Near-field Scanning Research Articles

Excitonic Insulator Enabled Ultrasensitive Terahertz Photodetection with Efficient Low-Energy Photon Harvesting.

Despite the interest toward the terahertz (THz) rapidly increasing, the high-efficient detection of THz photon is not widely available due to the low photoelectric conversion efficiency at this low-energy photon regime. Excitonic insulator (EI) states in emerging materials with anomalous optical transitions and renormalized valence band dispersions render their nontrivial photoresponse, which offers the prospect of harnessing the novel EI properties for the THz detection. Here, an EI-based photodetector is developed for efficient photoelectric conversion in the THz band. High-quality EI material Ta2 NiSe5 is synthesized and the existence of the EI state at room temperature is confirmed. The THz scanning near-field optical microscopy experimentally reveals the strong light-matter interaction in the THz band of EI state in the Ta2 NiSe5 . Benefiting from the strong light-matter interaction, the Ta2 NiSe5 -based photodetectors exhibit superior THz detection performances with a detection sensitivity of ≈42 pW Hz-1/2 and a response time of ≈1.1 µs at 0.1 THz at room temperature. This study provides a new avenue for realizing novel high-performance THz photodetectors by exploiting the emerging EI materials.

Excitation-dependent spatially resolved photoluminescence in InGaN/GaN LEDs with air-cavity arrays grown on patterned sapphire substrates

We investigated the excitation-dependent spatially resolved luminescence properties of InGaN/GaN light-emitting diodes (LEDs) with air-cavity arrays using scanning near-field optical microscopy (SNOM), and quantified the effect of the air-cavity structure on the spatial distributions of light-extraction efficiency, internal quantum efficiency, and external quantum efficiency through experiments and numerical simulations. We found that higher light-extraction efficiency and less stress were always observed in the air-cavity areas. Compared to flat areas, the average light-extraction efficiency of the air-cavity areas was improved by 65%, and the residual stress of the air-cavity center was reduced by 89 MPa, which resulted in increased PL intensity, reduced wavelength shift, and narrowed full width at half maximum. The highest external quantum efficiency and aggravated efficiency droop were found in the air-cavity center. At 3.1 W/cm2, the external quantum efficiency and internal quantum efficiency of the air-cavity center were enhanced by 255% and 223%, respectively.

Machine Learning for Optical Scanning Probe Nanoscopy.

The ability to perform nanometer-scale optical imaging and spectroscopy is key to deciphering the low-energy effects in quantum materials, as well as vibrational fingerprints in planetary and extraterrestrial particles, catalytic substances, and aqueous biological samples. These tasks can be accomplished by the scattering-type scanning near-field optical microscopy (s-SNOM) technique that has recently spread to many research fields and enabled notable discoveries. Herein, it is shown that the s-SNOM, together with scanning probe research in general, can benefit in many ways from artificial-intelligence (AI) and machine-learning (ML) algorithms. Augmented with AI- and ML-enhanced data acquisition and analysis, scanning probe optical nanoscopy is poised to become more efficient, accurate, and intelligent.

Near-field launching and mapping unidirectional surface plasmon polaritons using an automated dual-tip scanning near-field optical microscope

The spatial distribution of electromagnetic fields emitted from the aperture tip of a scanning near-field optical microscope (SNOM), which is called the emission pattern, depends on the geometry of the apex and the material composition of the tip’s coating. In previous works, experimental measurements of the emission pattern from the aperture tip were performed mostly in the far field. Moreover, the corresponding theoretical models were also developed based on these far-field measurements. Here, we have used the automated dual-tip SNOM to systematically characterize the emission from the aperture tip in the near field. In this regard, we have considered three different pairs of excitation and detection tips with distinct geometries. The emission patterns of the excitation tips were mapped using detection tips. Unidirectional surface plasmon polaritons (SPPs) at the surface of a gold platelet were launched by an excitation tip and measured in the near field by the detection tip. The experimental results were numerically reproduced by means of the Bethe–Bouwkamp model. This work puts into evidence the applicability of the automated dual-tip SNOM as the only available characterization technique to measure the emission from aperture tips in the near field. The reported asymmetric SPP radiation patterns can find applications in photonic integrated circuits or in biological and chemical sensing.

An Ultrawideband Multicomponent Differential Magnetic Probe for Near-Field Scanning

In this brief, a novel ultrawideband multi-component differential magnetic probe is proposed. Two pairs of differential loops and connecting vias were used to simultaneously sense two orthogonal magnetic-field components (Hx and Hy). Two rows of via fences next to the loops were used to prevent unwanted electromagnetic modes. The operating frequency band of the proposed probe ranges from the 1 MHz to 20 GHz. The accuracy of the proposed probe is verified by comparing its measurement and simulation results. This significantly improves the near-field scanning efficiency compared with the conventional single-component probe.

