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We study the amplitude and phase signals detected in infrared scattering-type near field optical microscopy (s-SNOM) when probing a thin sample layer on a substrate. We theoretically describe this situation by solving the electromagnetic scattering of a dipole near a planar sample consisting of a substrate covered by thin layers. We perform calculations to describe the effect of both weakly (Si and SiO(2)) and strongly (Au) reflecting substrates on the spectral s-SNOM signal of a thin PMMA layer. We theoretically predict, and experimentally confirm an enhancement effect in the polymer vibrational spectrum when placed on strongly reflecting substrates. We also calculate the scattered fields for a resonant tip-substrate interaction, obtaining a dramatic enhancement of the signal amplitude and spectroscopic contrast of the sample layer, together with a change of the spectral line shape. The enhanced contrast opens the possibility to perform ultra-sensitive near field infrared spectroscopy of monolayers and biomolecules.

The wave nature of light limits the spatial resolution in classical microscopy to about half of the illumination wavelength. Recently, a new approach capable of achieving subwavelength spatial resolution, called superlensing, was invented, challenging the already established method of scanning near-field optical microscopy (SNOM). We combine the advantages of both techniques and demonstrate a novel imaging system where the objects no longer need to be in close proxim-ity to a near-field probe, allowing for optical near-field microscopy of subsurface objects at sub-wavelength-scale lateral resolution.

The authors present a detection technique for scattering-type near-field optical microscopy capable of background interference elimination in the entire near-UV to far-IR spectral range. It simultaneously measures near-field optical signal amplitude and phase by interferometric detection of scattered light utilizing a phase-modulated reference wave. They compare its background suppression efficiency to other known methods and experimentally show that it provides a reliable near-field optical material contrast even in the case where both noninterferometric and homodyne interferometric detection methods fail.

We demonstrate that mid-infrared surface phonon polariton propagation on a SiC crystal can be imaged by scattering-type near-field optical microscopy. From the infrared images, we measure the wave vector and the propagation length of locally excited surface phonon polaritons. Our method can be also applied to surface plasmon polaritons and allows to study surface polaritons in subwavelength-scale structures.

We demonstrate that scattering-type scanning near-field optical microscopy (s-SNOM) allows nanoscale-resolved imaging of objects below transparent surface layers at both visible and mid-infrared wavelengths. We show topography-free subsurface imaging at lambda=633 nm. At lambda=10.7 microm, gold islands buried 50 nm below a polymer surface are imaged with a lateral resolution120 nm, corresponding to lambda/90. Studying oxide layers with systematically varied thicknesses we provide experimental evidence of mid-infrared near-field probing in depths80 nm.

We show that the scattering efficiency and image contrast of commercial Si tips used in “apertureless” scattering-type near-field optical microscopy can compare well with PtIr- or Au-covered tips as predicted by a simple dipole model. However, high scattering efficiency of Si tips is only achieved after etching an unexpected up to 20-nm-thick amorphous SiO2 layer generally found at the tip apex. The oxide was revealed by transmission electron microscopy. The unverified presence of SiO2 may explain contradicting reports published on the optical performance of Si probes. Bare Si tips offer advantages over metal probes due to reduced fluorescence quenching, low nonlinear optical coefficient, and less wear.

Ultrahigh-resolution (40 nm) near-field microscopy was demonstrated using 2.5 THz illumination, at 118 µm wavelength [1]. This was made possible by the extreme THz field concentration at the metallic probe tip. The THz nanoscope thus exceeds the diffraction limit of resolution by a factor of 2000. Its 40 nm resolving power matches the needs of modern nanoscience and technology.

A combination of experimental techniques is used to demonstrate that optical field concentration and Ohmic losses in planar chiral plasmonic metasurfaces depends on the handedness of circularly polarized light. Chiral dischroism in transmission is demonstrated.

We demonstrate the wide application potential of optical near-field microscopy for mapping samples with locally varying conductivity. The apertureless, scattering-type optical near-field microscope (s-SNOM) operates on an AFM basis with an added light-scattering channel, wherein a standard cantilevered tip suffices to accomodate the full spectral range from visible to THz frequencies. The optical maps appear in amplitude and phase contrast, simultaneously with the topography map, at a spatial resolution of typically 20 nm. Visible and near-infrared operation is best suited for distinguishing metals, while a sensitive response to semiconductors is ideally provided with infrared and THz operation. Various types of conductivity spectral features can be explored in specific regions, corresponding to the elementary excitation linked to e.g. superconductivity, electron correlation, or low-dimensional conduction.

