Near-field Optical Data Research Articles

Optimizing LED array irradiance uniformity with a particle swarm optimization-based scheme: application in a parallel photoreactor

This study proposes a particle swarm optimization (PSO)-based method for enhancing the irradiance uniformity of light emitting diode (LED) arrays in parallel photoreactors. We construct a spatial irradiance model, validating its accuracy with near-field optical test data and optical simulation results. An evaluation function for irradiance uniformity guides the iterative optimization of the LED arrangement, taking into consideration practical constraints such as chip size, heat dissipation, and circuit board wiring. The optimized LED array demonstrated a significant improvement in irradiance uniformity, increasing from 75.27% to 95.61%. This showcases the feasibility and effectiveness of our PSO-based method for practical applications in optimizing LED arrays.

A Novel Multimodal Fusion Framework Based on Point Cloud Registration for Near-Field 3D SAR Perception

This study introduces a pioneering multimodal fusion framework to enhance near-field 3D Synthetic Aperture Radar (SAR) imaging, crucial for applications like radar cross-section measurement and concealed object detection. Traditional near-field 3D SAR imaging struggles with issues like target–background confusion due to clutter and multipath interference, shape distortion from high sidelobes, and lack of color and texture information, all of which impede effective target recognition and scattering diagnosis. The proposed approach presents the first known application of multimodal fusion in near-field 3D SAR imaging, integrating LiDAR and optical camera data to overcome its inherent limitations. The framework comprises data preprocessing, point cloud registration, and data fusion, where registration between multi-sensor data is the core of effective integration. Recognizing the inadequacy of traditional registration methods in handling varying data formats, noise, and resolution differences, particularly between near-field 3D SAR and other sensors, this work introduces a novel three-stage registration process to effectively address these challenges. First, the approach designs a structure–intensity-constrained centroid distance detector, enabling key point extraction that reduces heterogeneity and accelerates the process. Second, a sample consensus initial alignment algorithm with SHOT features and geometric relationship constraints is proposed for enhanced coarse registration. Finally, the fine registration phase employs adaptive thresholding in the iterative closest point algorithm for precise and efficient data alignment. Both visual and quantitative analyses of measured data demonstrate the effectiveness of our method. The experimental results show significant improvements in registration accuracy and efficiency, laying the groundwork for future multimodal fusion advancements in near-field 3D SAR imaging.

Nano-FTIR Correlation Nanoscopy for Organic and Inorganic Material Analysis

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

(Invited) Nano-FTIR Correlation Nanoscopy for Organic and Inorganic Material Analysis

Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) is a scanning probe approach to optical microscopy and spectroscopy, bypassing the ubiquitous diffraction limit of light to achieve a spatial resolution below 20 nanometers. s-SNOM employs the strong confinement of light at the apex of a sharp metallic atomic force microscopy (AFM) tip to create a nanoscale optical hot-spot. Analyzing the scattered light from the tip enables the extraction of the optical properties (dielectric function) of the sample directly below the tip and yields nanoscale resolved images simultaneous to topography [1]. In addition, the technology has been advanced to enable Fourier-Transform Infrared Spectroscopy on the nanoscale (nano-FTIR) [2] using broadband radiation from the visible spectral range to THz frequencies. Recently, the combined analysis of complex nanoscale material systems by correlating near-field optical data with information obtained by other scanning probe microscopy (SPM)-based measurement methodologies has gained significant interest. For example, the material-characteristic nano-FTIR spectra of a phase-separated polystyrene/low-density polyethylene (PS/LDPE) polymer blend verifies sharp material interfaces by measuring a line profile across a ca. 1 µm sized LDPE island (Fig1). Near-field reflection/absorption imaging at 1500cm-1 of the ca. 50nm thin film allows to selectively highlight the distribution of PS in the blend and simultaneously map the mechanical properties like adhesion of the different materials [3,4]. Further, results will be presented that correlate the near-field optical response of semiconducting samples like graphene (2D) or functional SRAM devices (3D) in different frequency ranges (mid-IR & THz) to Kelvin Probe Force Microscopy (KPFM) measurements. Thus, s-SNOM systems represent an ideal platform to gain novel insights into complex material systems by different near-field and AFM-based method. [1] F. Keilmann, R. Hillenbrand, Phil. Trans. R. Soc. Lond. A 362, 787 (2004). [2] F. Huth, et al., Nano Lett. 12, 3973 (2012). [3] B. Pollard, et al., Beilstein J. of Nanotechn. 7, 605 (2016). [4] I. Amenabar, et al., Nature Commun. 8, 14402 (2017). Figure caption: Fig 1. Near-field correlation nanoscopy of a thin PS/LDPE polymer film, highlighting the phase separation of the materials by nano-FTIR measurements as well as the different mechanical properties of the polymers. Figure 1

