Scattering Scanning Near-field Optical Microscopy Research Articles

Revealing Mode Formation in Quasi-Bound States in the Continuum Metasurfaces via Near-Field Optical Microscopy.

Photonic metasurfaces offer exceptional control over light at the nanoscale, facilitating applications spanning from biosensing, and nonlinear optics to photocatalysis. Many metasurfaces, especially resonant ones, rely on periodicity for the collective mode to form, which makes them subject to the influences of finite size effects, defects, and edge effects, which have considerable negative impact at the application level. These aspects are especially important for quasi-bound state in the continuum (BIC) metasurfaces, for which the collective mode is highly sensitive to perturbations due to high-quality factors and strong near-field enhancement. Here, the mode formation in quasi-BIC metasurfaces on the individual resonator level using scattering scanning near-field optical microscopy (s-SNOM) in combination with a new image processing technique, is quantitatively investigated. It is found that the quasi-BIC mode is formed at a minimum size of 10×10-unit cells much smaller than expected from far-field measurements. Furthermore, it is shown that the coupling direction of the resonators, defects and edge states have pronounced influence on the quasi-BIC mode. This study serves as a link between the far-field and near-field responses of metasurfaces, offering crucial insights for optimizing spatial footprint and active area, holding promise for augmenting applications such as catalysis and biospectroscopy.

Spectral Information of EG-2D Ag Plasmonic Nanostructures Revealed by Analyzing Trajectory of Hyperspectral sSNOM Mapping in Argand Space

It is well known that optical microscopies, such as confocal Raman or PL spectroscopy, provide information regarding the optical properties of 2DM or modulation of the latter in their vdWHs, still, with a spatial resolution limited to the wavelength of excitation light. Since the size of the plasmonic nanostructures is usually order of magnitude smaller than the excitation wavelength, or even smaller, it is not feasible to directly map the field confinement with such optical microscopies. Scattering Scanning Near-Field Optical Microscopy, also known as “sSNOM”, is an AFM based imaging technique, which can not only directly probe the local optical surface impedance of the 2DMs or their vdWHs, but also capable of high-resolution mapping of optical response. This suggests it to be an ideal tool for visualizing the localized field of surface modes and associated charge distribution in plasmonic nanostructures, as well as for revealing other light-matter coupling phenomena at nanometer-scale resolution (~ 10 nm).Unlike confocal Raman or PL spectroscopy, it is not straight-forward for sSNOM to obtain spectral information. It is accomplished by one of two approaches: either by analysis of a quantitative sSNOM data, such as a signal intensity or the periodicity of standing wave fringes, measured at series of excitation wavelength – i.e., via a hyperspectral mapping of the region of interest. In another approach (nano-FTIR) a broad-band excitation is utilized to take single-point spectra, repeated at every pixel over the region of interest. Both approaches proved to provide consistent results though correlation of the resulting spectra with specific materials parameters is a tough problem due to complex mechanisms of near-field response.For either approach, proper referencing/normalization of the demodulated near-field signal has to be done before starting data analysis. This is due to the near-field signals are influenced by tuning of setup and may vary from wavelength to wavelength. Most commonly, near-field signal from the substrate (typically, Si or SiO2) or from the electrodes (Au or Pt) is used for referencing/normalization, anticipating that it either does not contribute to signal dispersion or the contribution is additive and can be subtracted by referencing. Such “trivial background” referencing procedure cannot be proved if the background signal itself is non-uniform vs. spatial coordinates and/or wavelength. Typical examples include: wave diffraction patterns, where local intensity of the pixel is a function of the excitation even in the absence of dispersion; or the case of the substrate’s “background” being a resonance strongly coupled to the sample signal.Most success was demonstrated in spectroscopic near-field analysis of standing waves – the method which is self-calibrated without any reference. Indeed, the periodicity of a standing wave pattern (the distance between wave nodes and peaks) is independent of the (non-calibrated) intensity of the sSNOM signal itself. Plotting periodicity vs. excitation wavelength gives directly the dispersion relation of surface modes. Said that, in order to get the periodicity data, analysis should be performed over a region of several wavelengths (of the surface mode) to have a few distinct nodes and peaks, which undermines the idea of sub-diffractional imaging of nanomaterials. Furthermore, if the polariton wavelength is comparable with the size of plasmonic nanostructure one can barely observe a single wavelength period. If the size is even smaller, instead of forming interference fringe patterns, the polariton wave will be confined within or in the vicinity of the nanostructures and appear as “hot spot”, not a wave. The period and, therefore, dispersion relation cannot be derived. Similarly, in highly disordered samples, the sSNOM map shows a complicated diffraction pattern which contains only short fragments of the waves, much less than a full period, obstructing dispersion analysis as well.To overcome this challenge, we developed an analytic approach for sSNOM hyperspectral mapping to reveal the local value of surface wave propagation constant and map it with substantially sub-wavelength resolution. It applies to the surface polaritons that are confined by the plasmonic nanostructures which have sub-diffraction size compared to the polariton wavelength, which is typically much shorter than the laser excitation wavelength. The proposed method is based on eikonal representation of the surface wave. By tracing the phase change of the wave and analyzing the trajectory of phasors (Argand or Nyquist plot of the sSNOM data) in complex space demodulated near-field signal allows natural referencing/normalization at each laser excitation wavelength. Thus, the hyperspectral sequence of the demodulated near-field signal yields the map of a propagation constant of surface polaritons, demonstrated below for (epitaxial graphene) EG-Ag plasmonic nanostructures with sub-wavelength sizes, lithographically fabricated on SiC substrate.

