Near-field Scanning Research Articles

Terahertz nanoscopy: Advances, challenges, and the road ahead

Exploring nanoscale material properties through light-matter interactions is essential to unveil new phenomena and manipulate materials at the atomic level, paving the way for ground-breaking advancements in nanotechnology and materials science. Various elementary excitations and low-energy modes of materials reside in the terahertz (THz) range of the electromagnetic spectrum (0.1–10 THz) and occur over various spatial and temporal scales. However, due to the diffraction limit, a slew of THz studies are restricted to drawing conclusions from the spatially varying THz responses around half of the probing wavelengths, i.e., from tens to a couple of hundred micrometers. To address this fundamental challenge, scanning near-field optical microscopy (SNOM), notably scattering-type SNOM (s-SNOM), combined with THz sources has been employed and is fueling growing interest in this technique across multiple disciplines. This review (1) provides an overview of the system developments of SNOM, (2) evaluates current approaches to understand and quantify light-matter interactions, (3) explores advances in THz SNOM applications, especially studies with THz nano-scale spatial responses employing an s-SNOM, and (4) envisions future challenges and potential development avenues for the practical use of THz s-SNOM.

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.

Optical Characterization of InGaN Quantum Structures at the Nanoscale

AbstractThis review paper presents an overview of the optical characterization techniques for Indium Gallium Nitride (InGaN) quantum well (QW) structures at a nanoscale. The two major techniques are reviewed—Electron Microscopy‐Cathodoluminescence (EM‐CL) and Scanning Near‐field Optical Microscopy (SNOM). It elucidates the critical role these methodologies play in revealing the complex properties of InGaN QWs, including their structural characteristics, optical properties, carrier dynamics, and the effects of defects and doping. The review highlights key findings from a variety of studies, demonstrating how EM‐CL and SNOM have contributed to the understanding of these micro‐/nano‐ structures and their potential applications in high‐efficiency optoelectronic devices.

Nano-infrared imaging of epitaxial graphene on SiC revealing doping and thickness inhomogeneities

We report on a nano-infrared (IR) imaging and spectroscopy study of epitaxial graphene on silicon carbide (SiC) by using scattering-type scanning near-field optical microscopy (s-SNOM). With nano-IR imaging, we reveal in real space microscopic domains with distinct IR contrasts. By analyzing the nano-IR, atomic force microscopy, and scanning tunneling microscopy imaging data, we conclude that the imaged domains correspond to single-layer graphene, bilayer graphene (BLG), and higher-doped BLG. With nano-IR spectroscopy, we find that graphene can screen the SiC phonon resonance, and the screening is stronger at more conductive sample regions. Our work offers insights into the rich surface properties of epitaxial graphene and demonstrates s-SNOM as an efficient and effective tool in characterizing graphene and possibly other two-dimensional materials.

Probing Temperature-Induced Plasmonic Nonlinearity: Unveiling Opto-Thermal Effects on Light Absorption and Near-Field Enhancement.

Precise measurement and control of local heating in plasmonic nanostructures are vital for diverse nanophotonic devices. Despite significant efforts, challenges in understanding temperature-induced plasmonic nonlinearity persist, particularly in light absorption and near-field enhancement due to the absence of suitable measurement techniques. This study presents an approach allowing simultaneous measurements of light absorption and near-field enhancement through angle-resolved near-field scanning optical microscopy with iterative opto-thermal analysis. We revealed gold thin films exhibit sublinear nonlinearity in near-field enhancement due to nonlinear opto-thermal effects, while light absorption shows both sublinear and superlinear behaviors at varying thicknesses. These observations align with predictions from a simple harmonic oscillation model, in which changes in damping parameters affect light absorption and field enhancement differently. The sensitivity of our method was experimentally examined by measuring the opto-thermal responses of three-dimensional nanostructure arrays. Our findings have direct implications for advancing plasmonic applications, including photocatalysis, photovoltaics, photothermal effects, and surface-enhanced Raman spectroscopy.

Observation of gapless corner modes of photonic crystal slabs in synthetic translation dimensions

Second-order topological photonic crystals support localized corner modes that deviate from the conventional bulk-edge correspondence. However, the frequency shift of corner modes spanning the photonic band gap has not been experimentally reported. Here, we observe the gapless corner modes of photonic crystal slabs within a parameter space by considering translation as an additional synthetic dimension. These corner modes, protected by topological pumping in synthetic translation dimensions, are found to exist independently of the specific corner configuration. The gapless corner modes are experimentally imaged via the near-field scanning measurement and validated numerically by full-wave simulations. We propose a topological rainbow with gradient translation, demonstrating the ability to extract and separate specific frequency components of light into different spatial locations. Our work contributes to the advancement of topological photonics and provides valuable insights into the exploration of gapless corner modes in synthetic dimensions.

