Near-field Scanning Research Articles

Probing higher order optical modes in all-dielectric nanodisk, -square, and -triangle by aperture type scanning near-field optical microscopy.

All-dielectric nanoantennas, consisting of high refractive index semiconductor material, are drawing a great deal of attention in nanophotonics. Owing to their ability to manipulate efficiently the flow of light within sub-wavelength volumes, they have become the building blocks of a wide range of new photonic metamaterials and devices. The interaction of the antenna with light is largely governed by its size, geometry, and the symmetry of the multitude of optical cavity modes it supports. Already for simple antenna shapes, unraveling the full modal spectrum using conventional far-field techniques is nearly impossible due to the spatial and spectral overlap of the modes and their symmetry mismatch with incident radiation fields. This limitation can be circumvented by using localized excitation of the antenna. Here, we report on the experimental near-field probing of optical higher order cavity modes (CMs) and whispering gallery modes (WGMs) in amorphous silicon nanoantennas with simple, but fundamental, geometrical shapes of decreasing rotational symmetry: a disk, square, and triangle. Tapping into the near-field using an aperture type scanning near-field optical microscope (SNOM) opens a window on a rich variety of optical patterns resulting from the local excitation of antenna modes of different order with even and odd parity. Numerical analysis of the antenna and SNOM probe interaction shows how the near-field patterns reveal the node positions of - and allows us to distinguish between - cavity and whispering gallery modes. As such, this study contributes to a richer and deeper characterization of the structure of light in confined nanosystems, and their impact on the structuring of the light fields they generate.

Mapping the spin angular momentum distribution of focused linearly and circularly polarized vortex fields.

Using the previously proposed spin-resolved near-field scanning optical microscopy (NSOM) technique, we mapped the spin angular momentum (SAM) axial component (Sz) distributions of tightly focused linearly and circularly polarized vortex beams. The system's effectiveness was confirmed in our previous article by mapping various tightly focused cylindrical vector vortex beams. The SAM of different focused vortex light fields is essential in the research of near-field spin optics and topological photonics. The SAM distributions of different orders of linearly and circularly polarized vortex beams were mapped by separating their right spin (IRCP) and left spin component (ILCP) using the relationship Sz∝IRCP-ILCP.

Experimental observation of multiple edge and corner states in photonic slabs heterostructures

The photonic topological insulator has become an important research topic with a wide range of applications. Especially the higher-order topological insulator, which possesses gapped edge states and corner or hinge states in the gap, provides a new scheme for the control of light in a hierarchy of dimensions. In this paper, we propose a heterostructure composed of ordinary-topological-ordinary (OTO) photonic crystal slabs. Two coupled edge states (CESs) are generated due to the coupling between the topological edge states of the ordinary-topological interfaces, which opens up an effective way for high-capacity photonic transport. In addition, we obtain a new band gap between the CESs, and the two kinds of coupled corner states (CCSs) appear in the OTO bend structure. In addition, the topological corner state is also found, which arises from the filling anomaly of a lattice. Compared with the previous topological photonic crystal based on C-4 lattice, CESs, CCSs, and the topological corner state are all directly observed in experiment by using the near-field scanning technique, which makes the manipulation of the electromagnetic wave more flexible. We also verify that the three corner states are all robust to defects. Our work opens up a new way for guiding and trapping the light flow and provides a useful case for the coupling of topological photonic states.

Terahertz scattering-type near-field microscopy quantitatively determines the conductivity and charge carrier density of optically doped and impurity-doped silicon

A terahertz scattering-type scanning near-field optical microscope is used for nano-scale non-invasive conductivity measurements on bulk silicon samples. We first investigate the case where the density of charge carriers is determined by optical interband excitation. We show that the amplitude and phase of the near-field signal are reproduced by simulations based on an established near-field interaction model, which takes the Drude conductivity, ambipolar carrier diffusion, and known recombination properties of photo-excited carrier pairs in Si into account. This study is then extended to impurity-doped Si. We demonstrate that the phase of the near-field signal, which can easily be measured in absolute terms, allows us to quantitatively determine the conductivity of the specimens, from which the carrier density is derived based on the known carrier momentum relaxation time. A measurement at a single properly chosen terahertz frequency is sufficient. The technique proposed here holds promise for the spatially resolved quantitative characterization of micro- and nanoelectronic materials and devices.

