Near-field Microscopy Research Articles

Chiral assemblies of pinwheel superlattices on substrates.

The unique topology and physics of chiral superlattices make their self-assembly from nanoparticles highly sought after yet challenging in regard to (meta)materials1-3. Here we show that tetrahedral gold nanoparticles can transform from a perovskite-like, low-density phase with corner-to-corner connections into pinwheel assemblies with corner-to-edge connections and denser packing. Whereas corner-sharing assemblies are achiral, pinwheel superlattices become strongly mirror asymmetric on solid substrates as demonstrated by chirality measures. Liquid-phase transmission electron microscopy and computational models show that van der Waals and electrostatic interactions between nanoparticles control thermodynamic equilibrium. Variable corner-to-edge connections among tetrahedra enable fine-tuning of chirality. The domains of the bilayer superlattices show strong chiroptical activity as identified by photon-induced near-field electron microscopy and finite-difference time-domain simulations. The simplicity and versatility of substrate-supported chiral superlattices facilitate the manufacture of metastructured coatings with unusual optical, mechanical and electronic characteristics.

Skate, overtravel, and contact force of tilted triangular cantilevers for microcantilever-based MEMS probe technologies

Microfabricated chip-edge microcantilevers are commonly used as surface probes, e.g. in near-field microscopy. Such probes normally function in the low-deflection regime, where their behaviour is very well understood and documented. In contrast, when microcantilevers are used for applications such as electrical testing probes, their deflection can be somewhat higher, taking them into the less well understood high-deflection regime of microelectromechanical systems (MEMS). Here, a scalable model for the relationship between the skate, overtravel, and resulting tip contact force in tilted triangular cantilevers—which are bending with high deflection and in contact with a flat surface—is presented. The model is tested experimentally using macroscopic triangular cantilevers—the experimental results agree well with the proposed model. The findings enable a practical solution for zero-skate in tapered MEMS probes to be suggested. It is hoped that the findings may be of use for probe engineers involved with on-wafer testing and designers of emerging MEMS micro cantilever-based probes.

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.

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.

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.

Incorporation of nano-features into surface photoactive arrays for broadband absorption of the solar radiation

Surface arrays of photoactive materials with subwavelength dimensions are important for energy harvesting applications due to their capacity for broadband and omnidirectional absorption of the solar radiation. It was recently proposed and demonstrated that the absorption performance of such arrays can be further amplified with the introduction of additional nano-features. In the current work we consider the incorporation of nano-features into nanopillar (NP) arrays. The considered nano-features are: quasi-nanolenses, sidewall nano-decorations, and decreasing the NP bottom diameter to transform it into a light funnel (LF). The underlying various light trapping mechanisms are examined using near-field microscopy. Near-field microscopy is used to obtained both high-resolution far-field imaging, as well as mapping of the near-field photon distributions of the arrays. Firstly, it is shown that the main contribution to far-field reflected photons is from the ambient surrounding the silicon structures. Secondly, the incorporation of the various nano-features induces a decrease in the reflected far-field photons which originate from this ambient region. Also, it is shown how the incorporation of quasi-nanolenses concludes a decrease in far-field photons reflected in the oblique directions with high polar angles, whereas the incorporation of sidewall nano-decorations leads to a decrease in far-field photons reflected in directions of small polar angles. Also, the incorporation of both qNL and sidewall nano-decorations is examined, and light trapping with the LF geometry is discussed and demonstrated. Finally, understanding the underlying light trapping mechanisms will support the deterministic design of subwavelength arrays incorporated with nano-features for efficient harvesting of the solar radiation.

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.

Two-dimensional quantitative near-field phase imaging using square and hexagonal interference devices

Abstract We demonstrate the formation of the near field with non-trivial phase distribution using surface plasmon interference devices, and experimental quantitative imaging of that phase with near-field phase microscopy. The phase distribution formed with a single device can be controlled by the polarization of the external illumination and the area of the device assigned to the object wave. A comparison of the experimental data to a numerical electromagnetic model and an analytical model assigns the origin of the near-field phase to the out-of-plane electric component of surface plasmon polaritons, and also verifies the predictive power of the models. We demonstrate a formation of near-field plane waves with different propagation directions on a single device, or even simultaneously at distinct areas of a single device. Our findings open the way to the imaging and tomography of phase objects in the near field.