Electric near-field scanning for electronic PCB electromagnetic radiation measurement

This paper introduces a design and test of an electric (E) near-field scanning (NFS) system by using innovative 4-layer miniature probe for electromagnetic compatibility (EMC) application. The probe works in the challenging frequency band up to 12 GHz with spatial resolution lower than 2 mm. The NFS is designed to operate by automation based on PC driven by LabVIEW® interface controlling a vector network analyzer (VNA). The PC communicates with the VNA through the local area network to read and save the measured data emitted by the device under test (DUT). The LabVIEW® interface is designed to control the E-NF positioning motion core by means of STM32® microcontroller. A stepper motor is used to move the E-NF probe along the scanning surface plan. Finally, the NFS is validated by visualizing the E-NF intensity maps generated in real time after calibration with a reference device under test represented by microstrip line and a microwave circuit.

A historical overview of Nano-optics: From Near-Field Optics to Plasmonics

Abstract Nano-optics is an emergent research field in physics that appeared in the 1980s, which deals with light-matter optical interactions at the nanometer scale. In early studies of nano-optics, the main concern focus was to obtain higher optical resolution over the diffraction limit. The researches of near-field imaging and spectroscopy based on Scanning Near-Field Optical Microscopy (SNOM) were developed. The exploration of improving SNOM probe for near-field detection led to the emergence of surface plasmons. In the sense of resolution and wider application, there has been a significant transition from seeking higher resolution microscopy to plasmonic near-field modulations in the nano-optics community during the nano-optic development. Nowadays, studies of nano-optics prefer the investigation of plasmonics in different material systems. In this article, the history of the development of near-field optics is briefly reviewed. The difficulties of conventional SNOM to achieve higher resolution are discussed. As an alternative solution, surface plasmons have shown the advantages of higher resolution, wider application, and flexible nano-optical modulation for new devices. The typical studies in different periods are introduced and analyzed characteristics of nano-optics in each stage. In this way, the evolution progress from near-field optics to plasmonics of nano-optics research is presented. The future development of nano-optics is discussed then.

Accelerated Nano-Optical Imaging through Sparse Sampling.

The integration time and signal-to-noise ratio are inextricably linked when performing scanning probe microscopy based on raster scanning. This often yields a large lower bound on the measurement time, for example, in nano-optical imaging experiments performed using a scanning near-field optical microscope (SNOM). Here, we utilize sparse scanning augmented with Gaussian process regression to bypass the time constraint. We apply this approach to image charge-transfer polaritons in graphene residing on ruthenium trichloride (α-RuCl3) and obtain key features such as polariton damping and dispersion. Critically, nano-optical SNOM imaging data obtained via sparse sampling are in good agreement with those extracted from traditional raster scans but require 11 times fewer sampled points. As a result, Gaussian process-aided sparse spiral scans offer a major decrease in scanning time.

Experimental study on subwavelength focusing optical field detection methods for micro-Fresnel zone plate

In this paper, the detection methods and detection systems for the subwavelength focusing optical field of the micro-Fresnel zone plate (FZP) are experimentally studied. First, a comparison is made between the micro/nanofabrication methods for micro-FZP, namely, focused ion beam (FIB) and electron beam lithography, and the results show that FIB is better suited for the amplitude-type micro-FZP fabrication. Subsequently, the experimental detection devices based on the wide-field microscopy amplification (WFMA) imaging method (indirect detection method) and scanning near-field optical microscopy (direct detection method) are, respectively, constructed for the detection of the subwavelength focusing optical field of micro-FZP. The experimental results are compared and analyzed with theoretical calculation results, indicating that the WFMA method is more suitable for the detection of micro-FZP subwavelength focusing optical field that is not sensitive to radial components. This study provides an experimental reference for the micro/nanofocusing optical field detection of micro/nano-optical components similar to micro-FZP and promotes the practical application of micro-FZP.

