Near-field Scanning Research Articles

Experimental Observation of ABCB Stacked Tetralayer Graphene.

In tetralayer graphene, three inequivalent layer stackings should exist; however, only rhombohedral (ABCA) and Bernal (ABAB) stacking have so far been observed. The three stacking sequences differ in their electronic structure, with the elusive third stacking (ABCB) being unique as it is predicted to exhibit an intrinsic bandgap as well as locally flat bands around the K points. Here, we use scattering-type scanning near-field optical microscopy and confocal Raman microscopy to identify and characterize domains of ABCB stacked tetralayer graphene. We differentiate between the three stacking sequences by addressing characteristic interband contributions in the optical conductivity between 0.28 and 0.56 eV with amplitude and phase-resolved near-field nanospectroscopy. By normalizing adjacent flakes to each other, we achieve good agreement between theory and experiment, allowing for the unambiguous assignment of ABCB domains in tetralayer graphene. These results establish near-field spectroscopy at the interband transitions as a semiquantitative tool, enabling the recognition of ABCB domains in tetralayer graphene flakes and, therefore, providing a basis to study correlation physics of this exciting phase.

The Differences in Spatial Luminescence Characteristics between Blue and Green Quantum Wells in Monolithic Semipolar (20-21) LEDs Using SNOM.

The differences in spatially optical properties between blue and green quantum wells (QWs) in a monolithic dual-wavelength semipolar (20-21) structure were investigated by scanning near-field optical microscopy (SNOM). The shortest wavelength for green QWs and the longest wavelength for blue QWs were both discovered in the region with the largest stress. It demonstrated that In composition, compared to stress, plays a negligible role in defining the peak wavelength for blue QWs, while for green QWs, In composition strongly affects the peak wavelength. For green QWs, significant photoluminescence enhancement was observed in the defect-free region, which was not found for blue QWs. Furthermore, the efficiency droop was aggravated in the defect-free region for green QWs but reduced for blue QWs. It indicates that carrier delocalization plays a more important role in the efficiency droop for QWs of good crystalline quality, which is experimentally pointed out for the first time.

The opening workspace control strategy of a novel manipulator-driven emission source microscopy system

Emission source microscopy (ESM) technique can be utilized for localization of electromagnetic interference sources in the electronic systems, but its accuracy is limited by the typical planar scanning mode. In order to increase the accuracy, this paper presents a novel cylinder-aperture ESM measurement system driven by 6-DOF manipulator, and investigated the control strategy to generate the maximum-area aperture and optimized scanning trajectory. Based on the multiple constraints of the cylinder-aperture ESM measurement, we proposes analyzing the impact of the constraints by steps. This can obtain the analytical solution of the manipulator workspace and support solving the maximum aperture area. Besides, a modified RRT*(Rapidly-exploring Random Trees) algorithm is addressed to optimize the manipulator trajectory. The simulation and tests have proven that this algorithm could obviously reduce the joint mutation and cumulative tracking error. In the experimental section, the near-field scanning (NFS) tests, planar-aperture ESM measurement and proposed cylinder-aperture ESM measurement were conducted to measure one benchmark emission source. The results have demonstrated that the cylinder-aperture ESM measurement has the best convergences on the radiation pattern of the emission source.

Design of a Power Splitter Based on a 3D MMI Coupler at the Fibre-Tip

Planar MMI couplers based on inorganic material platforms have played an essential role in photonic integrated circuits development. Advances in organic polymer fabrication techniques enable the design of components beyond a single plane, thus facilitating vertical integration for a wide range of components, including the MMI coupler. This paper presents the design of two 3D IP-dip polymer-based MMI power splitters operating in the near-infrared part of the spectrum at a wavelength of 1550 nm. The resulting output power ratio, modal field distributions, spectral characteristics, and the effects of input fibre misalignment are investigated using the beam propagation method. The fabrication method used to realise the designed splitters was direct laser writing. The function of the splitters was then verified by a highly resolved near-field scanning optical microscope.