Photobiology of lipofuscin granules in the retinal pigment epithelium cells of the eye: norm, pathology, age.

Lipofuscin granules (LGs) are accumulated in the retinal pigment epithelium (RPE) cells. The progressive LG accumulation can somehow lead to pathology and accelerate the aging process. The review examines composition, spectral properties and photoactivity of LGs isolated from the human cadaver eyes. By use of atomic force microscopy and near-field microscopy, we have revealed the fluorescent heterogeneity of LGs. We have discovered the generation of reactive oxygen species by LGs, and found that LGs and melanolipofuscin granules are capable of photoinduced oxidation of lipids. It was shown that A2E, as the main fluorophore (bisretinoid) of LGs, is much less active as an oxidation photosensitizer than other fluorophores (bisretinoids) of LGs. Photooxidized products of bisretinoids pose a much greater danger to the cell than non-oxidized one. Our studies of the fluorescent properties of LGs and their fluorophores (bisretinoids) showed for the first time that their spectral characteristics change (shift to the short-wavelength region) in pathology and after exposure to ionizing radiation. By recording the fluorescence spectra and fluorescence decay kinetics of oxidized products of LG fluorophores, it is possible to improve the methods of early diagnosis of degenerative diseases. Lipofuscin ("aging pigment") is not an inert "slag". The photoactivity of LGs can pose a significant danger to the RPE cells. Fluorescence characteristics of LGs are a tool to detect early stages of degeneration in the retina and RPE.

Defocus Phase Contrast in Photon-Induced Near-field Electron Microscopy

Defocus Phase Contrast in Photon-Induced Near-field Electron Microscopy

WSe2 as Transparent Top Gate for Infrared Near-Field Microscopy.

Independent control of carrier density and out-of-plane displacement field is essential for accessing novel phenomena in two-dimensional (2D) material heterostructures. While this is achieved with independent top and bottom metallic gate electrodes in transport experiments, it remains a challenge for near-field optical studies as the top electrode interferes with the optical path. Here, we characterize the requirements for a material to be used as the top-gate electrode and demonstrate experimentally that few-layer WSe2 can be used as a transparent, ambipolar top-gate electrode in infrared near-field microscopy. We carry out nanoimaging of plasmons in a bilayer graphene heterostructure tuning the plasmon wavelength using a trilayer WSe2 gate, achieving a density modulation amplitude exceeding 2 × 1012 cm-2. The observed ambipolar gate-voltage response allows us to extract the energy gap of WSe2, yielding a value of 1.05 eV. Our results provide an additional tuning knob to cryogenic near-field experiments on emerging phenomena in 2D materials and moiré heterostructures.

Collection of propagating electromagnetic fields by uncoated probe

Understanding light-matter interaction at the nanoscale by observation of fine details of electromagnetic fields is achieved by bringing nanoscale probes into the nearfield of light sources, capturing information that is lost in the far field. Although metal coated probes are often used for nearfield microscopy, they strongly perturb the electromagnetic fields under study. Here, through experiment and simulation, we detail light collection by uncoated fiber probes, which minimize such perturbation. Second-harmonic light from intensely-irradiated sub-wavelength sub-surface features was imaged to avoid otherwise dominating fundamental light background, yielding clear nearfield details through a 50 nm aperture uncoated probe with ∼23 nm optical resolution. Simulations show how a metallic coating distorts optical nearfields and limits optical coupling into the probe in comparison to an uncoated probe.

Langevin dynamics/Monte Carlo simulations method for calculating nanoscale dielectric functions of materials

We have developed a simulational methodology for calculating the nanoscale frequency-dependent complex dielectric function of a wide range of materials using a combination of Langevin dynamics and Metropolis Monte Carlo methods. The premise of such a scheme is to use the atomistic structure of materials and designate appropriate interatomic interactions as well as internal field couplings to accommodate correlated materials. We validate our model by recreating the dielectric functions of well-studied representative materials including insulator ${\text{SiO}}_{2}$ thin film that has phonon resonances in the midinfrared, and semiconductor monolayer ${\text{MoS}}_{2}$ that exhibits strong excitonic resonances in the visible frequency range. To further showcase the capability of the model in calculating nanoscale dielectric modulation of complex materials, we simulate the dielectric response of ${\text{SmNiO}}_{3}$, a correlated perovskite oxide, with respect to differing levels of hydrogenation, oxygen vacancy formation, and external fields. This is accomplished by inserting and tracking the movement of dopants at the nanoscale using Metropolis Monte Carlo methods that explicitly include interactions with each other as well as external fields. Simulated nanoscale dielectric spectra agree very well with high-resolution near-field experimental measurements based on scattering type scanning near-field microscopy. We find that this modeling scheme carries a broad utility in describing and predicting the nanoscale dielectric behavior of a broad range of materials exposed to changing local environments.