Research on a Two-stage Plane Adaptive Sampling Algorithm for Near-field Scanning Acceleration

As one of the most useful methods in electromagnetic interference (EMI) diagnosis, near-field (NF) scanning is widely used in electromagnetic compatibility (EMC) evaluation of complex devices under test (DUTs). In this paper, a two-stage plane adaptive sampling algorithm is proposed to reduce the acquisition time in the process of NF scanning and to make reconstruction of the radiation source more efficient. The sampling method is based on the region self-growth algorithm and the Voronoi subdivision principle, significantly reducing the number of NF samples in the stage of solving the radiation source model through uniform and non-uniform two-stage sampling. Two experiments were conducted to verify the correctness and effectiveness by comparing with the traditional uniform sampling method.

Simultaneous mapping of the magnetic field components using near-field microscopy

In this work, we present a near-field scanning optical microscope capable of simultaneously mapping the x− , y− , and z− components of the magnetic field generated by a set of magnetic coils in the kHz range. Using this device, we present two experiments: the observation of the well-known magnetic field of a circular coil and a resolution test. The observations achieved with our instrument are supported by numerically calculated simulations, which confirm the validity of our experimental results. In addition, our instrument is capable of imaging features of the order of 15mm , representing a resolution of λ/10000 . Our approach establishes an important avenue for the three-dimensional mapping of the electromagnetic field in any spectral range, using the correct configuration of the probe, and opens a new discussion on the resolution of scanning probe microscopes imaging more than one component of the studied field.

A Source Reconstruction Method for Unknown EMI Sources Inside a Shielding Enclosure Based on Magnitude-Only Near-Field Scanning

A Source Reconstruction Method for Unknown EMI Sources Inside a Shielding Enclosure Based on Magnitude-Only Near-Field Scanning

Mapping electric field components of superchiral field with photo-induced force.

Circular dichroism (CD) of materials, difference in absorbance of left- and right-circularly polarized light, is a standard measure of chirality. Detection of the chirality for individual molecules is a frontier in analytical chemistry and optical science. The usage of a superchiral electromagnetic field near metallic structure is one promising way because it boosts the molecular far-field CD signal. However, it is still elusive as to how such a field actually interacts with the molecules. The cause is that the distribution of the electric field vector is unclear in the vicinity of the metal surface. In particular, it is difficult to directly measure the localized field, e.g., using aperture-type scanning near-field optical microscope. Here, we calculate the three-dimensional (3D) electric field vector, including the longitudinal field, and reveal the whole figure of the near-field CD on a two-dimensional (2D) plane just above the metal surface. Moreover, we propose a method to measure the near-field CD of the whole superchiral field by photo-induced force microscopy (PiFM), where the optical force distribution is mapped in a scanning 2D plane. We numerically demonstrate that, although the presence of the metallic probe tip affects the 3D electric field distribution, the PiFM is sufficiently capable to evaluate the superchiral field. Unveiling the whole figure of near-field is significantly beneficial in obtaining rich information of single molecules with multiple orientations and in analyzing the boosted far-field CD signals.

Nano-compositional imaging of the lanthanum silicide system at THz wavelengths.

Terahertz scattering-type scanning near-field optical microscopy (THz-sSNOM) provides a noninvasive way to probe the low frequency conductivity of materials and to characterize material compositions at the nanoscale. However, the potential capability of atomic compositional analysis with THz nanoscopy remains largely unexplored. Here, we perform THz near-field imaging and spectroscopy on a model rare-earth alloy of lanthanum silicide (La-Si) which is known to exhibit diverse compositional and structural phases. We identify subwavelength spatial variations in conductivity that is manifested as alloy microstructures down to much less than 1 μm in size and is remarkably distinct from the surface topography of the material. Signal contrasts from the near-field scattering responses enable mapping the local silicon/lanthanum content differences. These observations demonstrate that THz-sSNOM offers a new avenue to investigate the compositional heterogeneity of material phases and their related nanoscale electrical as well as optical properties.

Generic arrays of surface-positioned and shallow-buried gold multi-shapes as reference samples to benchmark near-field microscopes. Part 1: Applications in s-SNOM depth imaging

