Near-field Scanning Research Articles

Scanning Near-Field Optical Microscopy of Ultrathin Gold Films.

Ultrathin metal films are an essential platform for two-dimensional (2D) material compatible and flexible optoelectronics. Characterization of thin and ultrathin film-based devices requires a thorough consideration of the crystalline structure and local optical and electrical properties of the metal-2D material interface since they could be dramatically different from the bulk material. Recently, it was demonstrated that the growth of gold on the chemical vapor deposited monolayer MoS2 leads to a continuous metal film that preserves plasmonic optical response and conductivity even at thicknesses below 10 nm. Here, we examined the optical response and morphology of ultrathin gold films deposited on exfoliated MoS2 crystal flakes on the SiO2/Si substrate via scattering-type scanning near-field optical microscopy (s-SNOM). We demonstrate a direct relationship between the ability of thin film to support guided surface plasmon polaritons (SPP) and the s-SNOM signal intensity with a very high spatial resolution. Using this relationship, we observed the evolution of the structure of gold films grown on SiO2 and MoS2 with an increase in thickness. The continuous morphology and superior ability with respect to supporting SPPs of the ultrathin (≤10 nm) gold on MoS2 is further confirmed with scanning electron microscopy and direct observation of SPP fringes via s-SNOM. Our results establish s-SNOM as a tool for testing plasmonic films and motivate further theoretical research on the impact of the interplay between the guided modes and the local optical properties on the s-SNOM signal.

Near-field measurements for wood anisotropy using cavity-backed slot antennas

Wood is becoming a very important structural material for buildings because it is a renewable resource and has a very small carbon footprint. This paper reports an experimental study on the effect of annual rings on wood anisotropy measurements using near-field microwave scanning for Pine, Hemlock, Red Cedar, and Hardwood laminated wood beams. Transmission measurements show significant anisotropy (>10 dB) for Pine wood compared to Hemlock wood and Red Cedar wood for linear polarization. The effect of annual rings in guiding the transmitted signal through wood fibers is observed through transmission measurements. Microwave guidance in transmission disappears with increasing the moisture content (17%–138%) of the Pine wood sample. The study provides a novel, accurate, fast, low-cost structural-wood testing method based on cavity-backed slot antennas for transmission and reflection measurements.

Deep-learning-aided extraction of optical constants in scanning near-field optical microscopy

Scanning near-field optical microscopy is one of the most effective techniques for spectroscopy of nanoscale systems. However, inferring optical constants from the measured near-field signal can be challenging because of a complicated and highly nonlinear interaction between the scanned probe and the sample. Conventional fitting methods applied to this problem often suffer from the lack of convergence or require human intervention. Here, we develop an alternative approach where the optical parameter extraction is automated by a deep learning network. The network provides an initial estimate that is subsequently refined by a traditional fitting algorithm. We show that this method demonstrates superior accuracy, stability against noise, and computational speed when applied to simulated near-field spectra.

Terahertz nanospectroscopy of plasmon polaritons for the evaluation of doping in quantum devices.

Terahertz (THz) waves are a highly sensitive probe of free carrier concentrations in semiconducting materials. However, most experiments operate in the far-field, which precludes the observation of nanoscale features that affect the material response. Here, we demonstrate the use of nanoscale THz plasmon polaritons as an indicator of surface quality in prototypical quantum devices properties. Using THz near-field hyperspectral measurements, we observe polaritonic features in doped silicon near a metal-semiconductor interface. The presence of the THz surface plasmon polariton indicates the existence of a thin film doped layer on the device. Using a multilayer extraction procedure utilising vector calibration, we quantitatively probe the doped surface layer and determine its thickness and complex permittivity. The recovered multilayer characteristics match the dielectric conditions necessary to support the THz surface plasmon polariton. Applying these findings to superconducting resonators, we show that etching of this doped layer leads to an increase of the quality factor as determined by cryogenic measurements. This study demonstrates that THz scattering-type scanning near-field optical microscopy (s-SNOM) is a promising diagnostic tool for characterization of surface dielectric properties of quantum devices.