In Situ Cell Detection Using Terahertz Near-Field Microscopy

The photoconductive antenna microprobe-based terahertz (THz) near-field scanning microscope has proven to overcome the diffraction limit and demonstrated the potential in distinguishing cellular types and states. Here, using the onion epidermal cells as a sample model, detection of single epithelial cell in the onion skin has been realized at amplitude signal around 1 THz. The boundaries of the target cells in a monolayer onion epidermis are clearly depicted with the spatial resolution of 9 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m (λ/33). Differentiated THz dielectric response shows clearly the distinctive intracellular and intercellular regions, mainly related to the biowater distribution and cellular heterogeneity, which can be used for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> subcellular identification <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">.</i> This finding may have significant implications for the advancement of THz near-field high-resolution bioimaging technology.

Measuring of Transverse Energy Flows in a Focus of an Aluminum Lens

In this study, we theoretically and experimentally investigate the propagation of a second-order cylindrical vector beam through an aluminum lens which forms a tight focus at the distance of the wavelength. Simulation by the finite-difference time-domain method and the Richards–Wolf formulae produces light field distributions which coincide with experimental measurements provided with scanning near-field optical microscopy. We demonstrate that a pyramidal metallized cantilever with a hole is more sensitive to the transversal component of intensity than to the full intensity or to the Umov–Poynting vector in areas of reverse energy flow.

Imaging Stacking-Dependent Surface Plasmon Polaritons in Trilayer Graphene

We report an infrared (IR) imaging study of trilayer graphene (TLG) with both ABA (Bernal) and ABC (rhombohedral) stacking orders using the scattering-type scanning near-field optical microscope (SSNOM). With the SSNOM operating in the mid-IR region, we map in real space the surface plasmon polaritons of ABA TLG and ABC TLG, which are tunable with electrical gating. Through quantitative modeling of plasmonic imaging data, we find that the plasmon wavelength of ABA TLG is significantly larger than that of ABC TLG, resulting in a sizable impedance mismatch and hence a strong plasmon reflection at the ABA/ABC lateral junction. Further analysis indicates that the different plasmonic responses of the two types of TLG are directly linked to their distinct electronic structures and carrier properties. Our work uncovers the physics behind the stacking-dependent plasmonic responses of TLG and sheds light on future applications of TLG and the ABA/ABC junctions in IR plasmonics and planar nano-optics.

Super-resolved discrimination of nanoscale defects in low-dimensional materials by near-field photoluminescence spectral imaging

Low-dimensional materials (LDMs), such as monolayer transition-metal dichalcogenides, have emerged as candidate materials for next-generation optoelectronics devices. Detection of the spatial heterogeneity caused by various nanoscale defects in LDMs, is crucial for their applications. Here, we report the super-resolved discrimination of various nanoscale defects in LDMs by near-field photoluminescence (NFPL) spectral imaging of LDMs with scanning near-field optical microscopy. As a demonstration example, a monolayer WS2 sample is characterized with a sub-diffraction spatial resolution of 140 nm in ambient environment. By performing topography and NFPL mapping, different defects, such as the stacks, bubbles, and wrinkles, can be identified through their light emission properties, which strongly correlate with the exciton emission modulation and tensile strain arising from local structural deformations.

Numerical characterization of optical properties of tapered plasmonic structure on a cantilever pyramidal tip for plasmon nanofocusing

The plasmon nanofocusing process has been widely implemented in near-field scanning optical microscopy (NSOM) recently because it allows generating a background-free nanolight source at the apex of a metallic tip, enabling high contrast imaging at the nanoscale. In plasmon nanofocusing-assisted NSOM, the metallic tip properties play a vital role in generating an intense and well-confined nanolight source by controlling the plasmons’ behavior. This is why various tip designs have been developed so far. Recently, our group has also developed a metallic tapered tip, composed of a dielectric pyramidal base and a thin metallic layer coated on one side of the pyramid, using a novel fabrication method that allows tuning the optical properties of a tip depending on the requirement. Although our metallic tip has a unique advantage of tuning its optical properties, it has not yet been well studied. In this work, we present a thorough study of the optical properties of our metallic tip that depends on its parameters, such as the dielectric material, metal thickness, and cone angle, using finite-difference time-domain simulations. This particular study will allow us to understand controlling the tip’s optical properties and expand it for a wide range of applications.