A wide palette of nanoscale imaging techniques operating in the near-field regime has been reported to date, enabling an important number of scientific breakthroughs. While the tuning and benchmarking of near-field microscopes represent a very important step for optimizing the outputs of the imaging sessions, no generally acknowledged standards exist yet in terms of calibration of near-field microscopes, which would play an important role in fully exploiting the potential of these instruments. With this work, we aim to contribute to filling in this gap, by introducing a prototypical sample, that holds potential for becoming a benchmark with respect to comparing the performances of diverse near-field measurement techniques, including traditional, aperture based, scanning near field microscopy (SNOM), or apertureless variants, such as scattering-type scanning nearfield optical microscopy (s-SNOM). The proposed samples have been thoroughly simulated, and an easy fabrication procedure is presented and demonstrated. In this latter context, Au-SiO2 samples sharing different configurations, in terms of geometry, number and depth of contrast yielding layers, enabling both surface and sub-surface nanoscopy measurements, were designed and fabricated. We argue that the proposed prototypical samples can be highly useful for benchmarking the outputs of various near-field microscopy techniques, as they facilitate a broad range of tests, relevant for comparing the performances and accuracy of many diverse investigation methods. We also introduce a methodology for numerically simulating the samples and their near-field after illuminating them with light of different wavelengths, as well as their simple process flow. This methodology can considerably augment their future use as a prototypical sample for the evaluation and calibration of current and next generation near-field nanoscopy techniques. Experimental evidence on the usefulness of these samples as s-SNOM testing and benchmarking tools is provided in the context of differentiation of surface and sub-surface structures, and influence of tip-sample distance on attainable amplitude and phase signals. We consider these efforts to represent an important, required step, in advancing the near-field imaging field, with important potential to augment the outputs of current near-field imaging systems, and to facilitate the development and benchmarking of next generation of near-field instrumentation.

Research advances in polaritonics based on near-field optical imaging technique

Polaritons, as hybrid excitations of light and matter, are important for miniaturizing the integrated nano-optoelectronic devices due to their capability of manipulating nanolight. Recently, the state-of-the-art nano-imaging technique (e.g. scattering-type scanning near-field optical microscope) has visualized various types of polaritons and revealed the physical mechanism behind them. The nanometer-resolution imaging not only deepens our understanding of fundamentals of polaritons but also promotes the studies of polariton manipulation and applications. In this review paper, we systematically summarize the recent near-field study of polaritons. Rather than other previous reviews focusing on polaritons in two-dimensional materials, our review extends the polaritonic systems to multiple dimensions (3D/2D/1D), at the same time we also collect the latest progress of polaritons in anisotropic systems. Moreover, we show the recent study of polariton manipulation and their corresponding applications, e.g. sub-diffractional imaging, focusing, optical modulator, nanostructure diagnosis and molecular sensing. Our review also look forward to future opportunities of polaritonics and its nanophotonic applications.

Infrared nanoimaging of neuronal ultrastructure and nanoparticle interaction with cells.

Here we introduce scattering-type scanning near-field optical microscopy (s-SNOM) as a novel tool for nanoscale chemical-imaging of sub-cellular organelles, nanomaterials and of the interactions between them. Our setup uses a tuneable mid-infrared laser and a sharp scanning probe to image at a resolution substantially surpassing the diffraction limit. The laser can be tuned to excite vibrational modes of functional groups in biomolecules, (e.g. amide moieties), in a way that enables direct chemical mapping without the need for labelling. We, for the first time, chemically image neuronal ultrastructure, identify neuronal organelles and sub-organelle structures as small as 10 nm and validate our findings using transmission electron microscopy (TEM). We produce chemical and morphological maps of neurons treated with gold nanospheres and characterize nanoparticle size and intracellular location, and their interaction with the plasma membrane. Our results show that the label-free nature of s-SNOM means it has a 'true' chemical resolution of up to 20 nm which can be further improved. We argue that it offers significant potential in nanomedicine for nanoscale chemical imaging of cell ultrastructure and the subcellular distribution of nanomaterials within tissues.

Nanotip design for high-resolution terahertz scattering-type scanning near-field optical microscopy

Nanotip design for high-resolution terahertz scattering-type scanning near-field optical microscopy

Nano-FTIR spectroscopy reveals SiO2 densification within fs-laser induced nanogratings.

This study explores the structural transformations induced by femtosecond (fs) laser inscriptions in glass, with a focus on type II modifications (so-called nanogratings), crucial for advanced optical and photonic technologies. Our novel approach employs scattering-type scanning near-field optical microscopy (s-SNOM) and synchrotron radiation nanoscale Fourier-transform infrared spectroscopy (nano-FTIR) to directly assess the nanoscale structural changes in the laser tracks, potentially offering a comprehensive understanding of the underlying densification mechanisms. The results reveal the first direct nanoscale evidence of densification driven by HP-HT within fs-laser inscribed tracks, characterized by a significant shift of the main infrared (IR) vibrational structural band of silica glass. It reveals moreover a complex interplay between type I and type II modifications.