Terahertz s-SNOM Imaging of a Single Cell with Nanoscale Resolution.

Terahertz scattering scanning near-field optical microscopy is a robust spectral detection technique with a nanoscale resolution. However, there are still major challenges in investigating the heterogeneity of cell membrane components in individual cells. Here, we present a novel and comprehensive analytical approach for detecting and investigating heterogeneity in cell membrane components at the single-cell level. In comparison to the resolution of the topographical atomic force microscopy image, the spatial resolution of the terahertz near-field amplitude image is 3 times that of the former. This ultrafine resolution enables the compositional distribution in the cell membrane, such as the distribution of extracellular vesicles, to be finely characterized. Furthermore, via extraction of the near-field absorption images at specific frequencies, the visualization and compositional difference analysis of cell membrane components can be presented in detail. These findings have significant implications for the intuitive and visual analysis of cell development and disease evolutionary pathways.

Rapid Deposition of the Biomimetic Hydroxyapatite-Polydopamine-Amino Acid Composite Layers onto the Natural Enamel.

In this work, we developed a technology that enables rapid deposition of biomimetic composite films onto natural enamel slices (known as biotemplates). These films are composed of polydopamine (PDA) and nanocrystalline carbonate-substituted hydroxyapatite (nano-cHAp) that have been functionalized with amino acid l-Arginine. We utilized atomic force microscopy (AFM) and scattering scanning near-field optical microscopy (s-SNOM) combined with infrared (IR) synchrotron to achieve nanoscale spatial resolution for both IR absorption and topography analyses. This combined analytical modality allowed us to understand how morphology connects to local changes in the chemical environment on the biotemplate surface during the deposition of the bioinspired coating. Our findings revealed that when using the proposed technology and after the deposition of the first PDA layer, the film formed on the enamel surface nearly covers the entire surface of the specimen whose thickness is larger on the surface of the emerging enamel prisms. Calculation of the crystallinity index for the biomimetic layer showed a multiple increase compared with natural enamel. This indicates regular and dense aggregation of nano-cHAp into larger crystals, imitating the morphology of natural enamel rods. The microhardness of the formed PDA-based biomimetic layer mineralized with nano-cHAp functionalized with amino acid l-Arginine deposited on natural enamel was practically the same as that of natural enamel. The characterization of nano-cHAp-amino acid-PDA layers using IR and Raman microspectroscopy showed that l-arginine acts as a conjunction agent in the formation of mineralized biomimetic composite coatings. The uniformity of the mechanisms of PDA layer formation under different deposition conditions and substrate types allows for the formation of coatings regardless of the macro- and micromorphology of the template. Therefore, the results obtained in this work have a high potential for future clinical applications in dental practice.