A sub-2 Kelvin cryogenic magneto-terahertz scattering-type scanning near-field optical microscope (cm-THz-sSNOM)

We have developed a versatile near-field microscopy platform that can operate at high magnetic fields and below liquid-helium temperatures. We use this platform to demonstrate an extreme terahertz (THz) nanoscope operation and to obtain the first cryogenic magneto-THz time-domain nano-spectroscopy/imaging at temperatures as low as 1.8 K, magnetic fields of up to 5 T, and with operation of 0–2 THz. Our Cryogenic Magneto-Terahertz Scattering-type Scanning Near-field Optical Microscope (or cm-THz-sSNOM) instrument is comprised of three main equipment: (i) a 5 T split pair magnetic cryostat with a custom made insert, (ii) a custom sSNOM instrument capable of accepting ultrafast THz excitation, and (iii) a MHz repetition rate, femtosecond laser amplifier for broadband THz pulse generation and sensitive detection. We apply the cm-THz-sSNOM to obtain proof of principle measurements of superconductors and topological semimetals. The new capabilities demonstrated break grounds for studying quantum materials that require an extreme environment of cryogenic operation and/or applied magnetic fields in nanometer space, femtosecond time, and THz energy scales.

Review on infrared nanospectroscopy of natural 2D phyllosilicates.

Phyllosilicates have emerged as a promising class of large bandgap lamellar insulators. Their applications have been explored from the fabrication of graphene-based devices to 2D heterostructures based on transition metal dichalcogenides with enhanced optical and polaritonics properties. In this review, we provide an overview of the use of infrared (IR) scattering-type scanning near-field optical microscopy (s-SNOM) for studying nano-optics and local chemistry of a variety of 2D natural phyllosilicates. Finally, we bring a brief update on applications that combine natural lamellar minerals into multifunctional nanophotonic devices driven by electrical control.

Strongly coupled Raman scattering enhancement revealed by scattering-type scanning near-field optical microscopy.

Recent advances in near-field technology with an ultrahigh spatial resolution breaking optical diffraction limit, make it possible to further identify surface-enhanced Raman scattering (SERS) enhancement theories, and to monitor the SERS substrates. Here we verify the electromagnetic enhancement mechanism for SERS with a close-up view, using scattering-type scanning near-field optical microscopy. The array of metal-insulator-metal (MIM) subwavelength structures is studied, in which the field enhancement comes from the strong coupling between gap plasmon polariton and surface plasmon polariton modes. The near-field optical measurements reveal that SERS enhancement factor (EF) varies from one MIM subwavelength unit to another in a finite array. Besides the enhancement of isolated unit, the loss exchange phenomenon in strong coupling with a large Rabi splitting can give rise to an additional enhancement of more than 2 orders of magnitude in periodic arrays and close to 3 orders of magnitude in finite arrays. The SERS EF of the array composed of only 5 units is demonstrated to yield the best SERS performance. Our near-field optical measurements show evidence that finite-size structures embodied with strong coupling effect are a key way to develop practical high-performance SERS substrates.

Nano-infrared imaging of metal insulator transition in few-layer 1T-TaS2.

Among the family of transition metal dichalcogenides, 1T-TaS2 stands out for several peculiar physical properties including a rich charge density wave phase diagram, quantum spin liquid candidacy and low temperature Mott insulator phase. As 1T-TaS2 is thinned down to the few-layer limit, interesting physics emerges in this quasi 2D material. Here, using scanning near-field optical microscopy, we perform a spatial- and temperature-dependent study on the phase transitions of a few-layer thick microcrystal of 1T-TaS2. We investigate encapsulated air-sensitive 1T-TaS2 prepared under inert conditions down to cryogenic temperatures. We find an abrupt metal-to-insulator transition in this few-layer limit. Our results provide new insight in contrast to previous transport studies on thin 1T-TaS2 where the resistivity jump became undetectable, and to spatially resolved studies on non-encapsulated samples which found a gradual, spatially inhomogeneous transition. A statistical analysis suggests bimodal high and low temperature phases, and that the characteristic phase transition hysteresis is preserved down to a few-layer limit.

