN and graphene flakes.

Where is the opportunity?

From the instrumentation standpoint going towards the terahertz region would allow the real detection of carrier concentrations, but currently the speed of far-infrared detectors makes this very hard. For the material scientists many systems can be investigated with this technique as it has been popularized only in the past five years. As a consequence, data interpretation is not completely established, therefore there are quite a few opportunities to add to the theory of the technique.


Topological valley transport at bilayer graphene domain walls.

Abstract: Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. A tunable bandgap can be induced in bilayer graphene by an external electric field, and such gapped bilayer graphene is predicted to be a topological insulating phase protected by no-valley mixing symmetry, featuring quantum valley Hall effects and chiral edge states. Observation of such chiral edge states, however, is challenging because inter-valley scattering is induced by atomic-scale defects at real bilayer graphene edges. Recent theoretical work has shown that domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators. Here we report an experimental observation of ballistic (that is, with no scattering of electrons) conducting channels at bilayer graphene domain walls. We employ near-field infrared nanometre-scale microscopy (nanoscopy) to image in situ bilayer graphene layer-stacking domain walls on device substrates, and we fabricate dual-gated field effect transistors based on the domain walls. Unlike single-domain bilayer graphene, which shows gapped insulating behaviour under a vertical electrical field, bilayer graphene domain walls feature one-dimensional valley-polarized conducting channels with a ballistic length of about 400 nanometres at 4 kelvin. Such topologically protected one-dimensional chiral states at bilayer graphene domain walls open up opportunities for exploring unique topological phases and valley physics in graphene.

Pub.: 23 Apr '15, Pinned: 30 Apr '17

Model for quantitative tip-enhanced spectroscopy and the extraction of nanoscale-resolved optical constants

Abstract: Near-field infrared spectroscopy by elastic scattering of light from a probe tip resolves optical contrasts in materials at dramatically sub-wavelength scales across a broad energy range, with the demonstrated capacity for chemical identification at the nanoscale. However, current models of probe-sample near-field interactions still cannot provide a sufficiently quantitatively interpretation of measured near-field contrasts, especially in the case of materials supporting strong surface phonons. We present a model of near-field spectroscopy derived from basic principles and verified by finite-element simulations, demonstrating superb predictive agreement both with tunable quantum cascade laser near-field spectroscopy of SiO$_2$ thin films and with newly presented nanoscale Fourier transform infrared (nanoFTIR) spectroscopy of crystalline SiC. We discuss the role of probe geometry, field retardation, and surface mode dispersion in shaping the measured near-field response. This treatment enables a route to quantitatively determine nano-resolved optical constants, as we demonstrate by inverting newly presented nanoFTIR spectra of an SiO$_2$ thin film into the frequency dependent dielectric function of its mid-infrared optical phonon. Our formalism further enables tip-enhanced spectroscopy as a potent diagnostic tool for quantitative nano-scale spectroscopy.

Pub.: 30 Jun '14, Pinned: 30 Apr '17

Nano-optical investigations of the metal-insulator phase behavior of individual VO(2) microcrystals.

Abstract: Despite the relatively simple stoichiometry and structure of VO(2), many questions regarding the nature of its famous metal-insulator transition (MIT) remain unresolved. This is in part due to the prevailing use of polycrystalline film samples and the limited spatial resolution in most studies, hindering access to and control of the complex phase behavior and its inevitable spatial inhomogeneities. Here, we investigate the MIT and associated nanodomain formation in individual VO(2) microcrystals subject to substrate stress. We employ symmetry-selective polarization Raman spectroscopy to identify crystals that are strain-stabilized in either the monoclinic M1 or M2 insulating phase at room-temperature. Raman measurements are further used to characterize the phase dependence on temperature, identifying the appearance of the M2 phase during the MIT. The associated formation and spatial evolution of rutile (R) metallic domains is studied with nanometer-scale spatial resolution using infrared scattering-scanning near-field optical microscopy (s-SNOM). We deduce that even for small crystals of VO(2), the MIT is influenced by the competition between the R, M1, and M2 crystal phases with their different lattice constants subjected to the external substrate-induced stress. The results have important implications for the interpretation of the investigations of conventional polycrystalline thin films where the mutual interaction of constituent crystallites may affect the nature of the MIT in VO(2).

Pub.: 10 Apr '10, Pinned: 30 Apr '17