Topological extraordinary optical transmission

The incumbent technology for bringing light to the nanoscale, the near-field scanning optical microscope, has notoriously small throughput efficiencies - of the order of 10^(-4) - 10^(-5), or less. We report on a broadband, topological, unidirectionally-guiding structure, not requiring adiabatic tapering and in principle enabling near-perfect (ideally, ~100%) optical transmission through an unstructured single (POTUS) arbitrarily-subdiffraction slit at its end. Specifically, for a slit width of just lambda_eff / 72 (lambda_0 / 138) the attained normalized transmission coefficient reaches a value of 1.52, while for a unidirectional-only (non-topological) device the normalized transmission through a lambda_eff / 21 (~lambda_0 / 107) slit reaches 1.14 - both, limited only by inherent material losses, and with zero reflection from the slit. The associated, under ideal conditions, near-perfect optical extraordinary transmission (POET) has implications, among diverse areas in wave physics and engineering, for high-efficiency, maximum-throughput nanoscopes and heat-assisted magnetic recording devices.

Terahertz-slicing - an all-optical synchronization for 4th generation light sources.

A conceptually new approach to synchronizing accelerator-based light sources and external laser systems is presented. The concept is based on utilizing a sufficiently intense accelerator-based single-cycle terahertz pulse to slice a thereby intrinsically synchronized femtosecond-level part of a longer picosecond laser pulse in an electro-optic crystal. A precise synchronization of the order of 10 fs is demonstrated, allowing for real-time lock-in amplifier signal demodulation. We demonstrate successful operation of the concept with three benchmark experiments using a 4th generation accelerator-based terahertz light source, i.e. (i) far-field terahertz time-domain spectroscopy, (ii) terahertz high harmonic generation spectroscopy, and (iii) terahertz scattering-type scanning near-field optical microscopy.

A near-field study of VO2/(100)TiO2 film and its crack-induced strain relief

Temperature-induced metal–insulator transition (MIT) in vanadium dioxide (VO2) has been under intense research interest for decades both theoretically and experimentally. Due to the complex nature of electron correlations, the underlying physics behind the MIT in VO2 has yet to be fully grasped. In this work, we utilize the fine resolution of the scattering-type scanning near-field optical microscope to investigate the MIT in an epitaxial VO2 thin film on the (100)R TiO2 substrate with mid-infrared light. Bidirectional tweed-like metal–insulator phase coexistence patterns are observed and understood under the Landau free energy paradigm. More interestingly, delayed metallic nucleation is observed near the surface cracks due to local strain relief. This research proposes ideas in investigating the temperature–pressure phase diagram and tuning the interplay between local strain and MIT in oxide thin films.

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.

(Invited) Persistently Luminescent Nanoscale Materials

Although alkaly earth aluminates (such as SrAl2O4 doped with Europium and Dysprosium) have been investigated as persistently luminescent (PL) materials for several decades, multiple questions still remain unanswered about their nanoscale optoelectronic properties, microscopic characterization, and their utilization in specific devices, such as, for example, after-glowing solar cells. In this talk, I will review what the scientific community, and my group, have been learning on SrAl2O4(:Eu:Dy) using nano-optical characterization tools, including scanning near field optical microscopy (SNOM) luminescence imaging to inform device fabrication. Specifically, I will review phase phase-modulated scanning near-field optical luminescence (PM-SNOL) imaging as new method for assessing the exciton diffusion length (LD) in nanoscale PL materials. Using PL we have determined that LD is of the order of 100 μm in SrAl2O4(:Eu:Dy), a surprisingly large value, that should enable exciton storage and transfer from SrAl2O4(:Eu:Dy) micro- and nano-crystals, as opposed to simple exciton recombination with photon re-emission. With this result in mind, in the second part of our talk, we will we introduce a new family of photovoltaics capable of generating power in the dark for several hours after illumination, thanks to the afterglow redox properties of strontium aluminate codoped with Eu2+ and Dy3+ [SrAl2O4(Eu,Dy)], a persistently luminescent material incorporated at the interface between the transparent anode and the active layer of dye-sensitized solar cells (DSSC). After 9-hr operation in the dark, our photovoltaics still produce dark short-circuit currents about 75% of the current generated immediately after illumination. Control DSSCs with identical architecture, but without SrAl2O4(Eu,Dy), do not exhibit any significant generation of power in the dark. Differently from previous unsuccesful attempts to design nighttime solar cells seeking use of the reabsorption of photons persistently emitted by SrAl2O4(Eu,Dy) in relatively inefficient processes, our devices rely on the more efficient integration of Eu2+/Eu3+ and Dy3+/Dy2+ redox processes of SrAl2O4(Eu,Dy) into the electrochemistry of DSSCs, with the suppression of persistent luminescence, the direct oxidation of the iodine electrolyte by Eu3+,and the reduction of the DSSC anode by Dy2+. With the discovery of charge transfer and redox processes between strontium aluminate, titania and iodine electrolytes, our work also offers unique advances in the photo-physics of PL materials at the nanoscale.