Nonlocal Noise Removal Technique to Improve the Electromagnetic Field Pattern in Near-Field Scanning of a Device

A nonlocal noise removal technique, known as block-matching 3-D (BM3D), is introduced to improve the quality of a blurry electromagnetic pattern generated from near field of a device. Noise distribution in electromagnetic pattern is analyzed. Taking the nonlocal self-similarity into account, the BM3D technique removes the noise of the pattern in a 3-D transformation domain based on sparse representation. First, the noise removal technique was validated well on the profile recovery of weak emission of a microstrip line driven by a square wave. Then, the BM3D technique is applied to improve source localization in a field programmable gate array (FPGA) and also to improve pattern clustering in a double data rate (DDR) synchronous dynamic random access memory (SDRAM) integrated circuit (IC). Both the applications demonstrate the effectiveness of the BM3D noise removal technique in the near-field scanning patterns of a device.

Imaging the Terahertz Nanoscale Conductivity of Polycrystalline CsPbBr3 Perovskite Thin Films.

Terahertz (THz) radiation is a valuable tool to investigate the electronic properties of lead halide perovskites (LHPs). However, attaining high-resolution information remains elusive, as the diffraction-limited spatial resolution (∼300 μm) of conventional THz methods prevents a direct analysis of microscopic effects. Here, we employ THz scattering scanning near-field optical microscopy (THz-sSNOM) for nanoscale imaging of cesium lead bromide (CsPbBr3) thin films down to the single grain level at 600 GHz. Adopting a scattering model, we are able to derive the local THz nanoscale conductivity in a contact-free fashion. Increased THz near-field signals at CsPbBr3 grain boundaries complemented by correlative transmission electron microscopy-energy-dispersive X-ray spectroscopy elemental analysis point to the formation of halide vacancies (VBr) and Pb-Pb bonds, which induce charge carrier trapping and can lead to nonradiative recombination. Our study establishes THz-sSNOM as a powerful THz nanoscale analysis platform for thin-film semiconductors such as LHPs.

Advanced characterization techniques for nanostructured materials in biomedical applications

Recent advancements in nanostructured materials have found widespread application across many domains, particularly in the biomedical field. Before using nanostructured materials in clinical applications, many important challenges, especially those related to their uses in biomedicine, must be resolved. Biological activity, compatibility, toxicity, and nano-bio interfacial characteristics are some of the major problems in biomedicine. We may therefore investigate the nanostructured materials for biomedical applications with the aid of modern characterization techniques. This overview article illustrates the present state of nanostructured materials in the biomedical field with uses and the importance of characterization methods through the use of cutting-edge characterization techniques. In this article, the techniques for analysing the topology of nanostructures, including Field Emission Scanning Electron Microscopy (FESEM), Dynamic Light Scattering (DLS), Scanning Probe Microscopy (SPM), Near-field Scanning Optical Microscopy (NSOM), and Confocal microscopy, are described. In addition, the internal structural investigation techniques X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), and Magnetic Resonance Force Microscopy (MRFM) are discussed. In addition, composition analysis techniques such as X-ray Photoelectron Spectroscopy (XPS), Energy Dispersive X-ray spectroscopy (EDS), Auger Electron Spectroscopy (AES), and Secondary Ion Mass Spectroscopy (SIMS) have been discussed. The essence of the nanomaterials as they relate to physics, chemistry, and biology is thoroughly explained in this overview along with characterization techniques through case studies. Additionally, the constraints and difficulties with specimen and analysis that are related to comprehending nanostructured materials have been identified and addressed in this study.

Nanoscopic Imaging of Photonic Modes Excited in Square-Shaped Perylene Microcrystals

Organic microcrystals have attracted much attention because of their light confinement and transport capability on the micrometer scale. The capability is closely related to the photonic modes resonantly excited in the organic microcrystal, and therefore, visualization of photonic modes is important not only for a deeper understanding of organic microcrystals but also for their practical application. Here, we directly visualized the photonic modes excited in square-shaped perylene microcrystals by using a scanning near-field optical microscope. From the near-field optical images and the electromagnetic simulations, we demonstrate that the Fabry–Pérot modes are predominantly excited compared with the whispering gallery modes in two-dimensional organic microcrystals. The findings provide a deeper understanding of the photonic modes and should be beneficial for the design of microscale waveguides and photonic integrated circuits.