Infrared nanoscopy and tomography of intracellular structures

Although techniques such as fluorescence-based super-resolution imaging or confocal microscopy simultaneously gather both morphological and chemical data, these techniques often rely on the use of localized and chemically specific markers. To eliminate this flaw, we have developed a method of examining cellular cross sections using the imaging power of scattering-type scanning near-field optical microscopy and Fourier-transform infrared spectroscopy at a spatial resolution far beyond the diffraction limit. Herewith, nanoscale surface and volumetric chemical imaging is performed using the intrinsic contrast generated by the characteristic absorption of mid-infrared radiation by the covalent bonds. We employ infrared nanoscopy to study the subcellular structures of eukaryotic (Chlamydomonas reinhardtii) and prokaryotic (Escherichia coli) species, revealing chemically distinct regions within each cell such as the microtubular structure of the flagellum. Serial 100 nm-thick cellular cross-sections were compiled into a tomogram yielding a three-dimensional infrared image of subcellular structure distribution at 20 nm resolution. The presented methodology is able to image biological samples complementing current fluorescence nanoscopy but at less interference due to the low energy of infrared radiation and the absence of labeling.

Development of a Magnetic Field Probe for Near-Field Measurement

Current electronic circuits are characterised by their compact design of miniaturisation; however, such a feature has also contributed to the rise of electromagnetic compatibility issues. This study introduces a technique to develop a magnetic field probe for near-field measurement. The probe aims at performing the debugging of electromagnetic interference on an integrated circuit. Certain processes, such as designing a microstrip line in simulation and a prototype for the validation as well as developing a functional magnetic field probe for near-field scanning and calibration measurement to determine the probe factor, have been performed under a frequency domain. The microstrip line model is built both as a simulation and a real prototype, and the handcrafted loop probe is developed with the same diameter as a commercial probe. Calibration measurement is set-up with a vector network analyser (VNA) to capture the magnetic field on radiated plane. The performance of the handcrafted probe in terms of measuring the configuration of the H-field source on the scattering region is conducted. Two types of measurements which are S11 and S21 will be conducted using a VNA. The comparison of the S11 and S21 results against simulation results are plotted using MATLAB. S11 represents the return loss of the microstrip line while S21 represents the power received by the magnetic field probe relative to power input to the microstrip line. The results of the S11 parameter indicate that the customised microstrip line board has a similar waveform pattern that matches the one on the simulation model. The S21 results for the handcrafted probe revealed that it can only function well up to the frequency of 2.644 GHz, the abnormal result obtained after the frequency mentioned. Some factors may have affected the results, such as material loss, fabrication tolerance and interferences from other devices. Therefore, calibration measurement is conducted under a less reflected radiation environment, and the designated substrate should be a perfect material correlated with the measurement.

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.

AFM-IR and s-SNOM-IR measurements of chemically addressable monolayers on Au nanoparticles.

The performance of catalysts depends on their nanoscale properties, and local variations in structure and composition can have a dramatic impact on the catalytic reactivity. Therefore, probing the localized reactivity of catalytic surfaces using high spatial resolution vibrational spectroscopy, such as infrared (IR) nanospectroscopy and tip-enhanced Raman spectroscopy, is essential for mapping their reactivity pattern. Two fundamentally different scanning probe IR nanospectroscopy techniques, namely, scattering-type scanning near-field optical microscopy (s-SNOM) and atomic force microscopy-infrared spectroscopy (AFM-IR), provide the capabilities for mapping the reactivity pattern of catalytic surfaces with a spatial resolution of ∼20nm. Herein, we compare these two techniques with regard to their applicability for probing the vibrational signature of reactive molecules on catalytic nanoparticles. For this purpose, we use chemically addressable self-assembled molecules on Au nanoparticles as model systems. We identified significant spectral differences depending on the measurement technique, which originate from the fundamentally different working principles of the applied methods. While AFM-IR spectra provided information from all the molecules that were positioned underneath the tip, the s-SNOM spectra were more orientation-sensitive. Due to its field-enhancement factor, the s-SNOM spectra showed higher vibrational signals for dipoles that were perpendicularly oriented to the surface. The s-SNOM sensitivity to the molecular orientation influenced the amplitude, position, and signal-to-noise ratio of the collected spectra. Ensemble-based IR measurements verified that differences in the localized IR spectra stem from the enhanced sensitivity of s-SNOM measurements to the adsorption geometry of the probed molecules.

