Scattering-type Scanning Near-field Optical Microscopy Research Articles

Ultrabroadband Nanospectroscopy with a Laser-Driven Plasma Source

Scattering-type scanning near-field optical microscopy (s-SNOM) enables infrared spectroscopy at 10–20 nm spatial resolution through elastic light scattering. Coupled with an infrared light source, s-SNOM characterizes chemical compositions or probes nanoscale photonic phenomena on length scales 2 orders of magnitude below the diffraction limit. However, widespread use of s-SNOM as an analytical standard tool has been restrained to a large extent by the lack of a bright and affordable broadband light source. Here we present a turnkey thermal emitter based on a laser-driven plasma that offers incoherent radiation of a broader bandwidth (>1000 cm–1) and ∼40-fold higher brilliance than previous blackbody radiators in addition to a compact size and at a fraction of the cost of alternative coherent laser systems or synchrotrons. We demonstrate a nearly 1 order of magnitude increase in signal-to-noise in near-field spectra compared to existing incoherent emitters, which allows probing of not only inorganic mate...

All-electronic terahertz nanoscopy

Probing conductivity in a contactless way with nanoscale resolution is a pressing demand in such active fields as quantum materials, superconductivity, and molecular electronics. Here, we demonstrate a laser- and cryogen-free microwave-technology-based scattering-type scanning near-field optical microscope powered by an easily aligned free-space beam with a tunable frequency up to 0.75 THz. It uses Schottky diode components to record background-free amplitude and phase nano-images, for the first time in the terahertz range, which is uniquely sensitive for assessing conduction phenomena. Images of Si with doped nanostructures prove a conductance sensitivity corresponding to 1016 cm−3 mobile carriers, at 50 nm spatial resolution.

High performance mixed matrix membranes (MMMs) composed of ZIF-94 filler and 6FDA-DAM polymer

Carbon capture and storage (CCS) using membranes for the separation of CO2 holds great promise for the reduction of atmospheric CO2 emissions from fuel combustion and industrial processes. Among the different process outlines, post-combustion CO2 capture could be easily implemented in existing power plants. However, for this technology to become viable, new membrane materials have to be developed. In this article we present the development of high performance mixed matrix membranes (MMMs) composed of ZIF-94 filler and 6FDA-DAM polymer matrix. The CO2/N2 separation performance was evaluated by mixed gas tests (15CO2:85N2) at 25°C and 1–4bar transmembrane pressure difference. The CO2 membrane permeability was increased by the addition of the ZIF-94 particles, maintaining a constant CO2/N2 selectivity of ~22. The largest increase in CO2 permeability of ~ 200% was observed for 40wt% ZIF-94 loading, reaching the highest permeability (2310 Barrer) at similar selectivity among 6FDA-DAM MMMs reported in literature. For the first time, the ZIF-94 metal organic framework crystals with particle size smaller than 500nm were synthesized using nonhazardous solvent (tetrahydrofuran and methanol) instead of dimethylformamide (DMF) in a scalable process. Membranes were characterized by three non-invasive image techniques, i.e. SEM, AFM and nanoscale infrared imaging by scattering-type scanning near-field optical microscopy (s-SNOM). The combination of these techniques demonstrates a very good dispersion and interaction of the filler in the polymer layer, even at very high loadings.

Rigorous numerical modeling of scattering-type scanning near-field optical microscopy and spectroscopy

Over the last decade, scattering-type scanning near-field optical microscopy and spectroscopy have been widely used in nano-photonics and material research due to their fine spatial resolution and broad spectral range. A number of simplified analytical models have been proposed to quantitatively understand the tip-scattered near-field signal. However, a rigorous interpretation of the experimental results is still lacking at this stage. Numerical modelings, on the other hand, are mostly done by simulating the local electric field slightly above the sample surface, which only qualitatively represents the near-field signal rendered by the tip-sample interaction. In this work, we performed a more comprehensive numerical simulation which is based on realistic experimental parameters and signal extraction procedures. By directly comparing to the experiments as well as other simulation efforts, our methods offer a more accurate quantitative description of the near-field signal, paving the way for future studies of complex systems at the nanoscale.

Near-Field Imaging of Infrared Nanoantenna Modes Under Oblique Illumination

Plasmonic nanoantennas provide powerful tools in enhancing light–matter interactions by linking propagating far fields with localized near fields. Microscopic understanding on the enhancements of light–matter interactions requires precise knowledges on the near-field distributions upon nanoantenna excitations. In this article, we study near-field distributions of nanoantennas under oblique illumination. We acquire the amplitude- and phase-resolved images of the near fields on Au nanorods in the infrared range, by use of the scattering-type scanning near-field optical microscopy and the interferometric homodyne detection. Asymmetric near-field distributions are experimentally visualized and theoretically reproduced. The asymmetry in the near-field distributions is attributed to the preferential excitation of the surface plasmon polariton in one direction and the finite loss of the surface plasmon polariton inside the cavity. Furthermore, we organize the excitation conditions of the nanoantenna modes under ...