Ultimate Limit for Optical Losses in Gold, Revealed by Quantitative Near-Field Microscopy.

We report thorough measurements of surface plasmon polaritons (SPPs) running along nearly perfect air-gold interfaces formed by atomically flat surfaces of chemically synthesized gold monocrystals. By means of amplitude- and phase-resolved near-field microscopy, we obtain their propagation length and effective mode index at visible wavelengths (532, 594, 632.8, 729, and 800 nm). The measured values are compared with the values obtained from the dielectric functions of gold that are reported in literature. Importantly, a reported dielectric function of monocrystalline gold implies ∼1.5 times shorter propagation lengths than those observed in our experiments, whereas a dielectric function reported for properly fabricated polycrystalline gold leads to SPP propagation lengths matching our results. We argue that the SPP propagation lengths measured in our experiments signify the ultimate limit of optical losses in gold, encouraging further comprehensive characterization of optical material properties of pure gold as well as other plasmonic materials.

Signatures of bright-to-dark exciton conversion in corrugated MoS2 monolayers

In this study, we investigated the effect of periodic uniaxial strains on electron and phonon transports of polycrystalline and single-crystal molybdenum disulphide (MoS2) monolayers on a periodically corrugated sapphire surface. Analysis of micro-Raman, polarized photoluminescence and second harmonic generation results shows the anisotropy of the corrugation-induced strain in both single- and polycrystalline MoS2 monolayers. AFM topography measurements show periodically-rippled surfaces of the MoS2 in the nanometre scale. Our results show that the application of the periodic strain produces two major effects on the band structure of MoS2 monolayers: modulations on the band gap anisotropy and reduction of out-of-plane spin-relaxation time due to substrate-induced bending of MoS2. Spin memory loss, in other words, shortening the spin relaxation time, enables an electron spin-flip scattering process that can convert a formerly bright exciton to a dark exciton. Such conversion is reflected in decreasing intensity of photoluminescence and in the light intensity collected by scanning near-field microscope. Our results demonstrate the ability to control both the bandgap and exciton character in monolayer MoS2 via periodic strains imposed by corrugated sapphire substrates. This approach offers an effective means in designing novel electronic devices for photovoltaic applications. The bright-to-dark excitons conversion in photovoltaic devices can boast longer lifetimes than their bright exciton counterparts so they can be more efficiently collected by external electrodes. The strain-induced conversion of the bright-to-dark excitons makes the hybrid MoS2/corrugated sapphire structure an interesting platform for future photovoltaic applications.

Super-resolution optical microscopy using cylindrical vector beams

AbstractSuper-resolution optical microscopy, which gives access to finer details of objects, is highly desired for fields of nanomaterial, nanobiology, nanophotonics, etc. Many efforts, including tip optimization and illumination optimization etc., have been made in both near-field and far-field super-resolution microscopy to achieve a spatial resolution beyond the diffraction limit. The development of vector light fields opens up a new avenue for super-resolution optical microscopy via special illumination modes. Cylindrical vector beam (CVB) has been verified to enable resolution improvement in tip-scanning imaging, nonlinear imaging, stimulated emission depletion (STED) microscopy, subtraction imaging, superoscillation imaging, etc. This paper reviews recent advances in CVB-based super-resolution imaging. We start with an introduction of the fundamentals and properties of CVB. Next, strategies for CVB based super-resolution imaging are discussed, which are mainly implemented by tight focusing, depletion effect, plasmonic nanofocusing, and polarization matching. Then, the roadmap of super-resolution imaging with CVB illumination in the past two decades is summarized. The typical CVB-based imaging techniques in fields of both near-field and far-field microscopy are introduced, including tip-scanning imaging, nonlinear imaging, STED, subtraction imaging, and superoscillation imaging. Finally, challenges and future directions of CVB-illuminated super-resolution imaging techniques are discussed.