Non‐Local Electrostatic Gating Effect in Graphene Revealed by Infrared Nano‐Imaging

AbstractElectrostatic gating lies in the heart of field effect transistor (FET) devices and modern integrated circuits. To achieve efficient gate tunability, the gate electrode has to be placed very close to the conduction channel, typically a few nanometers. Remote control of a FET device through a gate electrode located far away is highly desirable, because it not only reduces the complexity of device fabrication, but also enables the design of novel devices with new functionalities. Here, a non‐local electrostatic gating effect in graphene devices using scanning near‐field optical microscopy (SNOM)—a technique that can probe local charge density in graphene—is reported. Remarkably, the charge density of the graphene region tens of micrometers away from a local gate can be efficiently tuned. The observed non‐local gating effect is initially driven by an in‐plane electric field induced by the quantum capacitance of graphene, and further largely enhanced by adsorbed polarized water molecules. This study reveals a non‐local phenomenon of Dirac electrons, provides a deep understanding of in‐plane screening from Dirac electrons, and paves the way for designing novel electronic devices with remote gate control.

Imaging of magnetic excitations in nanostructures with near-field microwave microscopy

We present images of spin-wave excitations in a patterned yttrium iron garnet (YIG) thin film obtained by use of near-field microwave microscopy, which can achieve spatial resolution as high as 50 nm. Visualization of magnetic excitations is an enticing prospect for high-speed, high-density magnetic logic and storage applications, which has spurred the development of new magnetic microscopy and imaging techniques in recent years. Here we present a novel approach for local imaging of magnetic modes excited at room temperature with sub-diffraction-limited spatial resolution. This approach is based on a special atomic force microscope with broadband GHz capability for imaging the spatial distribution of dynamic magnetic excitations in patterned magnetic structures. Due to the inherent sub-wavelength, nanometer-scale spatial resolution of near-field scanning probe techniques, this approach has potential for a significant improvement in spatial resolution over optical techniques.

Preparation and characteristic analysis of nanofacula array

The development of nanofacula array is an effective methods to improve the performance of Near-field Scanning Optical Microscopy (NSOM) and achieve high-throughput array scanning. The nanofacula array is realized by preparing metal nanopore array through the "two etching-one development" method of double-layer resists and the negative lift-off process after metal film coating. The shading property of metal film plays important rules in nanofacula array fabrication. We investigate the shading coefficient of three kinds of metal films (gold–palladium alloy (Au/Pd), platinum (Pt), chromium (Cr)) under different coating times, and 3.5 min Au/Pd film is determined as the candidate of the nanofacula array fabrication for its lower thickness (about 23 nm) and higher shading coefficient (≥ 90%). The nanofacula array is obtained by irradiating with white light (central wavelength of 500 nm) through the metal nanopore array (250/450 nm pore diameter, 2 μm pore spacing and 7 μm group spacing). Moreover, the finite difference and time domain (FDTD) simulation proves that the combination of nanopore array and microlens array achieves high-energy focused nanofacula array, which shows a 3.2 times enhancement of electric field. It provides a new idea for NSOM to realize fast super-resolution focusing facula array.

Rapid simulations of hyperspectral near-field images of three-dimensional heterogeneous surfaces.

The scattering-type scanning near-field optical microscope (s-SNOM) has emerged as a powerful tool for resolving nanoscale inhomogeneities in laterally heterogeneous samples. However, most analytical models used to predict the scattering near-field signals are assuming homogenous landscapes (bulk materials), resulting in inconsistencies when applied to samples with more complex configurations. In this work, we combine the point-dipole model (PDM) to the finite-element method (FEM) to account for the lateral and vertical heterogeneities while keeping the computation time manageable. Full images, spectra, or hyperspectral line profiles can be simulated by calculating the self-consistent dipole radiation demodulated at higher harmonics of the tip oscillation, mimicking real experimental procedures. Using this formalism, we clarify several important yet puzzling experimental observations in near-field images on samples with rich typography and complex material compositions, heterostructures of two-dimensional material flakes, and plasmonic antennas. The developed method serves as a basis for future investigations of nano-systems with nontrivial topography.