Nanoscale infrared imaging and spectroscopy of few-layer hexagonal boron nitride

Nanoscale evaluation of the number of layers and boundaries in two-dimensional (2D) materials is crucial for understanding relationships between structure and property. Here, using scattering-type scanning near-field optical microscopy, we systematically studied on a nanoscale the infrared spectra and imaging of hexagonal boron nitride (h-BN), an ideal 2D insulating material. We revealed that the main factor determining the infrared amplitude changes at an optical frequency of about 1370 cm−1, corresponding to the in-plane phonon mode of h-BN. At lower frequencies, the amplitude is mainly determined by the local dielectric function of a sample and depends on the number of h-BN layers. At higher frequencies, it is affected by the phonon polariton waves of h-BN, and thus edges and grain boundaries of h-BN can be visualized due to the reflection of the waves at the boundary. The infrared spectra show a shoulder peak at higher frequencies, derived from the resonance with the phonon polaritons, in addition to a peak due to the in-plane phonon mode.

Passive dual-probe near-field microscopy.

Accurate and simultaneous multiposition near-field measurements are essential to study the time-dependent local dynamics, including heat and carrier transfer. The existing passive long-wavelength infrared (LWIR) scattering-type scanning near-field optical microscopy (s-SNOM) systems with a single probe cannot perform precise near-field measurements of the heat or carrier transporting process at the nanoscale level. Therefore, in this study, we developed a passive LWIR s-SNOM system with two probes. To test the effectiveness of the proposed passive LWIR dual-probe s-SNOM system, each probe was precisely controlled using a shear-force feedback system, and the mechanical interference between the probes was used to monitor the distance between the probes. We achieved simultaneous near-field measurements at two different positions 500nm apart using the proposed passive LWIR dual-probe s-SNOM system. The simultaneously detected near-field signals from two different points were extracted individually, making this technique an effective nanoscale analysis tool for local carrier dynamics.

Learning-Based Subsurface Quantitative Imaging via Near-Field Scanning Microwave Microscopy

Accurate characterization of sample dielectric property is essential for scientists and engineers to analyze and evaluate the performance of the invested sample. This article proposes a learning-based (LB) method for quantitative subsurface imaging via coaxial-resonator-based near-field scanning microwave microscopy (CR-NFSMM) in a nondestructive way. To efficiently generate the database, a well-designed forward solver is critical since many samples will be included in the database, each sample needs a scanning procedure, and each scanning point involves solving the forward problem once. A fast forward problem solver is used to evaluate the tip–sample interaction in CR-NFSMM that avoids repeated meshing during the scanning process and it can apply to an arbitrary tip shape. Once the neural network is trained, it generates the reconstructed image within 1 s. Numerical and experimental results show that the proposed LB method could improve the resolution of the image and recover the dielectric properties of the subsurface perturbation pixel-by-pixel. Moreover, the proposed method outperforms the traditional objective function approach in terms of image resolution and time cost. The proposed method is promising to realize a nondestructive and real-time local dielectric evaluation of the subsurface object.

Near-field interference map due to a dipolar emission near the edge of a monocrystalline gold platelet

Point source excitation and point detection in the near-field provides new perspective to study the near-field optical phenomena of plasmonic nanostructures. Using the automated dual-tip scanning near-field optical microscope (SNOM), we have measured the optical near-field response of a dipolar emission near the edge of a monocrystalline gold platelet. The image dipole method was used to analytically calculate the interference pattern due to surface plasmon polaritons excited at the position of aperture tip and those reflected from edges of the gold platelet. The near-field enhancement was observed on the edges of the gold platelet. Our results verify that automated dual-tip SNOM is an intriguing technique for quantum plasmonic studies where deterministic coupling of quantum emitters and the detection of the near-field enhancement are of great interest.

Side chain engineering and film uniformity: two key parameters for the rational design of dopant-free polymeric hole transport materials for efficient and stable perovskite solar cells

Herein, we introduce a series of new (X–DADAD)n-type conjugated polymers comprised of benzo[1,2-b:4,5-b′]dithiophene (X), benzo[c] [1,2,5]thiadiazole (acceptor, A), and thiophene (donor, D) units as highly promising hole transport materials for perovskite solar cells (PSCs). The rational engineering of the backbone structure and the side chains enabled high power conversion efficiencies of up to 20% in n-i-p perovskite solar cells in combination with impressive operational stability: the devices preserved initial performance after >2500 h of continuous illumination at open-circuit conditions. The origins of the observed long-term perovskite solar cells stability enabled by the newly designed polymeric hole transport materials have been revealed through the use of infrared scattering-type scanning near-field optical microscopy and further elucidated by density functional theory calculations. The presented findings point to novel strategies of designing advanced charge transport materials to simultaneously endow the PSCs with high performance and durability.

Oxidized Alginate Dopamine Conjugate: A Study to Gain Insight into Cell/Particle Interactions.