Characterization of Nanoporous Copper Using Scanning Probe Techniques

This study explores the mechanical and optical properties of nanoporous copper (NPC) films dealloyed in acid and base using qualitative and quantitative measurements. Two types of NPC films (NPC 1 and NPC 2) were prepared by dealloying precursor films in different corrosive solutions. The films were characterized using atomic force microscopy (AFM) and scattering scanning near-field optical microscopy (s-SNOM). The AFM analysis revealed distinct nanoporous structures in both films, with NPC 2 showing larger pore sizes and the presence of copper aggregations on the surface. Rugosity measures showed that NPC 2 had higher rugosity than NPC 1 and indicated fundamental differences in the height distributions of the surface nano-topography. The Minkowski connectivity analysis highlighted differences in connectedness and topological characteristics between the two samples. The analysis of the GLCM features showed clear distinctions in ASM, Contrast, Dissimilarity, Energy, and Homogeneity between NPC 1 and NPC 2. The s-SNOM results demonstrated altered optical properties, with NPC 1 showing higher contrast at 1550 nm, while NPC 2 exhibited reversed-phase contrast at 561 nm. The phase contrast at 638 nm indicated a red-shifted optical absorption peak in NPC 2 compared to NPC 1.

Modeling scanning near-field optical photons scattered from an atomic force microscope for quantum metrology

Scattering scanning near-field optical microscopy (s-SNOM) is a promising technique for overcoming Abbe diffraction limit and substantially enhancing the spatial resolution in spectroscopic imaging. The s-SNOM works by exposing an atomic force microscope (AFM) tip to an optical electromagnetic (EM) field, while the tip is so close to a dielectric sample that the incident beam lies within the near-field regime and displays nonlinear behavior. We suggest replacing the incident EM field by photons generated by a single photon emitter, and propose a theoretical model for the suggested system by employing electric-dipole approximation, image theory, and perturbation theory. The count rate of the scattered photons from the AFM tip is extracted through a single photon detector, which contains information about electrical permittivity of the dielectric material beneath the tip. The permittivity of the sample can be extracted through spectroscopic setups. Our proposed scheme is useful for enhancing the spatial resolution of the modern quantum spectroscopy configurations that utilize entangled single photons.

Quantitative Modeling of Near-Field Interactions in Terahertz Near-Field Microscopy

Terahertz scattering-scanning near-field optical microscopy (THz s-SNOM), combining the best features of terahertz technology and s-SNOM technology, has shown unique advantages in various applications. Consequently, building a model to characterize near-field interactions and investigate practical issues has become a popular topic in THz s-SNOM research. In this study, a finite element model (FEM) is proposed to quantify the near-field interactions, and to investigate the edge effect and antenna effect in THz s-SNOM. Our results indicate that the proposed model can give us a better understanding of the near-field interactions and direct the parameter design of the probe for THz s-SNOM.

Imaging the Terahertz Nanoscale Conductivity of Polycrystalline CsPbBr3 Perovskite Thin Films.

Terahertz (THz) radiation is a valuable tool to investigate the electronic properties of lead halide perovskites (LHPs). However, attaining high-resolution information remains elusive, as the diffraction-limited spatial resolution (∼300 μm) of conventional THz methods prevents a direct analysis of microscopic effects. Here, we employ THz scattering scanning near-field optical microscopy (THz-sSNOM) for nanoscale imaging of cesium lead bromide (CsPbBr3) thin films down to the single grain level at 600 GHz. Adopting a scattering model, we are able to derive the local THz nanoscale conductivity in a contact-free fashion. Increased THz near-field signals at CsPbBr3 grain boundaries complemented by correlative transmission electron microscopy-energy-dispersive X-ray spectroscopy elemental analysis point to the formation of halide vacancies (VBr) and Pb-Pb bonds, which induce charge carrier trapping and can lead to nonradiative recombination. Our study establishes THz-sSNOM as a powerful THz nanoscale analysis platform for thin-film semiconductors such as LHPs.

Imaging of THz Photonic Modes by Scattering Scanning Near-Field Optical Microscopy

We investigated the near-field distribution associated to the photonic mode of terahertz photonic micro-resonators by scattering scanning near-field optical microscopy. Probing individual THz micro-resonators concentrating electric fields is important for high-sensitivity chemical and biochemical sensing and fundamental light-matter interactions studies at the nanoscale. We imaged both electric field concentration predicted by numerical simulations and unexpected patterns that deviate from intuitive assumptions. We propose a scenario based on the combination of the near-field with the far-field pattern of the probe/resonator ensemble that is in excellent agreement with the experimental data and propose an image analysis procedure to recover the near-field of such structures.