Infrared-spectroscopic, dynamic near-field microscopy of living cells and nanoparticles in water

Infrared fingerprint spectra can reveal the chemical nature of materials down to 20-nm detail, far below the diffraction limit, when probed by scattering-type scanning near-field optical microscopy (s-SNOM). But this was impossible with living cells or aqueous processes as in corrosion, due to water-related absorption and tip contamination. Here, we demonstrate infrared s-SNOM of water-suspended objects by probing them through a 10-nm thick SiN membrane. This separator stretches freely over up to 250 µm, providing an upper, stable surface to the scanning tip, while its lower surface is in contact with the liquid and localises adhering objects. We present its proof-of-principle applicability in biology by observing simply drop-casted, living E. coli in nutrient medium, as well as living A549 cancer cells, as they divide, move and develop rich sub-cellular morphology and adhesion patterns, at 150 nm resolution. Their infrared spectra reveal the local abundances of water, proteins, and lipids within a depth of ca. 100 nm below the SiN membrane, as we verify by analysing well-defined, suspended polymer spheres and through model calculations. SiN-membrane based s-SNOM thus establishes a novel tool of live cell nano-imaging that returns structure, dynamics and chemical composition. This method should benefit the nanoscale analysis of any aqueous system, from physics to medicine.

Microdomain homogeneity evaluation of perpendicular lamellar structures in block copolymer films: X-ray scattering and IR nanospectroscopy analyses

Self-assembled block copolymer (BCP) thin films necessarily require a distinct chemical contrast between the blocks (or microdomain homogeneity) for the purpose of high-resolution pattern transfer applications. Despite its importance, however, there has been difficulty in measuring the chemical distribution associated with the self-assembling processes, i.e. solvent vapor annealing (SVA) or thermal annealing (TA). Herein, we present a simple yet effective method to probe the chemical distribution over polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) films that underwent the SVA or SVA + TA processes. An IR nanospectroscopy using scattering-type scanning near-field optical microscopy (s-SNOM), in concert with X-ray scattering analyses, provides the relative chemical concentration of the perpendicular lamellar patterns. The chemical mapping for the components based on the s-SNOM reveals that the SVA process leaves a considerable portion of the intermixed zones of PS and PMMA blocks, whereas the subsequent TA process facilitates the chain segregation towards high-contrast chemical distribution. Interestingly, such a microdomain homogeneity critically affects the quality of the unidirectional Al2O3 lines that were substituted from topographically guided lamellae. Our results substantiate an important insight into understanding the chain redistribution behavior in the self-assembled BCP films using non-destructive s-SNOM technique.

Microring Zone Structure for Near-Field Probes

Recent advances in Surface Plasmon Resonance (SPR) technologies have shown the possibility of transmission enhancement of localized modes propagating through sub-diffraction wide slits and apertures, resulting in the strong near-field focusing of metallic planar nanostructures. This work presents a new approach to the fabrication of high-resolution near-field optical probes using 3D lithography in combination with numerical finite difference time domain (FDTD) simulations. A narrow 500 nm depth of field focus area was observed both by numerical analysis and near field scanning optical microscopy (NSOM) measurements. Further research and optimization are planned in order to achieve subwavelength focal regions and increased signal intensities.

Subwavelength probing of surface plasmons in magnetoplasmonic crystals

In this work, we report on near-field studying of propagating surface plasmons (SPs) in one-dimensional magnetoplasmonic crystals (MPCs) by aperture type scanning near-field optical microscopy (SNOM). Optical near-field around the aperture probe is used to drive SPs in the MPC locally. The SNOM signal represents the scattered intensity caused by the interaction of the SNOM probe near-field with the MPC. Scanning the MPC surface with polarization resolving of the scattered radiation shows decreasing of the intensity due to the SP excitation. The observed polarization dependence of the scattered SNOM signal is associated with the selective coupling of the near-field components of the SNOM probe with SPs. Finite-difference time-domain simulations reproduce the experimental SNOM signal. It is shown the excitation of SPs with symmetric (even parity) field distribution, which is forbidden for plane wave source at normal incidence.

Nanotextured Dynamics of a Light-Induced Phase Transition in VO2.

We investigate transient nanotextured heterogeneity in vanadium dioxide (VO2) thin films during a light-induced insulator-to-metal transition (IMT). Time-resolved scanning near-field optical microscopy (Tr-SNOM) is used to study VO2 across a wide parameter space of infrared frequencies, picosecond time scales, and elevated steady-state temperatures with nanoscale spatial resolution. Room temperature, steady-state, phonon enhanced nano-optical contrast reveals preexisting "hidden" disorder. The observed contrast is associated with inequivalent twin domain structures. Upon thermal or optical initiation of the IMT, coexisting metallic and insulating regions are observed. Correlations between the transient and steady-state nano-optical textures reveal that heterogeneous nucleation is partially anchored to twin domain interfaces and grain boundaries. Ultrafast nanoscopic dynamics enable quantification of the growth rate and bound the nucleation rate. Finally, we deterministically anchor photoinduced nucleation to predefined nanoscopic regions by locally enhancing the electric field of pump radiation using nanoantennas and monitor the on-demand emergent metallicity in space and time.