Nano-FTIR spectroscopy based on Fourier transform infrared near-field spectroscopy allows for label-free chemical nanocharacterization of organic and inorganic composite surfaces. The potential capability for subsurface material analysis, however, is largely unexplored terrain. Here, we demonstrate nano-FTIR spectroscopy of subsurface organic layers, revealing that nano-FTIR spectra from thin surface layers differ from that of subsurface layers of the same organic material. Further, we study the correlation of various nano-FTIR peak characteristics and establish a simple and robust method for distinguishing surface from subsurface layers without the need of theoretical modeling or simulations (provided that chemically induced spectral modifications are not present). Our experimental findings are confirmed and explained by a semi-analytical model for calculating nano-FTIR spectra of multilayered organic samples. Our results are critically important for the interpretation of nano-FTIR spectra of multilayer samples, particularly to avoid that geometry-induced spectral peak shifts are explained by chemical effects., Nano-FTIR spectroscopy allows chemical characterization of composite surfaces, but its capability in subsurface analysis is not much explored. The authors show that spectra from thin surface layers differ from those of subsurface layers of the same organic material, and establish a method for distinguishing them in experiments.

Resumen del trabajo presentado a la NanoSpain Conference, celebrada en San Sebastián del 7 al 10 de marzo de 2017., Van der Waals materials (vdW) are a wide class of 2D systems in which individual atomic layers are only weakly bound by van der Waals interaction. Much of the vdW materials functionality results from the large anisotropy in the bonding strength of atoms in the direction parallel to atomic layers and across them, and is often intimately connected with the corresponding phonons. The investigation of phononic excitation in vdW materials thus requires low mid-IR energies with nanoscale spatial resolution. Electron energy loss spectroscopy performed in scanning transmission electron microscopy (STEM-EELS) is a versatile technique capable of performing spectroscopy at the nanoscale. However, conventional STEM-EELS has limited capabilities for mid-IR spectroscopy primarily due to poor monochromaticity of the primary electron beam and limited resolution of the detection system, which mask low energy excitations below 200 meV with the zero loss peak (ZLP) originating from the elastic electron scattering. Here, we reduced the ZLP width of a conventional STEM-EELS system down to 50 meV which allowed us to probe low-energy phononic excitations (down to 100 meV) in vdW materials. Particularly, we performed a nanoscale mapping of electron energy loss (EEL) in hBN flakes, a representative vdW material. Surprisingly, we observe a variation in EEL peak position as a function of flake thickness and the edge prximity, which can not be explained by the bulk phonon excitations. Instead, our developed theory reveiles that the energy loss is dominated by the excitation of phonon polaritons, which exhibit a hyperbolic dispersion owing to the layered crystal structure of hBN. The propagating nature of these hyperbolic polaritons explains the observed phenomena and suggests that STEM-EELS is highly suitable for the investigation of optical/polaritonic properties of vdW materials, with our work laying the foundation for such investigations. With ongoing developments in the monochromator and electron gun designs, we expect further improvement of STEM-EELS spectral resolution and the applicable energy ranges in near future, allowing for correlative studies of polaritons and structural properties in vdW materials.

We have demonstrated a mid-IR superlens fabricated from a SiC membranes and incorporated into an NSOM for subsurface imaging. Enhanced transmission/absorption through hole arrays in a perforated SiC membranes will also be reported.

Recent results on the development of active metallic nanocavities integrated with organic dye molecules in a J-aggregate state will be presented. Highly efficient SERS, the photoluminescence enhancement and photoluminescence lifetime modification have been observed at nanowire junctions.

Phonon-polaritons, mixed excitations of light coupled to lattice vibrations (phonons), are emerging as a powerful platform for nanophotonic applications. This is because of their ability to concentrate light into extreme sub-wavelength scales and because of their longer phonon lifetimes compared to their plasmonic counterparts. In this work, the infrared properties of phonon-polaritonic nanoresonators made of monoisotopic 10B hexagonal boron nitride (h-BN) are explored, a material with increased phonon-polariton lifetimes compared to naturally abundant h-BN due to reduced photon scattering from randomly distributed isotopes. An average relative improvement of 50% of the quality factor of monoisotopic h-BN nanoresonators is obtained with respect to nanoresonators made of naturally abundant h-BN, allowing for the sensing of nanometric-thick films of molecules through both surface-enhanced absorption spectroscopy and refractive index sensing. Further, even strong coupling between molecular vibrations and the phonon-polariton resonance in monoisotopic h-BN ribbons can be achieved., The authors acknowledge funding from the Graphene Flagship (Core2 and Core3), the Spanish Ministry of Science and Innovation (projects MDM-2016-0618 of the Maria de Maeztu Units of Excellence Programme and national projects (RTI2018-094830-B-100, RTI2018-094861-B-100, and PID2019-107432GB-I00)), and the Basque Government (project GIU18/202, Ekartek project KK-2018/00001, IT1164-19, and PhD fellowships PRE_2018_2_0253 and PRE_2017_2_0052). The h-BN crystal growth was supported by the National Science Foundation, grant CMMI 1538127 and the II-VI Foundation.