Nano-FTIR Correlation Nanoscopy for Organic and Inorganic Material Analysis

Significance: Correlation scanning probe techniques to complement nanoscale IR measurements for next generation sample characterizationScattering-type Scanning Near-field Optical Microscopy (s-SNOM) is a scanning probe approach to optical microscopy and spectroscopy, bypassing the ubiquitous diffraction limit of light to achieve a spatial resolution below 20 nanometers. s-SNOM employs the strong confinement of light at the apex of a sharp metallic atomic force microscopy (AFM) tip to create a nanoscale optical hot-spot. Analyzing the scattered light from the tip enables the extraction of the optical properties of the sample directly below the tip and yields nanoscale resolved images simultaneous to topography [1]. In addition, the technology has been advanced to enable Fourier-Transform Infrared Spectroscopy on the nanoscale (nano-FTIR) [2] using broadband radiation from the visible spectral range to THz frequencies.Recently, the combined analysis of complex nanoscale material systems by correlating near-field optical data with information obtained by other scanning probe microscopy (SPM)-based measurement methodologies has gained significant interest. For example, the material-characteristic nano-FTIR spectra of a phase-separated polystyrene/low-density polyethylene (PS/LDPE) polymer blend verifies sharp material interfaces by measuring a line profile across a ca. 1 μm sized LDPE island. Near-field reflection/absorption imaging at 1500cm-1 of the ca. 50nm thin film allows to selectively highlight the distribution of PS in the blend and simultaneously map the mechanical properties like adhesion of the different materials [3,4].Further, we present results that correlate the near-field optical response of semiconducting samples like graphene (2D) or functional SRAM devices (3D) in different frequency ranges (mid-IR & THz) to Kelvin Probe Force Microscopy (KPFM) measurements. Thus, s-SNOM systems represent an ideal platform to gain novel insights into complex material systems by different near-field and AFM-based method.[1] F. Keilmann, R. Hillenbrand, Phil. Trans. R. Soc. Lond. A 362, 787 (2004).[2] F. Huth, et al., Nano Lett. 12, 3973 (2012).[3] B. Pollard, et al., Beilstein J. of Nanotechn. 7, 605 (2016).[4] I. Amenabar, et al., Nature Commun. 8, 14402 (2017).

Far-Field and Non-Intrusive Optical Mapping of Nanoscale Structures.

Far-field high-density optics storage and readout involve the interaction of a sub-100 nm beam profile laser to store and retrieve data with nanostructure media. Hence, understanding the light–matter interaction responding in the far-field in such a small scale is essential for effective optical information processing. We present a theoretical analysis and an experimental study for far-field and non-intrusive optical mapping of nanostructures. By a comprehensive analytical derivation for interaction between the modulated light and the target in a confocal laser scanning microscopy (CLSM) configuration, it is found that the CLSM probes the local density of states (LDOSs) in the far field rather than the sample geometric morphology. With a radially polarized (RP) light for illumination, the far-field mapping of LDOS at the optical resolution down to 74 nm is obtained. In addition, it is experimentally verified that the target morphology is mapped only when the far-field mapping of LDOS coincides with the geometric morphology, while light may be blocked from entering the nanostructures medium with weak or missing LDOS, hence invalidating high-density optical information storage and retrieval. In this scenario, nanosphere gaps as small as 33 nm are clearly observed. We further discuss the characterization for far-field and non-intrusive interaction with nanostructures of different geometric morphology and compare them with those obtainable with the projection of near-field LDOS and scanning electronic microscopic results.