Properties of sub-band edge states in AlInN studied by time-resolved photoluminescence of a AlInN/GaN heterostructure

Time-resolved photoluminescence (PL) measurements of Al0.82In0.18N/GaN heterostructures revealed a large enhancement of low temperature PL from the GaN layer and a strong temperature dependence of this effect. Analysis of different phenomena that might affect the photoexcited carrier dynamics suggests that the enhanced GaN PL should be attributed to photoexcited hole transfer from the AlInN layer. The hole transport most probably takes place via dense sub-band edge valence band states related to nanoscale In-rich clusters. Scanning near-field optical microscopy data support this assignment.

Scanning Near-Field EllipsometryMicroscopy: imaging nanomaterials with resolution below the diffraction limit

We introduce a simple Scanning Near-Field Ellipsometer Microscopy (SNEM) setup to address the rapidly increasing need for simple, routine optical imaging techniques with resolution well below the diffraction limit. Our setup is based on the combination of commercially available atomic force microscope (AFM) and ellipsometry equipment with gold-coated AFM tips to obtain near-field optical images with a demonstrated resolution below λ/10. AFM topographical data, obtained in contact mode, and near-field optical data were acquired simultaneously using a combined AFM-ellipsometer. The highly enhanced field due to lightning-rod effects and localized surface plasmons excited at the end of the gold-coated tip allowed us to resolve and identify metallic nanoparticles embedded in poly(methyl methacrylate) as well as microphases in microphase-separated block copolymer films.

A Hyperlens-embedded solid immersion lens for beam focusing beyond the diffraction limit

A novel beam focusing device called hyperlens-embedded solid immersion lens (HESIL) is proposed in this letter. In the device, a hyperlens which supports propagating waves with very large wavenumbers is embedded into a super-hemispherical solid immersion lens and a super-resolution focusing spot can be formed at the bottom planar plane of the HESIL. The working principles and characteristics of the device are discussed in detail. Numerical simulations verify that a nano-scale spot with a diameter of 57.5 nm (1/6.5 of the vacuum wavelength <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\lambda_{0}$</tex> </formula> ) is achieved (the numerical aperture of the objective is 0.45) and the system transmission exceeds 34%, which validates the effectiveness of our proposed device for beam focusing. The device is expected to find numerous applications in high-density near-field optical data storage, nano-photolithography, and so on.

Quantum dot labeling based on near-field optical imaging of CD44 molecules

The lateral organization of membrane proteins and lipids domains has a direct impact on many cellular processes, but generally these domains are too small to be resolved by diffraction-limited resolution of fluorescence microscopy. Here, we use quantum dot (QD) labeling based on near-field optical imaging, to study the nanoscale organization of hyaluronan receptor CD44 molecules of fixed mesenchymal stem cells (MSCs) in air, with a optical resolution down to 50 nm. The photostability and high luminance of QD evidently improve the signal-to-noise ratio and reproducibility of near-field optical data. Importantly, the blinking-intensity analysis was proposed to identify single QD, providing a calibration to relate intensity to numbers of antibody for the first time. Additionally, the fluorescence-topographic imaging enables us to investigate the topographic location pattern. Our results demonstrate that CD44 molecules on MSCs are enriched into nanosized domain and they predominantly locate on the peak of the membrane protrusions, which may contribute to clarify the underlying mechanism of functions ascribed to these molecules.

