Near-field Optical Microscopy Research Articles

Polarization-resolved near-field characterization of coupling between a bus waveguide and a ring resonator

Light propagation in Photonic integrated circuits (PICs), which can nowadays involve complex systems of light-guiding structures, is measured with different approaches(Nuzhdin et al., 2020, Lončar et al., 2002, Hopman et al., 2007, Morichetti et al., 2014, Sapienza et al., 2012, Vesseur et al., 2007). An interferometric polarization-sensitive measurement of the evanescent fields of these structures provides insight in the performance of the circuit detects possible malfunction with sub-wavelength precision(Engelen et al., 2007, Gersen et al., 2005, Barwick et al., 2009). We demonstrate a Near-field scanning optical microscopy (NSOM) measurement on coupled ring resonators that shows how the guided modes evolve as they propagate across the optical chip. Analysis of the measurements provides information about intensity distribution of the two polarization components (TE and TM) and their spatial confinement. The data validates our methodology and opens new possibilities for analysis of signal propagation in prototyping and optimization of integrated optical systems. Direct observation of polarization state of the guided modes allows for clear understanding of mode conversion in coupling systems.

Detection of surface enhanced Raman scattering active hotspot using near field scanning optical microscopy

Hotspots are high-intensity electromagnetic zones that form, for example, at the interstitials of plasmonic nanoaggregates, resulting in a considerable rise in the enhancement factor. However, it is inevitable to achieve specific nanometric geometry as well as a suitable technique to capture the details of hotspots. We report near-field surface-enhanced Raman scattering (SERS) spectroscopy of a well-defined gold nanoaggregate of a few nanoparticles adsorbed with a small number of target analytes. A spectrally and spatially resolved SERS measurement setup using an aperture near-field scanning optical microscope (a-NSOM) facilitated the direct observation of localized electromagnetic (EM) fields at the interstitials through SERS. Correlated optical image and corresponding nanometric geometry were captured through the home-built a-NSOM setup. Near-field SERS spectra were recorded at different sites of interest. It was evident that the interstitial positioned at the center of the tetramer provided the most intense Raman scattering, implying the possibility of a SERS-active hotspot therein. SERS bands of the spectrum of the Raman-active dye Rhodamine 6G recorded at the same hotspot coincided well with those reported so far. It was noteworthy that most of the SERS bands in such scenery got enhanced. Such direct observation with high spatial resolution is indispensable to understanding the origin of localized EM fields at “hotspots” and the EM enhancement factor in the SERS process. A finite-difference time-domain (FDTD) analysis was carried out to validate the results.

Propagation constant-based diameter measurement technique for a submicrometer-scale optical fiber.

Diameter is a critical parameter for determining the physical properties of a submicrometer optical fiber and requires an accurate measurement. In this study, we proposed, to our knowledge, a novel diameter measurement technique derived from the waveguide theory, utilizing the pitch of a standing-wave near-field light generated by two counter-propagating lights within the submicrometer optical fiber. In a submicrometer optical fiber, the propagating light extends into the surrounding air as near-field light, existing within a range approximately equivalent to one wavelength from the surface of the fiber. By generating the standing-wave near-field light with the incident lights from both ends of the fiber, the pitch of the standing-wave near-field light can be measured by scanning along the fiber's central axis with a scanning near-field optical microscopy probe. The fiber diameter is subsequently acquired by solving the optical fiber eigenvalue equation. Based on the feasibility verification experiment, a high-precision measurement of approximately 0.50 µm was realized for the diameter of the optical fiber.

Probing Inherent Optical Anisotropy in Substrates via Direct Nanoimaging of Mie Scattering.

In this study, we investigated the optical properties of a transition metal dichalcogenide (TMD) substrate via Mie-scattering-induced surface analysis (MISA). Employing near-field optical microscopy and finite-difference time-domain (FDTD) simulations, we systemically prove and directly visualize the Mie scattering of superspherical gold nanoparticles (s-AuNPs) at the nanoscale. Molybdenum disulfide substrates exhibited optical isotropy, while rhenium disulfide (ReS2) substrates showed anisotropic behavior attributed to the interaction with incident light's electric field. Our study revealed substantial anisotropic trends in Mie scattering, particularly in the near-infrared energy range, with ReS2 exhibiting more pronounced spectral and angular responses in satellite peaks. Our results emphasize the application of Mie scattering, exploring the optical properties of substrates and contributing to a deeper understanding of nanoscale light-matter interactions.