Sharp, high numerical aperture (NA), nanoimprinted bare pyramid probe for optical mapping.

The ability to correlate optical hyperspectral mapping and high resolution topographic imaging is critically important to gain deep insight into the structure-function relationship of nanomaterial systems. Scanning near-field optical microscopy can achieve this goal, but at the cost of significant effort in probe fabrication and experimental expertise. To overcome these two limitations, we have developed a low-cost and high-throughput nanoimprinting technique to integrate a sharp pyramid structure on the end facet of a single-mode fiber that can be scanned with a simple tuning-fork technique. The nanoimprinted pyramid has two main features: (1) a large taper angle (∼70°), which determines the far-field confinement at the tip, resulting in a spatial resolution of 275nm, an effective numerical aperture of 1.06, and (2) a sharp apex with a radius of curvature of ∼20nm, which enables high resolution topographic imaging. Optical performance is demonstrated through evanescent field distribution mapping of a plasmonic nanogroove sample, followed by hyperspectral photoluminescence mapping of nanocrystals using a fiber-in-fiber-out light coupling mode. Through comparative photoluminescence mapping on 2D monolayers, we also show a threefold improvement in spatial resolution over chemically etched fibers. These results show that the bare nanoimprinted near-field probes provide simple access to spectromicroscopy correlated with high resolution topographic mapping and have the potential to advance reproducible fiber-tip-based scanning near-field microscopy.

Faculty interviews—traps and tips

Faculty interviews—traps and tips

Damage analysis and mechanism study of sol–gel coating over KDP crystal under multi-pulse of laser irradiation at low flux

The purpose of this study is to analyze the damage of antireflective (AR) coating over potassium dihydrogen phosphate (KDP) crystal subjected to multi-pulse laser irradiation at low flux under vacuum. Fresh silica AR was characterized as a reference; Atomic Force Microscope (AFM), Scanning Electron Microscopy (SEM), profilometer, and Scanning Near-Field Optical Microscope Photo-induced Force Microscope (SNOM-PiFM) were employed to analyze the characteristics of coatings. The experimental results indicated that the damage of AR coating over the KDP crystal was mainly caused by partial exfoliation, which exposed silica particles beneath the surface. It was found that the accumulated tensile stress led to coating damage with the increase of laser pulse. The initial coating damage was observed to extend and interconnect to form large-area exfoliation. Splitting mechanism of SiO–Si TO3 was observed at vibration mode peaks of 1064 cm−1 and 1096 cm−1showing progressing irradiation damage. Based on this study, it would be helpful to suppress the damage probability of AR coating over KDP crystal applied in high-power laser systems. Moreover, the applicability of SNOM-PiFM method to study the Infrared Radiation (IR) spectra of ultra-thin coatings with transparent substrates was proposed.

Quick identification of ABC trilayer graphene at nanoscale resolution via a near-field optical route

ABC-stacked trilayer graphene has exhibited a variety of correlated phenomena owing to its relatively flat bands and gate-tunable bandgap. However, convenient methods are still lacking for identifying ABC graphene with nanometer-scale resolution. Here we demonstrate that the scanning near-field optical microscope working in ambient conditions can provide quick recognition of ABC trilayer graphene with no ambiguity and excellent resolution (∼20 nm). The recognition is based on the difference in their near-field infrared (IR) responses between the ABA and ABC trilayers. We show that in most frequencies, the response of the ABC trilayer is weaker than the ABA trilayer. However, near the graphene phonon frequency (∼1585 cm−1), ABC’s response increases dramatically when gated and exhibits a narrow and sharp Fano-shape resonant line, whereas the ABA trilayer is largely featherless. Consequently, the IR contrast between ABC and ABA becomes reversed and can even be striking (ABC/ABA ∼ 3) near the graphene phonon frequency. The observed near-field IR features can serve as a golden rule to quickly distinguish ABA and ABC trilayers with no ambiguity, which could largely advance the exploration of correlation physics in ABC-stacked trilayer graphene.