Excitation of dielectric-loaded surface plasmon polariton observed by using near-field optical microscopy

The excitation of dielectric-loaded surface plasmon polariton (DLSPP) by a CdS nanostripe placed on a Ag layer is observed by using a scanning near-field optical microscopy (SNOM). The spectroscopic redshift of 30 meV of photoluminescence is observed at different positions of the stripe by SNOM and can be explained as the result of Franz–Keldysh effect. Finite difference time domain calculations confirm the observations of the near-field optical images and spectroscopic data. This DLSPP excitation structure is important in the implementation of photonic integrated circuits as well as the optical guiding modes in plasmonic devices.

Local versus global absorption in thin-film solar cells with randomly textured surfaces

Enhanced light absorption in amorphous silicon thin films deposited on randomly textured zinc-oxide surfaces is investigated by means of a rigorous diffraction theory taking into account measured surface profiles and near-field optical data. Global absorption enhancement is obtained in the calculations for particular modifications of the random texture. We furthermore spatially resolve local domains of the surface texture, which show the strongest contribution to the absorption. Criteria on how random surfaces should look like to enhance absorption in thin-film solar cells are derived.

Fabrication of Pure Silica Core Multimode Ultraviolet OpticalFibre Probes by Tube Etching

As a light wave-guide component for transmitting ultraviolet (UV) laserpulses, pure silica core UV fibre probes have attracted a great deal of attentionin the near-field optical data storage and bio-medical studies. Wefabricate UV fibre probes with tips in dimension of about 2–5 μm andtaper angle 16° by the tube etching method, using 40% HF acid asetching solution and xylene as overlayer. Probes produced have curvyconfiguration with smooth surface. The yield of fine probes is ratherhigh and etching operation greatly simplified. With higher damagethreshold, pure silica core multimode UV fibre probes can be coupledinto more laser power. In addition, using UV light reduces the cutoffwavelength of the fibre probes, which is in favour of increasing thetransmission efficiency of the probe. Furthermore, the larger tipdimension helps to enhance the light throughput either. The advancesof fabrication technique of UV optical fibre probe may further supportthe studies of UV light data storage, pulsed laser biosurgery and UVphotolithography.

Fabrication of Nanohole Array via Nanodot Array Using Simple Self-Assembly Process of Diblock Copolymer

We present a simple self-assembly process for fabricating a nanohole array via a nanodot array on a glass substrate by dripping ethanol onto the nanodot array. It is found that well-aligned arrays of nanoholes as well as nanodots are formed on the whole surface of the glass. A dot is transformed into a hole, and the alignment of the nanodots strongly reflects that of the nanoholes. We find that the change in the depth of holes agrees well with the change in the surface energy with the ethanol concentration in the aqueous solution. We believe that the interfacial energy between the nanodots and the dripped ethanol causes the transformation from nanodots into nanoholes. The nanohole arrays are directly applicable to molds for nanopatterned media used in high-density near-field optical data storage. The bit data can be stored and read out using probes with small apertures.

High-intensity C-shaped nanoaperture vertical-cavity surface-emitting laser with controlled polarization

The authors designed and demonstrated a C-shaped nanoaperture (C aperture) vertical-cavity surface-emitting laser (VCSEL) with a maximum far-field power of 157μW coming from a 70nm C aperture. Simulation shows that the near-field full width at half maximum spot size at 30nm away from the C aperture is 94×108nm2 in X and Y directions. The authors estimate the peak near-field intensity from the C-aperture VCSEL to be as high as 19mW∕μm2. This high intensity and small spot size are promising for realizing near-field applications such as near-field imaging and ultradense near-field optical data storage.