Single-Sided Microwave Near-Field Scanning of Pine Wood Lumber for Defect Detection

Defects and cracks in dried natural timber (relative permittivity 2–5) may cause structural weakness and enhanced warping in structural beams. For a pine wood beam (1200 mm × 70 mm × 70 mm), microwave reflection (S11) and transmission (S21) measurements using a cavity-backed slot antenna on the wood surface showed the variations caused by imperfections and defects in the wood. Reflection measurements at 4.4 GHz increased (>5 dB) above a major knot evident on the wood surface when the E-field was parallel to the wood grain. Similar results were observed for air cavities, independent of depth from the wood surface. The presence of a metal bolt in an air hole increased S11 by 2 dB. In comparison, transmission measurements (S21) were increased by 6 dB for a metal screw centered in the cavity. A kiln-dried pine wood sample was saturated with water to increase its moisture content from 17% to 138%. Both parallel and perpendicular E-field measurements showed a difference of more than 15 dB above an open saw-cut slot in the water-saturated beam. The insertion of a brass plate in the open slot created a 7 dB rise in the S11 measurement (p < 0.0003), while there was no significant variation for perpendicular orientation. By measuring the reflection coefficient, it was possible to detect the location of a crack through a change in its magnitude without a noticeable change (<0.01 GHz) in resonant frequency. These microwave measurements offer a simple, single-frequency non-destructive testing method of structural timber in situ, when one or more plane faces are accessible for direct antenna contact.

3D printed metasurface for generating a Bessel beam with arbitrary focusing directions.

In this Letter, a metasurface combined with emerging 3D printing technology is proposed. The proposed metasurface regards the simple cube as the unit cell, and the height of the cube is the only variable. A nearly linear transmission phase range covering 360° operating at 20 GHz is obtained when the height is regulated in [2.26 mm, 11.20 mm]. Therefore, the proposed unit cell can be adopted to any metasurface with various functions. Taking the generation of a non-diffractive Bessel beam as an example, two metasurfaces composed of 30×30 units with different focusing directions are designed based on non-diffractive theory and the generalized law of refraction. Two prototypes are 3D printed and measured by a near-field scanning system. The measured results validate our design with satisfactory focusing and beam deflection performance. Additionally, the 3D printed metasurface has lower cost and a shorter processing cycle, and avoids metal loss. Therefore, a 3D printed metasurface is an excellent candidate that can be applied in millimeter wave or even higher frequency bands.

Experimental observation of the short-range surface plasmon polariton mode and its longitudinal adiabatic compression in a metallic wedge.

In this study, we explore analytically and experimentally long- and short-range surface plasmon polariton (LR-SPP and SR-SPP, respectively) modes in gold wedges. Especially, we aim to observe the 2-dimensional confinement of the electromagnetic field in gold wedges as it could enhance the light-matter interaction by offering a local density of states which depends on the propagation constant, consequently on the wedge height. The LR-SPP mode can propagate over a long distance, but the real part of the propagation constant remains relatively insensitive to the decreasing wedge height. This mode also experiences cut-off at a wedge height of about 50 nm in our experimental condition. Meanwhile, the SR-SPP mode has a large propagation constant that increases further with decreasing wedge height. As a result, the effective wavelength of the mode shrinks confining the electromagnetic wave longitudinally along the propagation direction in addition to enhancing the transverse confinement of SR-SPP. In the experiment, we use gold wedges with different edge heights to excite each SPP mode individually and image the electromagnetic near field by using a pseudo-heterodyne scattering scanning near-field optical microscope. By imaging the LR-SPP mode field, we demonstrate that the theoretical and measured values of the effective wavelength agree quite well. By using short wedges, we measure the SR-SPP mode field and demonstrate that the effective wavelength decreases to 47% in about half a micrometer of propagation distance. This corresponds to a 3.5 times decrease of the vacuum wavelength or an effective index of 3.5. It is important to note that this value is, by no means, the limit of the electromagnetic field's longitudinal confinement in a gold wedge. Rather, we were only able to measure the electromagnetic field up to this point due to our measurement limitations. The electromagnetic field will be propagating further, and the longitudinal confinement will increase as well. In conclusion, we measured the SR-SPP in a gold wedge and demonstrate the electromagnetic field confinement in the visible spectrum in gold wedges.