Numerical calculation of near-field in vicinity of three-dimensional nano-optical probes using boundary element method

The diffraction limit is one of the main obstacles in the development of microscopes to analyze the morphology and structure of materials. The main idea of near field scanning optical microscopy (NSOM) is to overcome the diffraction limit using sub-wavelength apertures. In this work, the near-field is simulated in the vicinity of three-dimensional nano-optical apertureless probes. For this purpose, the Helmholtz equation is solved using the boundary element method (BEM). The effects of different parameters on the near field generated in the vicinity of the optical probe are studied. These parameters consist of the length and radius of the probe, the size of the aperture, the angle of the tapered tip, and the geometry of the probe tip. The main advantages of the proposed method are the high accuracy, the very short calculation time, and the ability to calculate the near field inside and outside the optical probe without any approximation.

Surface plasmons induce topological transition in graphene/α-MoO3 heterostructures

Polaritons in hyperbolic van der Waals materials—where principal axes have permittivities of opposite signs—are light-matter modes with unique properties and promising applications. Isofrequency contours of hyperbolic polaritons may undergo topological transitions from open hyperbolas to closed ellipse-like curves, prompting an abrupt change in physical properties. Electronically-tunable topological transitions are especially desirable for future integrated technologies but have yet to be demonstrated. In this work, we present a doping-induced topological transition effected by plasmon-phonon hybridization in graphene/α-MoO3 heterostructures. Scanning near-field optical microscopy was used to image hybrid polaritons in graphene/α-MoO3. We demonstrate the topological transition and characterize hybrid modes, which can be tuned from surface waves to bulk waveguide modes, traversing an exceptional point arising from the anisotropic plasmon-phonon coupling. Graphene/α-MoO3 heterostructures offer the possibility to explore dynamical topological transitions and directional coupling that could inspire new nanophotonic and quantum devices.

Cloaked near-field probe for non-invasive near-field optical microscopy

Near-field scanning optical microscopy is a powerful technique for imaging below the diffraction limit, which has been extensively used in biomedical imaging and nanophotonics. However, when the electromagnetic fields under measurement are strongly confined, they can be heavily perturbed by the presence of the near-field probe itself. Here, taking inspiration from scattering-cancellation invisibility cloaks, Huygens–Kerker scatterers, and cloaked sensors, we design and fabricate a cloaked near-field probe. We show that, by suitably nanostructuring the probe, its electric and magnetic polarizabilities can be controlled and balanced. As a result, probe-induced perturbations can be largely suppressed, effectively cloaking the near-field probe without preventing its ability to measure. We experimentally demonstrate the cloaking effect by comparing the interaction of conventional and nanostructured probes with a representative nanophotonic structure, namely, a 1D photonic-crystal cavity. Our results show that, by engineering the structure of the probe, one can systematically control its back action on the resonant fields of the sample and decrease the perturbation by &gt; --> 70 % with most of our modified probes, and by up to 1 order of magnitude for the best probe, at probe-sample distances of 100 nm. Our work paves the way for non-invasive near-field optical microscopy of classical and quantum nanosystems.

Scanning near-field optical spectroscopy and carrier transport based analysis in mesoscopic regions for two-dimensional semiconductors

The measurements of photoexcited transport in mesoscopic regimes reveal the states and properties of mesoscopic systems. In this study, we focused on direct measurements of electromagnetic energy transports in the mesoscopic regions and constructed a scanning tunnelling microscope-assisted multi-probe scanning near-field optical microscope spectroscopy system. After producing an emission energy map through a single-probe measurement, two-probe measurement enables us to observe and analyse carrier transport characteristics. It suggests that exciton generation and transport in the mesoscopic region of semiconductors with quantum structure changes, such as the bias of dopant, affect the excited carrier emission recombination process. The measured probability density of the carrier transported with quantum effects can be used for applications in natural intelligence research by combining it with the analysis using tournament structures. Our developed measurement and analysis methods are expected to clarify the details of carrier's behaviour in the mesoscopic region in various materials and lead to applications for novel optoelectronic devices.