Cover-layer effects on contamination in near-field optical data storage

Contamination at the head-disk interface (HDI) was observed for first-surface near-field recording (NFR) under accumulation over a long writing time with a high writing power. The contamination was caused by laboratory dust coupled with the heat from the focus of the writing laser at the HDI. One suggested solution to the problem is that a cover layer should be laminated for cover-layer-incident NFR. A uniform and edge-bead-free UV-curable cover layer was laminated by using an outer-ring technique. Diamondlike carbon films were optimized and coated on the cover layer, and then a lubricant film was applied to reduce the tribological problems at the HDI.

High transmission through ridge nano-apertures on Vertical-Cavity Surface-Emitting Lasers

We report high-intensity nano-aperture Vertical-Cavity Surface- Emitting Lasers (VCSELs) with sub-100nm near-field spots using ridge apertures. Power transmission efficiency through different ridge apertures, including bowtie, C, H and I-shaped apertures on VCSELs were studied. Significantly higher transmission efficiencies were obtained from the ridge apertures than those from conventional square apertures. Mechanisms for high transmission through the ridge apertures are explained through simulation and waveguide theory. A new quadruple-ridge aperture is proposed and designed via simulation. With the high-intensity and small spot size, VCSELs using these ridge nano-apertures are very promising means to realize applications such as ultrahigh-density near-field optical data storage and ultrahigh-resolution near-field imaging etc.

Scanning near-field optical data contrast measurement: a tomographylike near-field reconstruction

A method for the reconstruction of near-field optical signals is proposed to produce a tomographylike map of the field above the nanostructures being studied. The data obtained through lock-in detection are processed employing Chebyshev's polynomial function of the distance between the probe and the sample. The method is first applied to numerically generated near-field evanescent data, with three different decreasing lengths, and then applied to an experimental signal. Therefore the contrast of the signal above nanostructures is discussed to underline the discrepancy between the scanning near-field optical microscopy data and the reconstructions.

Design of high-performance supersphere solid immersion lenses

Two types of novel solid immersion lens are designed and investigated theoretically using the vector diffraction theory. The advantages of these so-called high-performance supersphere solid immersion lenses (HPSILs) are that they can improve the Strehl ratio of the focused spot and increase the focal depth of near-field optical systems. Both the spot size and the sidelobe intensity are not increased, however, compared with those of the standard Weierstrass solid immersion lens. These HPSILs will be useful for near-field optical data storage and photolithography.

Field enhancement by surface plasmon polariton in self-assembling nanopatterned media

We present the fabrication of nanodot arrays on metal films by self-assembly of diblock copolymer. The nanodots are uniform and aligned regularly with hexagonal close-packed structure on both gold film and glass. Gold is specifically a good candidate as the metal film excites surface plasmon polariton in the near-field optical media. Surface plasmon polariton is excited with the Kretschmann configuration at the gold-nanodot interface. We found that the intensity at the interface was enhanced 53.8 times to that of the incident light. It is expected to improve the carrier-to-noise ratio of the readout signal in the case of nanopatterned near-field optical data storage under collection mode.

Achieving superresolution in near-field optical data readout systems using surface plasmons

The effects of surface plasmons and enhanced transmission on the readout contrast in a superresolving near-field optical data system are studied numerically using an exact Green’s tensor formulation. It is shown that plasmon effects can both help and hinder such a readout system, and the system geometry must be chosen carefully to produce optimal effects. Under certain conditions, the system can have a readout contrast approaching 50% and a resolution of at least λ∕3.

Identification of unknown experimental parameters from noisy apertureless scanning near-field optical microscope data with an evolutionary procedure

We determine a set of experimental parameters through the application of an evolutionary inversion procedure. The input to the algorithm is experimental apertureless scanning near-field optical microscope data. The performance of our inversion procedure is assessed by means of a comparison with a nonevolutionary technique.