Quantitative modeling of near-field interactions incorporating polaritonic and electrostatic effects.

As scattering-scanning near-field optical microscopy (s-SNOM) continues to grow in prominence, there has been great interest in modeling the near-field light-matter interaction to better predict experimental results. Both analytical and numerical models have been developed to describe the near-field response, but thus far models have not incorporated the full range of phenomena accessible. Here, we present a finite element model (FEM), capable of incorporating the complex physical and spatial phenomena that s-SNOM has proved able to probe. First, we use electromagnetic FEM to simulate the multipolar response of the tip and illustrate the impact of strong coupling on signal demodulation. We then leverage the multiphysics advantage of FEM to study the electrostatic effect of metallic tips on semiconductors, finding that THz s-SNOM studies are most impacted by this tip-induced band-bending. Our model is computationally inexpensive and can be tailored to specific nanostructured systems and geometries of interest.

Fabry-Pérot Phonon Polaritons in Boron Nitride Nanotube Resonators.

Phonon polaritons (PhPs) offer extreme confinement of optical fields and strong dispersion in the mid-infrared spectral region. To study the propagation and interference of PhPs in a 1-D system, we employ scattering scanning near-field optical microscopy (s-SNOM), analytical, and computational techniques to describe the resonance behavior observed in boron nitride nanotubes (BNNTs). In BNNTs of a sufficiently small length, the reflected standing waves from both terminals strongly interfere with one another, leading to large constructive enhancement at select wavelengths through the Fabry-Pérot interference. This 1-D nanoresonant behavior illustrates methods to increase and localize field strength at positions on a BNNT nanotube.

Near-field scanning optical microscopy of molecular aggregates: The role of light polarization

We consider theoretically near-field absorption spectra of molecular aggregates stemming from a scattering scanning near-field optical microscopy type setup. Our focus is on the dependence on the direction and polarization of the incoming electromagnetic radiation, which induces a Hertz dipole with a specific orientation at the tip-apex. Within a simple description, which is based on the eigenstates of the aggregate, absorption spectra are calculated for the near field created by this dipole. We find that the spatial patterns of the spectra have a strong dependence on the orientation of this tip-dipole, which can be understood by considering three basic functions that only depend on the arrangement of the aggregate and the molecule tip distance, but not on the orientation of the tip-dipole. This allows direct access to spatial dependence of the aggregate eigenstates. For the important cases of one- and two-dimensional systems with parallel molecules, we discuss these spectra in detail. The simple numerically efficient approach is validated by a more detailed description where the incoming radiation and the interaction between the tip and molecules are explicitly taken into account.

Phase Control of an Infrared-Frequency Current Wave on a Microstrip Line with dc Diode Bias

Antenna tuning using voltage-controlled elements such as diodes or varactors has been challenging in the infrared. We present measurements of an antenna-coupled metal-oxide-metal (MOM) diode with a time constant fast enough to rectify at infrared frequencies. This diode loads a microstrip infrared antenna, and the dc diode impedance is a function of voltage. When a dc bias voltage was applied across the diode, phase shifts of the current standing waves on the antenna were observed using scattering scanning near-field optical microscopy. This is the first experimental evidence of a dc bias voltage on a MOM diode controlling the phase of current waves at infrared frequencies.

Simultaneous multimethod scanning probe microscopy of complex nano-systems

In the twenty-first century, scanning probe microscopy characterization techniques have seen significant progress and are capable of probing complex structures and devices for a variety of near-surface features and phenomena with nanometer scale resolution. With modest customization, we can deploy these techniques for industrial metrology purposes in a simultaneous and multimethod system capable of shedding light on device function and failure modes, as well as determining the most efficient methods for data collection. To demonstrate this concept with a current, complex industrial device under development, several scanning probe microscopy techniques advantageous to the progress of heat-assisted magnetic recording heads were selected. This work describes simultaneous and multimethod approaches for performing heat-assisted magnetic recording head characterization using atomic force microscopy with scattering scanning near-field optical microscopy simultaneously performed with magnetic force microscopy or photo-induced force microscopy that could be extended to applications of other complex nanoscale devices. We demonstrate that the optical and magnetic fields are overlapping for fabricated heads, which is necessary for performing heat-assisted magnetic recording. We also observed that the multimethod atomic force microscopy methods show strong agreement between the measured optical and magnetic fields and the locale of their associated parts on the head.