Direct programming of confined surface phonon polariton resonators with the plasmonic phase-change material In3SbTe2

Tailoring light-matter interaction is essential to realize nanophotonic components. It can be achieved with surface phonon polaritons (SPhPs), an excitation of photons coupled with phonons of polar crystals, which also occur in 2d materials such as hexagonal boron nitride or anisotropic crystals. Ultra-confined resonances are observed by restricting the SPhPs to cavities. Phase-change materials (PCMs) enable non-volatile programming of these cavities based on a change in the refractive index. Recently, the plasmonic PCM In3SbTe2 (IST) was introduced which can be reversibly switched from an amorphous dielectric state to a crystalline metallic one in the entire infrared to realize numerous nanoantenna geometries. However, reconfiguring SPhP resonators to modify the confined polaritons modes remains elusive. Here, we demonstrate direct programming of confined SPhP resonators by phase-switching IST on top of a polar silicon carbide crystal and investigate the strongly confined resonance modes with scanning near-field optical microscopy. Reconfiguring the size of the resonators themselves result in enhanced mode confinements up to a value of λ/35.\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{{{{\\boldsymbol{\\lambda }}}}}}{{{{{\\boldsymbol{/}}}}}}{{{{{\\boldsymbol{35}}}}}}{{{{{\\boldsymbol{.}}}}}}$$\\end{document} Finally, unconventional cavity shapes with complex field patterns are explored as well. This study is a first step towards rapid prototyping of reconfigurable SPhP resonators that can be easily transferred to hyperbolic and anisotropic 2d materials.

Infrared spectroscopy across scales in length and time at BESSY II.

The infrared beamline at BESSY II storage ring was upgraded recently to extend the capabilities of infrared microscopy. The endstations available at the beamline are now facilitating improved characterization of molecules and materials at different length scales and time resolutions. Here, the current outline of the beamline is reported and an overview of the endstations available is given. In particular, the first results obtained by using a new microscope for nano-spectroscopy that was implemented are presented. The capabilities of the scattering-type near-field optical microscope (s-SNOM) are demonstrated by investigating cellulose microfibrils, representing nanoscopic objects of a hierarchical structure. It is shown that the s-SNOM coupled to the beamline allows imaging to be performed with a spatial resolution of less than 30 nm and infrared spectra to be collected from an effective volume of less than 30 nm × 30 nm × 12 nm. Potential steps for further optimization of the beamline performance are discussed.

Optically driven plasmons in graphene/hBN van der Waals heterostructures: simulating s-SNOM measurements

Abstract Converting transverse photons into longitudinal two-dimensional plasmon-–polaritons (2D-PP) and vice versa presents a significant challenge within the fields of photonics and plasmonics. Therefore, understanding the mechanism which increases the photon – 2D-PP conversion efficiency could significantly contribute to those efforts. In this study, we theoretically examine how efficiently incident radiation, when scattered by a silver spherical nanoparticle (Ag-NP), can be transformed into 2D-PP within van der Waals (vdW) heterostructures composed of hexagonal boron nitride and graphene (hBN/Gr composites). We show that the Dirac plasmon (DP) excitation efficiency depends on the Ag-NP radius as R 3, and decreases exponentially with Ag-NP height h, so that for a certain Ag-NP geometry up to 25 % of the incident electrical field is channeled into the DP. We demonstrate that the linear plasmons (LPs) excitation efficiency can be manipulated by changing the graphene–graphene distance Δ (or hBN thickness) or by changing the number of graphene layers N. By increasing Δ and/or N the LPs move towards smaller wave vectors Q and become accessible by the Ag-NP dipole field, so that for N ≥ 5 the excitation of more than one LP is possible. These results are supported by recent scattering-type scanning near-field optical microscopy (s-SNOM) measurements. Furthermore, we show that Ag-NPs with specific parameters preferentially hybridizes with DPs of a particular wavelength λ D , facilitating selective excitation of DPs. The obtained tuning possibilities could have a significant impact on applied plasmonics, photonics or optoelectronics.