Background: We had previously synthetized a macromolecular prodrug consisting of oxidized Alginate and dopamine (AlgOx-Da) for a potential application in Parkinson disease (PD). Methods: In the present work, we aimed at gaining an insight into the interactions occurring between AlgOx-Da and SH-SY5Y neuronal cell lines in view of further studies oriented towards PD treatment. With the scope of ascertaining changes in the external and internal structure of the cells, multiple methodologies were adopted. Firstly, fluorescently labeled AlgOx-Da conjugate was synthetized in the presence of fluorescein 5(6)-isothiocyanate (FITC), providing FITC-AlgOx-Da, which did not alter SH-SY5Y cell viability according to the sulforhodamine B test. Furthermore, the uptake of FITC-AlgOx-Da by the SH-SY5Y cells was studied using scanning near-field optical microscopy and assessments of cell morphology over time were carried out using atomic force microscopy. Results: Notably, the AFM methodology confirmed that no relevant damage occurred to the neuronal cells. Regarding the effects of DA on the intracellular reactive oxygen species (ROS) production, AlgOx-Da reduced them in comparison to free DA, while AlgOx did almost not influence ROS production. Conclusions: these findings seem promising for designing in vivo studies aiming at administering Oxidized Alginate Dopamine Conjugate for PD treatment.

Phase-resolved all-fiber reflection-based s-NSOM for on-chip characterization.

We report on a phase-resolved, reflection-based, scattering-type near-field scanning optical microscope technique with a convenient all-fiber configuration. Exploiting the flexible positioning of the near-field probe, our technique renders a heterodyne detection for phase measurement and point-to-point frequency-domain reflectometry for group index and loss measurement of waveguides on a chip. The important issue of mitigating the measurement errors due to environmental fluctuations along fiber-optic links has been addressed. We perform systematic measurements on different types of silicon waveguides which demonstrate the accuracy and precision of the technique. With a phase compensation approach on the basis of a common-path interferometer, the phase drift error is suppressed to ∼ 0.013°/s. In addition, characterizations of group index, group velocity dispersion, propagation loss, insertion loss, and return loss of component waveguides on a chip are all demonstrated. The measurement accuracy of the propagation loss of a ∼ 0.2 cm long nano-waveguide reaches ±1 dB/cm. Our convenient and versatile near-field characterization technique paves the way for in-detail study of complex photonic circuits on a chip.

Super-resolved three-dimensional near-field mapping by defocused imaging and tracking of fluorescent emitters

Abstract Near-field optics is essential in many nanotechnology applications, such as implementing sensitive biosensing and imaging systems with extreme precision. Understanding optical near-fields at the nanoscale has so attracted the considerable research interest, which use a variety of analytical approaches, most notably near-field scanning microscopy. Here, we show defocused point localization mapped accumulation (DePLOMA), which can overcome many weaknesses of conventional analytical methods. DePLOMA is based on imaging fluorescence emitters at an out-of-focal plane. The acquisition, collection, and accumulation of the position and fluorescence intensity of emitters moving above nanostructures can generate three-dimensional near-field maps of light distribution. The idea enables super-resolution liquid-phase measurements, as demonstrated by reconstruction of near-field created by nanoslits with a resolution determined by emitter size. We employed fluorescent emitters with a radius of 50 and 100 nm for confirmation. The axial resolution was found to be enhanced by more than 6 times above that of diffraction-limited confocal laser scanning microscopy when DePLOMA was used.

Infrared imaging of surface confluent polydopamine (PDA) films at the nanoscale

Polydopamine (PDA) is one of the most commonly used materials for the preparation of protective adhesive layers for biomedical and tribological applications. Despite its widespread use, the way in which the polymer binds to the substrate is yet to be fully understood. At the nanometre level, the spatial arrangement of individual molecules and the initial growth of PDA layers are expected to be influenced by the utilized substrate material and PDA deposition time. To investigate these hypotheses, we have prepared PDA layers with various times of deposition on surfaces of gold and oxygen-terminated materials (silicon/silicon dioxide and nitrogen-doped titanium oxide). The prepared samples were subsequently analysed using a scattering-type scanning near-field optical microscope utilizing four irradiation energies in the mid-infrared region to detect the chemical contrast originating from vibrational modes of selected chemical moieties. It was found that the polymerization process leads to a formation of a surface confluent PDA layer and deposition of PDA nanoaggregates. The differences in the optical contrast observed at irradiation energies corresponding to the vibrations of indole units of PDA and quinoid groups of polymerization intermediates indicated a slightly different composition of the nanoaggregates and the surrounding confluent layer.

Topological phononics arising from fluid-solid interactions

Nontrivial band topologies have been discovered in classical systems and hold great potential for device applications. Unlike photons, sound has fundamentally different dynamics and symmetries in fluids and solids, represented as scalar and vector fields, respectively. So far, searches for topological phononic materials have only concerned sound in either fluids or solids alone, overlooking their intricate interactions in “mixtures”. Here, we report an approach for topological phononics employing such unique interplay, and demonstrate the realization of type-II nodal rings, elusive in phononics, in a simple three-dimensional phononic crystal. Type-II nodal rings, as line degeneracies in momentum space with exotic properties from strong tilting, are directly observed through ultrasonic near-field scanning. Strongly tilted drumhead surface states, the hallmark phenomena, are also experimentally demonstrated. This phononic approach opens a door to explore topological physics in classical systems, which is easy to implement that can be used for designing high-performance acoustic devices.

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.