2D Vibrational Exciton Nanoimaging of Domain Formation in Self-Assembled Monolayers.

Order, disorder, and domains affect many of the functional properties in self-assembled monolayers (SAMs). However, carrier transport, wettability, and chemical reactivity are often associated with collective effects, where conventional imaging techniques have limited sensitivity to the underlying intermolecular coupling. Here we demonstrate vibrational excitons as a molecular ruler of intermolecular wave function delocalization and nanodomain size in SAMs. In the model system of a 4-nitrothiophenol (4-NTP) SAM on gold, we resolve coupling-induced peak shifts of the nitro symmetric stretch mode with full spatio-spectral infrared scattering scanning near-field optical microscopy. From modeling of the underlying 2D Hamiltonian, we infer domain sizes and their distribution ranging from 3 to 12 nm across a field of view on the micrometer scale. This approach of vibrational exciton nanoimaging is generally applicable to study structural phases and domains in SAMs and other molecular interfaces.

In situ plant materials hyperspectral imaging by multimodal scattering near-field optical microscopy

Plant cells are elaborate three-dimensional polymer nano-constructs with complex chemistry. The bulk response of plants to light, in the far-field, is ultimately encoded by optical scattering from these nano-constructs. Their chemical and physical properties may be acquired through their interaction with a modulated nano-tip using scattering scanning near-field optical microscopy. Here, using this technique, we present 20 nm spatial resolution mechanical, spectral and optical mappings of plant cell walls. We first address the problem of plant polymers tracking through pretreatment and processing. Specifically, cellulose and lignin footprints are traced within a set of delignified specimen, establishing the factors hindering complete removal of lignin, an important industrial polymer. Furthermore, we determine the frequency dependent dielectric function {epsilon }(omega)={(n+ik)}^{2} of plant material in the range 28 ≤ ω ≤ 58 THz, and show how the environmental chemical variation is imprinted in the nanoscale variability of n and k. This nanometrology is a promise for further progress in the development of plant-based (meta-)materials.

(Invited) Hyperspectral Imaging of MoS2-hBN-Carbon Heteronanotubes: Novel Analytical Method with Optical Resolution of λ/3700

Low-dimensional nanostructures of electronic and/or optical materials possess a large number of new properties due to space quantization, thus exceeding the bulk “parent” materials. Similarly, nanoheterostructures of these materials provide even better control of properties and allow to explore new physics and utilize novel phenomena for device applications. In such systems, the materials properties can vary at the size scale of several nanometers.To meet the challenge of studying nanoheterostructures, the optical materials characterization tools have been recently extended beyond bulk spectroscopy or imaging. A number of Near-Field Optical Microscopy methods were developed. In this online presentation, we will report on a new approach to hyperspectral imaging with scattering Scanning Near-Field Optical Microscopy (sSNOM) which allowed us to perform materials spectroscopy with resolution not restricted by Abbe (diffraction) limit.In 2020, a new class of low-dimensional nanoheterostructure materials – two-dimensional sandwich materials (semiconductor and insulator van der Waals heterostructures) wrapped around carbon nanotube – has been synthesized [1]. Although confirmed by the structural studies (TEM) and elemental analysis (macro- to micro-scale), the heterostructures themselves or their devices were hardly imaged, except for AFM or SEM, not giving materials information. This is due to the diameter of the smallest part ca. 1-2 nm, much beyond the limitations of typical analytical methods.Here we present results on imaging different materials containing the nanoheterostructure with the sSNOM hyperspectral mapping [2]. The maximum resolution (imaging of 2 nm heteronanotube) achieved in these experiments exceeds the inverse wavelength of excitation mid-IR laser ~3,700 times, thus beating diffraction limits of regular optical spectroscopy. Heteronanotubes have been studied on the substrate as well as in real devices (diodes). Imaging of the heteronanotubes through the over-deposited metal electrodes was demonstrated.Acknowledgments: sSNOM work was supported by the National Science Foundation (Grant DMR-2011839). Part of this work was supported by JSPS KAKENHI Grant Numbers JP18H05329, JP20H00220, and by JST, CREST Grant Number JPMJCR20B5, Japan. Part of this work was conducted at Takeda Sentanchi Supercleanroom, supported by “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, Grant number JPMXP09F19UT0006.[1] Xiang, R.; Inoue, T.; Zheng, Y.; Kumamoto, A.; Qian, Y.; Sato, Y.; Liu, M.; Tang, D.; Gokhale, D.; Guo, J.; Hisama, K.; Yotsumoto, S.; Ogamoto, T.; Arai, H.; Kobayashi, Y.; Zhang, H.; Hou, B.; Anisimov, A.; Maruyama, M.; Miyata, Y.; Okada, S.; Chiashi, S.; Li, Y.; Kong, J.; Kauppinen, E. I.; Ikuhara, Y.; Suenaga, K.; Maruyama, S., One-dimensional van der Waals heterostructures. Science 2020, 367 (6477), 537.[2] Y. Feng, H. Li, T. Inoue, S. Chiashi, S. V. Rotkin, R. Xiang, and S. Maruyama, “One-dimensional van der waals hetero-junction diode”, (2020), arXiv:2012.03180.