Terahertz nanoscopy: Advances, challenges, and the road ahead

Exploring nanoscale material properties through light-matter interactions is essential to unveil new phenomena and manipulate materials at the atomic level, paving the way for ground-breaking advancements in nanotechnology and materials science. Various elementary excitations and low-energy modes of materials reside in the terahertz (THz) range of the electromagnetic spectrum (0.1–10 THz) and occur over various spatial and temporal scales. However, due to the diffraction limit, a slew of THz studies are restricted to drawing conclusions from the spatially varying THz responses around half of the probing wavelengths, i.e., from tens to a couple of hundred micrometers. To address this fundamental challenge, scanning near-field optical microscopy (SNOM), notably scattering-type SNOM (s-SNOM), combined with THz sources has been employed and is fueling growing interest in this technique across multiple disciplines. This review (1) provides an overview of the system developments of SNOM, (2) evaluates current approaches to understand and quantify light-matter interactions, (3) explores advances in THz SNOM applications, especially studies with THz nano-scale spatial responses employing an s-SNOM, and (4) envisions future challenges and potential development avenues for the practical use of THz s-SNOM.

Origins and consequences of asymmetric nano-FTIR interferograms

Infrared scattering-type near-field optical microscopy, IR s-SNOM, and its broadband variant, nano-FTIR, are pioneering, flagship techniques for their ability to provide molecular identification and material optical property information at a spatial resolution well below the far-field diffraction limit, typically less than 25 nm. While s-SNOM and nano-FTIR instrumentation and data analysis have been discussed previously, there is a lack of information regarding experimental parameters for the practitioner, especially in the context of previously developed frameworks. Like conventional FTIR spectroscopy, the critical component of a nano-FTIR instrument is an interferometer. However, unlike FTIR spectroscopy, the resulting interference patterns or interferograms are typically asymmetric. Here, we unambiguously describe the origins of asymmetric interferograms recorded with nano-FTIR instruments, give a detailed analysis of potential artifacts, and recommend optimal instrument settings as well as data analysis parameters.

Rapid Deposition of the Biomimetic Hydroxyapatite-Polydopamine-Amino Acid Composite Layers onto the Natural Enamel.

In this work, we developed a technology that enables rapid deposition of biomimetic composite films onto natural enamel slices (known as biotemplates). These films are composed of polydopamine (PDA) and nanocrystalline carbonate-substituted hydroxyapatite (nano-cHAp) that have been functionalized with amino acid l-Arginine. We utilized atomic force microscopy (AFM) and scattering scanning near-field optical microscopy (s-SNOM) combined with infrared (IR) synchrotron to achieve nanoscale spatial resolution for both IR absorption and topography analyses. This combined analytical modality allowed us to understand how morphology connects to local changes in the chemical environment on the biotemplate surface during the deposition of the bioinspired coating. Our findings revealed that when using the proposed technology and after the deposition of the first PDA layer, the film formed on the enamel surface nearly covers the entire surface of the specimen whose thickness is larger on the surface of the emerging enamel prisms. Calculation of the crystallinity index for the biomimetic layer showed a multiple increase compared with natural enamel. This indicates regular and dense aggregation of nano-cHAp into larger crystals, imitating the morphology of natural enamel rods. The microhardness of the formed PDA-based biomimetic layer mineralized with nano-cHAp functionalized with amino acid l-Arginine deposited on natural enamel was practically the same as that of natural enamel. The characterization of nano-cHAp-amino acid-PDA layers using IR and Raman microspectroscopy showed that l-arginine acts as a conjunction agent in the formation of mineralized biomimetic composite coatings. The uniformity of the mechanisms of PDA layer formation under different deposition conditions and substrate types allows for the formation of coatings regardless of the macro- and micromorphology of the template. Therefore, the results obtained in this work have a high potential for future clinical applications in dental practice.

Nano-infrared imaging of epitaxial graphene on SiC revealing doping and thickness inhomogeneities

We report on a nano-infrared (IR) imaging and spectroscopy study of epitaxial graphene on silicon carbide (SiC) by using scattering-type scanning near-field optical microscopy (s-SNOM). With nano-IR imaging, we reveal in real space microscopic domains with distinct IR contrasts. By analyzing the nano-IR, atomic force microscopy, and scanning tunneling microscopy imaging data, we conclude that the imaged domains correspond to single-layer graphene, bilayer graphene (BLG), and higher-doped BLG. With nano-IR spectroscopy, we find that graphene can screen the SiC phonon resonance, and the screening is stronger at more conductive sample regions. Our work offers insights into the rich surface properties of epitaxial graphene and demonstrates s-SNOM as an efficient and effective tool in characterizing graphene and possibly other two-dimensional materials.