Mapping Free-Carriers in Multijunction Silicon Nanowires Using Infrared Near-Field Optical Microscopy.

We report the use of infrared (IR) scattering-type scanning near-field optical microscopy (s-SNOM) as a nondestructive method to map free-carriers in axially modulation-doped silicon nanowires (SiNWs) with nanoscale spatial resolution. Using this technique, we can detect local changes in the electrically active doping concentration based on the infrared free-carrier response in SiNWs grown using the vapor-liquid-solid (VLS) method. We demonstrate that IR s-SNOM is sensitive to both p-type and n-type free-carriers for carrier densities above ∼1 × 1019 cm-3. We also resolve subtle changes in local conductivity properties, which can be correlated with growth conditions and surface effects. The use of s-SNOM is especially valuable in low mobility materials such as boron-doped p-type SiNWs, where optimization of growth has been difficult to achieve due to the lack of information on dopant distribution and junction properties. s-SNOM can be widely employed for the nondestructive characterization of nanostructured material synthesis and local electronic properties without the need for contacts or inert atmosphere.

Infrared Nanoimaging Reveals the Surface Metallic Plasmons in Topological Insulator

Surface plasmons make a high degree of localization of electromagnetic fields achievable at the vicinity of metal surfaces. Topological insulators (TIs) are a family of materials which are insulating in the bulk but have metallic surfaces caused by the strong spin–orbit coupling. Surface plasmons supported by the surface state on topological insulators have attracted incredible interests from ultraviolet to mid-infrared frequencies. In this work, we experimentally investigate the near-field properties of Bi2Te3 nanosheets using scattering-type scanning near-field optical microscopy (s-SNOM). The s-SNOM tip enables to detect significantly enhanced intensity in its near field at precisely controlled positions with regards to Bi2Te3 structure. With the help of highly position-selective excitation and high-pixel real-space mapping, we discover near-field patterns of bright outside fringes which are associated with its surface-metallic, plasmonic behavior at mid-infrared frequency. Thereby, we experimentally d...

Near-field edge fringes at sharp material boundaries.

We have studied the formation of near-field fringes when sharp edges of materials are imaged using scattering-type scanning near-field optical microscope (s-SNOM). The materials we have investigated include dielectrics, metals, a near-perfect conductor, and those that possess anisotropic permittivity and hyperbolic dispersion. For our theoretical analysis, we use a technique that combines full-wave numerical simulations of tip-sample near-field interaction and signal demodulation at higher orders akin to what is done in typical s-SNOM experiments. Unlike previous tip-sample interaction near-field models, our advanced technique allows simulation of the realistic tip and sample structure. Our analysis clarifies edge imaging of recently emerged layered materials such as hexagonal boron nitride and transition metal dichalcogenides (in particular, molybdenum disulfide), as well as traditional plasmonic materials such as gold. Hexagonal boron nitride is studied at several wavelengths, including the wavelength where it possesses excitation of phonon-polaritons and hyperbolic dispersion. Based on our results of s-SNOM imaging in different demodulation orders, we specify resonant and non-resonant types of edges and describe the edge fringes for each case. We clarify near-field edge-fringe formation at material sharp boundaries, both outside bright fringes and the low-contrast region at the edge, and elaborate on the necessity of separating them from propagating waves on the surface of polaritonic materials.

Nanoscale mapping of refractive index by using scattering-type scanning near-field optical microscopy

We present a novel method for nanoscale reconstruction of complex refractive index by using scattering-type Scanning Near-field Optical Microscopy (s-SNOM). Our method relies on correlating s-SNOM experimental image data with computational data obtained through simulation of the classical oscillating point-dipole model. This results in assigning a certain dielectric function for every pixel of the s-SNOM images, which further results in nanoscale mapping of the refractive index. This method is employed on human erythrocytes to demonstrate the approach in a biologically relevant manner. The presented results advance the current knowledge on the capabilities of s-SNOM to extract quantitative information with nanoscale resolution from optical data sets with biological application.

Mechanical Detection and Imaging of Hyperbolic Phonon Polaritons in Hexagonal Boron Nitride.

Mid-infrared nanoimaging and spectroscopy of two-dimensional (2D) materials have been limited so far to scattering-type scanning near-field optical microscopy (s-SNOM) experiments, where light from the sample is scattered by a metallic-coated atomic force microscope (AFM) tip interacting with the material at the nanoscale. These experiments have recently allowed imaging of plasmon polaritons in graphene as well as hyperbolic phonon polaritons in hexagonal boron nitride (hBN). Here we show that the high mechanical sensitivity of an AFM cantilever can be exploited for imaging hyperbolic phonon polaritons in hBN. In our imaging process, the lattice vibrations of hBN micrometer-sized flakes are locally enhanced by the launched phonon polaritons. These enhanced vibrations are coupled to the AFM tip in contact with the sample surface and recorded during scanning. Imaging resolution of Δ/20 is shown (Δ being the polaritonic fringes' separation distance), comparable to the best resolution in s-SNOM. Importantly, this detection mechanism is free from light background, and it is in fact the first photonless detection of phonon polaritons.