A Systematic Material Characterization Method via Near-Field Scanning Microwave Microscopy

Accurate measurement of material dielectric property is essential for engineers to synthesize microwave devices. This article proposes a systematic material characterization method via the coaxial resonator (CR)-based near-field scanning microwave microscopy (NFSMM) with an arbitrary tip shape, from the modeling method to the inversion algorithm. The effective interaction region in the CR-based NFSMM is investigated and determined, which contributes most to the contrast capacitance caused by the tip-sample interaction. Then the Huygens’ principle is applied in the forward solver of NFSMM when the samples under test are homogeneous. The relative error of our model is less than 2.1% in numerical validations. Based on the proposed forward solver, a dielectric characterization method via the conjugate gradient (CG) algorithm is proposed and verified with experimental results. The retrieved permittivity of unknown samples matches well with that measured by the transmission line method, and the discrepancy between the two results is less than 3%. The proposed systematic method is promising to provide a new pathway to quantitatively determine the dielectric property of homogeneous samples.

Energy-filtered dark-field imaging of nanoparticles by PINEM in 4D electron microscopy

Time-resolved dark-field imaging of alpha Fe2O3 nanoparticle and Ag nanowires using scattered electrons by selected crystal planes are realized by photon-induced near-field electron microscopy (PINEM) selecting probing electrons which absorb energy from a transient laser field during their passage through the target particles in a four-dimensional transmission electron microscope (4D-TEM). The high laser fluence illuminated on the particles causes significant part of probing swift electrons exchange energy with the laser light, creating enough PINEM electrons required for the dark-field imaging of particles with high spatiotemporal resolution at nanometer and femtosecond scale. Different from the bright-field PINEM imaging of particles where the outerspace with a close distance to the particle are illuminated by transmitted PINEM electrons, illumination is confined on the particles by selected scattered PINEM electrons, leading to a much more defined and sharp imaging of particles compared with a bright-field PINEM image. In combination with PINEM temporal gating and dark-field selective imaging, the PINEM dark-field imaging technique in the 4D-TEM enables the studies of structural dynamics of selective crystal planes or elements with high spatiotemporal resolution.

Terahertz near-field microscopy based on an air-plasma dynamic aperture

Terahertz (THz) near-field microscopy retains the advantages of THz radiation and realizes sub-wavelength imaging, which enables applications in fundamental research and industrial fields. In most THz near-field microscopies, the sample surface must be approached by a THz detector or source, which restricts the sample choice. Here, a technique was developed based on an air-plasma dynamic aperture, where two mutually perpendicular air-plasmas overlapped to form a cross-filament above a sample surface that modulated an incident THz beam. THz imaging with quasi sub-wavelength resolution (approximately λ/2, where λ is the wavelength of the THz beam) was thus observed without approaching the sample with any devices. Damage to the sample by the air-plasmas was avoided. Near-field imaging of four different materials was achieved, including metallic, semiconductor, plastic, and greasy samples. The resolution characteristics of the near-field system were investigated with experiment and theory. The advantages of the technique are expected to accelerate the advancement of THz microscopy.

Visualization of Band Shifting and Interlayer Coupling in WxMo1–xS2 Alloys Using Near-Field Broadband Absorption Microscopy

Beyond-diffraction-limit optical absorption spectroscopy provides in-depth information on the graded band structures of composition-spread and stacked two-dimensional materials, in which direct/indirect bandgap, interlayer coupling, and defects significantly modify their optoelectronic functionalities such as photoluminescence efficiency. We here visualize the spatially varying band structure of monolayer and bilayer transition metal dichalcogenide alloys by using near-field broadband absorption microscopy. The near-field spectral and spatial information manifests the excitonic band shift that results from the interplay of composition spreading and interlayer coupling. These results enable us to identify, notably, the top layer of the bilayer alloy as pure WS2. We also use the aberration-free near-field transmission images to demarcate the exact boundaries of alloyed and pure transition metal dichalcogenides. This technology can offer valuable insights on various layered structures in the era of "stacking science" in the quest of quantum optoelectronic devices.