Modeling of plasmonic and polaritonic effects in photocurrent nanoscopy

We present a basic framework for modeling collective mode effects in photocurrent measurements performed on two-dimensional materials using nano-optical scanned probes. We consider photothermal, photovoltaic, and bolometric contributions to the photocurrent. We show that any one of these can dominate depending on frequency, temperature, applied bias, and sample geometry. Our model is able to account for periodic spatial oscillations (fringes) of the photocurrent observed near sample edges or inhomogeneities. For the case of a non-absorbing substrate, we find a direct relation between the spectra measured by the photocurrent nanoscopy and its parental scanning technique near-field optical microscopy.

History-dependent nano-photoisomerization by optical near-field in photochromic single crystals

The history dependence of physical systems is promising for solving complex problems. To exploit spontaneous nanometre-scale optical memory for computation, the history dependence must be elucidated. Here, we demonstrate history-dependent or dynamic nano-photoisomerization through the sequential formation of multiple memory pathways in photochromic crystals via optical near-field interactions. Incident photons passing through photoisomerization memory pathways are observed using a double-probe optical near-field microscope, with one probe located on the front surface for local excitation and the other on the rear surface for near-field observations. By performing localised near-field excitation twice but at spatially different positions, negatively correlated near-field output patterns are observed between the first and second memory pathways. We also confirm that the first memory pathway is preserved after the formation of the second memory pathway. This result indicates that photoisomerization using an optical near field in diarylethene crystals has a history dependence, leading to brain-like dynamic information memorisation.

Probing Temperature-Induced Plasmonic Nonlinearity: Unveiling Opto-Thermal Effects on Light Absorption and Near-Field Enhancement.

Precise measurement and control of local heating in plasmonic nanostructures are vital for diverse nanophotonic devices. Despite significant efforts, challenges in understanding temperature-induced plasmonic nonlinearity persist, particularly in light absorption and near-field enhancement due to the absence of suitable measurement techniques. This study presents an approach allowing simultaneous measurements of light absorption and near-field enhancement through angle-resolved near-field scanning optical microscopy with iterative opto-thermal analysis. We revealed gold thin films exhibit sublinear nonlinearity in near-field enhancement due to nonlinear opto-thermal effects, while light absorption shows both sublinear and superlinear behaviors at varying thicknesses. These observations align with predictions from a simple harmonic oscillation model, in which changes in damping parameters affect light absorption and field enhancement differently. The sensitivity of our method was experimentally examined by measuring the opto-thermal responses of three-dimensional nanostructure arrays. Our findings have direct implications for advancing plasmonic applications, including photocatalysis, photovoltaics, photothermal effects, and surface-enhanced Raman spectroscopy.

A historical overview of Nano-optics: From Near-Field Optics to Plasmonics

Abstract Nano-optics is an emergent research field in physics that appeared in the 1980s, which deals with light-matter optical interactions at the nanometer scale. In early studies of nano-optics, the main concern focus was to obtain higher optical resolution over the diffraction limit. The researches of near-field imaging and spectroscopy based on Scanning Near-Field Optical Microscopy (SNOM) were developed. The exploration of improving SNOM probe for near-field detection led to the emergence of surface plasmons. In the sense of resolution and wider application, there has been a significant transition from seeking higher resolution microscopy to plasmonic near-field modulations in the nano-optics community during the nano-optic development. Nowadays, studies of nano-optics prefer the investigation of plasmonics in different material systems. In this article, the history of the development of near-field optics is briefly reviewed. The difficulties of conventional SNOM to achieve higher resolution are discussed. As an alternative solution, surface plasmons have shown the advantages of higher resolution, wider application, and flexible nano-optical modulation for new devices. The typical studies in different periods are introduced and analyzed characteristics of nano-optics in each stage. In this way, the evolution progress from near-field optics to plasmonics of nano-optics research is presented. The future development of nano-optics is discussed then.

Accelerated Nano-Optical Imaging through Sparse Sampling.