A Dual-Component Electric Probe Embedded With a 0°/180° Hybrid Coupler for Near-Field Scanning

In this article, a Ka -band dual-component electric probe is proposed in low temperature co-fired ceramic for near-field scanning, which consists of a sensing part of two symmetric L-shaped strips and a 0°/180° hybrid coupler. The responses to the tangential and normal electric components are outputted to the difference and sum ports without post-processing, to avoid the errors due to phase measurements at high frequencies. A grounded coplanar waveguide (GCPW) is used as a standard kit. Equations are derived carefully to take the imperfect terminations into account. To further improve the sensitivity and measurement efficiency, a low noise amplifier (LNA) and an absorptive single pole double throw (SPDT) switch are integrated with the probe. Simulated and measured results indicate that the probe can characterize the near electric field distributions within the band of 27.2–30 GHz, and the sensitivity of the active probe prototype is improved by 17–18 dB at 28 GHz when compared with the passive one. The passive probe prototype is utilized to scan the near electric field of an integrated transmitting front-end module at 28.5 GHz.

Nanoscale infrared imaging and spectroscopy of hot-electron plasmons in graphene

We report a nanoscale infrared (IR) imaging and spectroscopy study of hot-electron plasmons in graphene, which are excited by the sharp metallic probe of the scattering-type scanning near-field optical microscope (s-SNOM) illuminated with a mid-IR femtosecond pulsed laser. We found the average electron temperature $({T}_{e})$ can reach as high as $\ensuremath{\sim}1400\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ within the pulse duration, which can be controlled by tuning the laser power. With s-SNOM, we monitored both the plasmon interference fringes and the hybrid plasmon-phonon resonances of graphene. When graphene is heavily doped, a higher ${T}_{e}$ leads to a smaller plasmon wavelength and a weaker plasmon-phonon resonance intensity. At the charge neutrality point, on the other hand, the plasmon-phonon resonance intensity is enhanced when ${T}_{e}$ increases. With quantitative modeling and theoretical analysis, we concluded that the observed plasmonic responses of hot electrons are governed by the temperature dependencies of chemical potential, electron scattering, and thermal carrier generation. The competition of these factors leads to distinct ${T}_{e}$ dependence of graphene plasmons at various doping levels.

Cubic-wavelength mode volume photonic crystal nanobeam cavities in a monolithic CMOS platform.

We report on the design, fabrication, and experimental characterization of photonic crystal (PhC) nanobeam cavities with the smallest footprint, largest intrinsic quality factor, and smallest mode volume to be demonstrated to date in a monolithic CMOS platform. Two types of cavities were designed, with opposite spatial mode symmetries. The opposite mode symmetry, combined with evanescent coupling, allows the nanobeam cavities to be used in reflectionless topologies, desirable in complex photonic integrated circuits (PICs). The devices were implemented and fabricated in a 45 nm monolithic electronics-photonics CMOS platform optimized for silicon photonics (GlobalFoundries 45CLO) and do not require any post-processing. Quality factors exceeding 100 000 were measured for both devices, the highest, to the best of our knowledge, among fully cladded PhC nanobeam cavities in any silicon-on-insulator (SOI) platform. Additionally, the ability of the cavities to confine light into small mode volumes, of the order of (λ/n)3, was confirmed experimentally using near-field scanning optical microscopy (NSOM). These types of cavities are an important step toward realizing ultra-low energy active devices required for the next generation of integrated optical links beyond the current microring resonator-based links and other CMOS PICs.

Identification of weak molecular absorption in single-wavelength s-SNOM images.

Scattering-type scanning near-field optical microscopy (s-SNOM) allows for nanoscale optical mapping of manifold material properties. It is based on interferometric recording of the light scattered at a scanning probe tip. For dielectric samples such as biological materials or polymers, the near-field amplitude and phase signals of the scattered field reveal the local reflectivity and absorption, respectively. Importantly, absorption in s-SNOM imaging corresponds to a positive phase contrast relative to a non-absorbing reference sample. Here, we describe that in certain conditions (weakly or non- absorbing material placed on a highly reflective substrate), a slight negative phase contrast may be observed, which can hinder the recognition of materials exhibiting a weak infrared absorption. We first document this effect and explore its origin using representative test samples. We then demonstrate straightforward simple correction methods that remove the negative phase contrast and that allow for the identification of weak absorption contrasts.