Correlative nano-spectroscopic imaging of heterogeneity in migrated petroleum in unconventional reservoir pores

Low permeability unconventional oil and gas resources may experience formation damage due to the emplacement of viscous petroleum in narrow pore throats during petroleum migration and production. The composition of such pore-clogging molecules remains under-characterized, as standard analytical techniques in organic geochemistry either lack the desired nano-scale spatial resolution with simultaneous chemical specificity, or require extraction and therefore averaging of the suite of compounds that make up bulk crude oil. A new correlative imaging technique with high spatial resolution is demonstrated for migrated residual petroleum in nanopores of Niobrara chalk samples. Imaging combines Infrared scattering-Scanning Near-field Optical Microscopy (IR s-SNOM) with nano-mechanical atomic force microscopy to map both chemical and mechanical properties at ~30 nm spatial resolution. These correlative images demonstrate that heterogeneity exists within emplaced petroleum in situ on the scale of hundreds of nanometers, visualized as clusters of petroleum having distinct adhesive forces. The chemical composition of these clusters remains ambiguous, but their geometry and mechanical properties may imply that they are phase-separated asphaltene preferentially associated with petroleum-attractive nano-domains on calcite pore walls. Despite outstanding challenges, correlative IR s-SNOM and nano-mechanical imaging offer a promising and novel analytical approach to understanding fluid behavior in nanoporous reservoir rocks.

Impedance-Matching Technique for an Infrared Folded Dipole Antenna

Modifications of the geometry of a folded dipole antenna operating in the infrared were studied with the objective to increase the antenna input impedance. These modified folded dipoles were designed and optimized using numerical simulations. Candidate antennas were fabricated, and input impedance was measured experimentally using scattering-scanning near-field optical microscopy. Numerical and experimental results confirm that the choice of an appropriate antenna geometry leads to an input impedance of 1 kΩ or above. With this impedance, the modified folded dipole antenna could be implemented as a solution for the impedance mismatch challenge that arises in antenna-coupled infrared detectors.

Experimental verification of a plasmonic hook in a dielectric Janus particle

We report on the experimental observation of the curved plasmonic beam, a plasmonic hook (PH), for surface plasmon-polariton (SPP) waves. The SPP PH effect could be obtained with a cuboid particle with broken shape symmetry fabricated with a relatively simple routine. This has a pronounced difference with fabrication of the structure for generation of the Airy SPPs, which require complex techniques to compensate the wave vector mismatch. We confirmed the existence of SPP PH by amplitude scattering scanning near-field optical microscopy. The experimental results agree well with our predictions. Importantly, the SPP PH demonstrates the smallest curvature of the beam ever recorded for SPPs compared to that for the Airy-family plasmonic beams, which potentially can strongly impact many useful applications from nanoparticle manipulation to nanoscale bio-sensing.