The integration time and signal-to-noise ratio are inextricably linked when performing scanning probe microscopy based on raster scanning. This often yields a large lower bound on the measurement time, for example, in nano-optical imaging experiments performed using a scanning near-field optical microscope (SNOM). Here, we utilize sparse scanning augmented with Gaussian process regression to bypass the time constraint. We apply this approach to image charge-transfer polaritons in graphene residing on ruthenium trichloride (α-RuCl3) and obtain key features such as polariton damping and dispersion. Critically, nano-optical SNOM imaging data obtained via sparse sampling are in good agreement with those extracted from traditional raster scans but require 11 times fewer sampled points. As a result, Gaussian process-aided sparse spiral scans offer a major decrease in scanning time.

Hybrid Molecular Beam Epitaxy for Single-Crystalline Oxide Membranes with Binary Oxide Sacrificial Layers.

The advancement in thin-film exfoliation for synthesizing oxide membranes has led to possibilities for creating artificially assembled heterostructures with structurally and chemically incompatible materials. The sacrificial layer method is a promising approach to exfoliate as-grown films from a compatible material system, allowing for their integration with dissimilar materials. Nonetheless, the conventional sacrificial layers often possess an intricate stoichiometry, thereby constraining their practicality and adaptability, particularly when considering techniques such as molecular beam epitaxy (MBE). This is where easy-to-grow binary alkaline-earth-metal oxides with a rock salt crystal structure are useful. These oxides, which include (Mg, Ca, Sr, Ba)O, can be used as a sacrificial layer covering a much broader range of lattice parameters compared to conventional sacrificial layers and are easily dissolvable in deionized water. In this study, we show the epitaxial growth of the single-crystalline perovskite SrTiO3 (STO) on sacrificial layers consisting of crystalline SrO, BaO, and Ba1-xCaxO films, employing a hybrid MBE method. Our results highlight the rapid (≤5 min) dissolution of the sacrificial layer when immersed in deionized water, facilitating the fabrication of millimeter-sized STO membranes. Using high-resolution X-ray diffraction, atomic-force microscopy, scanning transmission electron microscopy, impedance spectroscopy, and scattering-type near-field optical microscopy (SNOM), we demonstrate single-crystalline STO membranes with bulk-like intrinsic dielectric properties. The employment of alkaline earth metal oxides as sacrificial layers is likely to simplify membrane synthesis, particularly with MBE, thus expanding the research and application possibilities.

Simultaneous mapping of the magnetic field components using near-field microscopy

In this work, we present a near-field scanning optical microscope capable of simultaneously mapping the x− , y− , and z− components of the magnetic field generated by a set of magnetic coils in the kHz range. Using this device, we present two experiments: the observation of the well-known magnetic field of a circular coil and a resolution test. The observations achieved with our instrument are supported by numerically calculated simulations, which confirm the validity of our experimental results. In addition, our instrument is capable of imaging features of the order of 15mm , representing a resolution of λ/10000 . Our approach establishes an important avenue for the three-dimensional mapping of the electromagnetic field in any spectral range, using the correct configuration of the probe, and opens a new discussion on the resolution of scanning probe microscopes imaging more than one component of the studied field.

Безапертурная ближнепольная микроскопия упругого рассеяния света

Безапертурная ближнепольная микроскопия упругого рассеяния света

Experimental study on subwavelength focusing optical field detection methods for micro-Fresnel zone plate

In this paper, the detection methods and detection systems for the subwavelength focusing optical field of the micro-Fresnel zone plate (FZP) are experimentally studied. First, a comparison is made between the micro/nanofabrication methods for micro-FZP, namely, focused ion beam (FIB) and electron beam lithography, and the results show that FIB is better suited for the amplitude-type micro-FZP fabrication. Subsequently, the experimental detection devices based on the wide-field microscopy amplification (WFMA) imaging method (indirect detection method) and scanning near-field optical microscopy (direct detection method) are, respectively, constructed for the detection of the subwavelength focusing optical field of micro-FZP. The experimental results are compared and analyzed with theoretical calculation results, indicating that the WFMA method is more suitable for the detection of micro-FZP subwavelength focusing optical field that is not sensitive to radial components. This study provides an experimental reference for the micro/nanofocusing optical field detection of micro/nano-optical components similar to micro-FZP and promotes the practical application of micro-FZP.