Imaging the Localized Plasmon Resonance Modes in Graphene Nanoribbons.

We report a nanoinfrared (IR) imaging study of the localized plasmon resonance modes of graphene nanoribbons (GNRs) using a scattering-type scanning near-field optical microscope (s-SNOM). By comparing the imaging data of GNRs that are aligned parallel and perpendicular to the in-plane component of the excitation laser field, we observed symmetric and asymmetric plasmonic interference fringes, respectively. Theoretical analysis indicates that the asymmetric fringes are formed due to the interplay between the localized surface plasmon resonance (SPR) mode excited by the GNRs and the propagative surface plasmon polariton (SPP) mode launched by the s-SNOM tip. With rigorous simulations, we reproduce the observed fringe patterns and address quantitatively the role of the s-SNOM tip on both the SPR and SPP modes. Furthermore, we have seen real-space signatures of both the dipole and higher-order SPR modes by varying the ribbon width.

Efficiency of Launching Highly Confined Polaritons by Infrared Light Incident on a Hyperbolic Material.

We investigated phonon-polaritons in hexagonal boron nitride-a naturally hyperbolic van der Waals material-by means of the scattering-type scanning near-field optical microscopy. Real-space nanoimages we have obtained detail how the polaritons are launched when the light incident on a thin hexagonal boron nitride slab is scattered by various intrinsic and extrinsic inhomogeneities, including sample edges, metallic nanodisks deposited on its top surface, random defects, and surface impurities. The scanned tip of the near-field microscope is itself a polariton launcher whose efficiency proves to be superior to all the other types of polariton launchers we studied. Our work may inform future development of polaritonic nanodevices as well as fundamental studies of collective modes in van der Waals materials.

Broadband near-field infrared spectroscopy with a high temperature plasma light source.

Scattering-type scanning near-field optical microscopy (S-SNOM) has enormous potential as a spectroscopy tool in the infrared spectral range where it can probe phonon resonances and carrier dynamics at the nanometer lengths scales. However, its applicability is limited by the lack of practical and affordable table-top light sources emitting intense broadband infrared radiation in the 100 cm-1 to 2,500 cm-1 spectral range. This paper introduces a high temperature plasma light source that is both ultra-broadband and has much more radiant power in the infrared spectral range than conventional, table-top thermal light sources such as the globar. We implement this plasma lamp in our near-field optical spectroscopy set up and demonstrate its capability as a broadband infrared nano-spectroscopy light source by obtaining near-field infrared amplitude and phase spectra of the phonon resonances of SiO2 and SrTiO3.

Anisotropic excitation of surface plasmon polaritons on a metal film by a scattering-type scanning near-field microscope with a non-rotationally-symmetric probe tip

Abstract We investigated the excitation of surface plasmon polaritons on gold films with the metallized probe tip of a scattering-type scanning near-field optical microscope (s-SNOM). The emission of the polaritons from the tip, illuminated by near-infrared laser radiation, was found to be anisotropic and not circularly symmetric as expected on the basis of literature data. We furthermore identified an additional excitation channel via light that was reflected off the tip and excited the plasmon polaritons at the edge of the metal film. Our results, while obtained for a non-rotationally-symmetric type of probe tip and thus specific for this situation, indicate that when an s-SNOM is employed for the investigation of plasmonic structures, the unintentional excitation of surface waves and anisotropic surface wave propagation must be considered in order to correctly interpret the signatures of plasmon polariton generation and propagation.

Mapping three-dimensional near-field responses with reconstruction scattering-type scanning near-field optical microscopy

Scattering-type scanning near-field optical microscopy (s-SNOM) enables mapping of nanoscale field distributions in two dimensions. However, the standard s-SNOM technique lacks direct resolving ability along the vertical direction, therefore unable to provide full three-dimensional near-field responses. Here, we develop a reconstruction technique that enables s-SNOM to collect a three-dimensional response cube of near-field interaction. The technique also allows a new operational mode of s-SNOM based on the characteristic decay range of near-field interactions. As a demonstration, the bound near-field at the sides of a polaritonic boron nitride nanotube is revealed through the collection of the near-field response cube. The graphene boundary and discontinuities are revealed by the near-field decay range mapping. The reconstruction s-SNOM technique extends the capability of s-SNOM and is generally applicable for a wide range of nanoscale characterizations that are suitable for s-SNOM, such as characterizations of plasmonic and polaritonic nanostructures.

Dual-Scattering Near-Field Microscope for Correlative Nanoimaging of SERS and Electromagnetic Hotspots.

Surface-enhanced Raman spectroscopy (SERS) enables sensitive chemical studies and materials identification, relying on electromagnetic (EM) and chemical-enhancement mechanisms. Here we introduce a tool for the correlative nanoimaging of EM and SERS hotspots, areas of strongly enhanced EM fields and Raman scattering, respectively. To that end, we implemented a grating spectrometer into a scattering-type scanning near-field optical microscope (s-SNOM) for mapping of both the elastically and inelastically (Raman) scattered light from the near-field probe, that is, a sharp silicon tip. With plasmon-resonant gold dimers (canonical SERS substrates) we demonstrate with nanoscale spatial resolution that the enhanced Raman scattering from the tip is strongly correlated with its enhanced elastic scattering, the latter providing access to the EM-field enhancement at the illumination frequency. Our technique has wide application potential in the correlative nanoimaging of local-field enhancement and SERS efficiency as well as in the investigation and quality control of novel SERS substrates.

A high signal-to-noise ratio passive near-field microscope equipped with a helium-free cryostat.

We have developed a passive long-wavelength infrared (LWIR) scattering-type scanning near-field optical microscope (s-SNOM) installed in a helium-free mechanically cooled cryostat, which facilitates cooling of an LWIR detector and optical elements to 4.5 K. To reduce mechanical vibration propagation from a compressor unit, we have introduced a metal bellows damper and a helium gas damper. These dampers ensure the performance of the s-SNOM to be free from mechanical vibration. Furthermore, we have introduced a solid immersion lens to improve the confocal microscope performance. To demonstrate the passive s-SNOM capability, we measured thermally excited surface evanescent waves on Au/SiO2 gratings. A near-field signal-to-noise ratio is 4.5 times the improvement with an acquisition time of 1 s/pixel. These improvements have made the passive s-SNOM a more convenient and higher-performance experimental tool with a higher signal-to-noise ratio for a shorter acquisition time of 0.1 s.

Mapping of Bernal and non-Bernal stacking domains in bilayer graphene using infrared nanoscopy.

Bilayer graphene (BLG) shows great potential as a new material for opto-electronic devices because its bandgap can be controlled by varying the stacking orders, as well as by applying an external electric field. An imaging technique that can visualize and characterize various stacking domains in BLG may greatly help in fully utilizing such properties of BLG. Here we demonstrate that infrared (IR) scattering-type scanning near-field optical microscopy (sSNOM) can visualize Bernal and non-Bernal stacking domains of BLG, based on the stacking-specific inter- and intra-band optical conductivities. The method enables nanometric mapping of stacking domains in BLG on dielectric substrates, augmenting current limitations of Raman spectroscopy and electron microscopy techniques for the structural characterization of BLG.

Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures.

The possibility of hybridizing collective electronic motion with mid-infrared light to form surface polaritons has made van der Waals layered materials a versatile platform for extreme light confinement and tailored nanophotonics. Graphene and its heterostructures have attracted particular attention because the absence of an energy gap allows plasmon polaritons to be tuned continuously. Here, we introduce black phosphorus as a promising new material in surface polaritonics that features key advantages for ultrafast switching. Unlike graphene, black phosphorus is a van der Waals bonded semiconductor, which enables high-contrast interband excitation of electron-hole pairs by ultrashort near-infrared pulses. Here, we design a SiO2/black phosphorus/SiO2 heterostructure in which the surface phonon modes of the SiO2 layers hybridize with surface plasmon modes in black phosphorus that can be activated by photo-induced interband excitation. Within the Reststrahlen band of SiO2, the hybrid interface polariton assumes surface-phonon-like properties, with a well-defined frequency and momentum and excellent coherence. During the lifetime of the photogenerated electron-hole plasma, coherent hybrid polariton waves can be launched by a broadband mid-infrared pulse coupled to the tip of a scattering-type scanning near-field optical microscopy set-up. The scattered radiation allows us to trace the new hybrid mode in time, energy and space. We find that the surface mode can be activated within ∼50 fs and disappears within 5 ps, as the electron-hole pairs in black phosphorus recombine. The excellent switching contrast and switching speed, the coherence properties and the constant wavelength of this transient mode make it a promising candidate for ultrafast nanophotonic devices.

Visible Range Plasmonic Modes on Topological Insulator Nanostructures

Phase-resolved real-space imaging of localized plasmons in Bi1.5Sb0.5Te1.8Se1.2 topological insulator nanostructures is performed by scattering-type scanning near-field optical microscopy. Dipolar and higher order surface plasmon modes with well-defined field amplitude and phase profiles are detected in the visible part of the spectrum.