Near-field Properties Research Articles

An experimental study of trailing edge noise from a heaving airfoil.

In this study, the far-field noise and near-field flow properties from a heaving NACA 0012 airfoil at the Reynolds number of 6.6×104 were investigated experimentally in a 0.4 m2 anechoic wind tunnel. The airfoil had an incident angle of 0° and followed a sinusoidal heaving motion. The Strouhal number, controlled by changing the heaving frequency and amplitude, varied from 0.0024 to 0.008. The acoustic properties were measured by a free-field microphone placed at a distance of 1.2 m away from the tunnel central line, and the flow structures near the trailing edge were acquired using the particle image velocimetry. It was found that the heaving motion could reduce the sound pressure level (SPL) of the primary peak in the time-averaged spectra. The spectrograms obtained by the short-time Fourier transform revealed that the discrete tones were produced when the airfoil passed through the maximum heaving position. During the corresponding period, a sequence of large-scaled vortices convected on the airfoil surface was observed, and then was shed from the trailing edge to the wake region at the same frequency as the primary tone of the induced sound. With the increase of Strouhal number, the sound signals tended to be broadband, and the overall SPL was increased in the far field.

Field enhancement of optical bowtie nano‐antenna using exponential tapered profile

In this work, both the near-field and far-field optical properties of curved sides bowtie nano-antenna are investigated in the near-infrared range. The curvature of the proposed structure is based on the Vivaldi antenna tapering shape. The effect of tapering rate on the near-field enhancement is studied, and the results show that the proposed antenna structure produces better field enhancement compared with conventional bowtie antenna with linear sides and hence it is promising for energy harvesting and microscopy applications. Also, the far-field properties are studied to decide the validity of the structure in optical communication applications.

Symmetry selective directionality in near-field acoustics.

Understanding unidirectional and topological wave phenomena requires the unveiling of intrinsic geometry and symmetry for wave dynamics. This is essential yet challenging for the flexible control of near-field evanescent waves, highly desirable in broad practical scenarios ranging from information communication to energy radiation. However, exploitations of near-field waves are limited by a lack of fundamental understanding about inherent near-field symmetry and directional coupling at sub-wavelengths, especially for longitudinal waves. Here, based on the acoustic wave platform, we show the efficient selective couplings enabled by near-field symmetry properties. Based on the inherent symmetry properties of three geometrically orthogonal vectors in near-field acoustics, we successfully realize acoustic Janus, Huygens, spin sources and quadrupole hybrid sources, respectively. Moreover, we experimentally demonstrate fertile symmetry selective directionality of those evanescent modes, supported by two opposite meta-surfaces. The symmetry properties of the near-field acoustic spin angular momenta are revealed by directly measuring local vectorial fields. Our findings advance the understanding of symmetries in near-field physics, supply feasible approaches for directional couplings, and pave the way for promising acoustic devices in the future.

Tuning of far-field and near-field via fs-laser in various hybrid oligomers

Oligomers have attracted intense attention due to the mode interference. We have simulated the tuning effect of far- and near-field properties of four different compositions of hybrid metal/dielectric oligomers before and after the gold components modified by femtosecond laser. The achieved results could be applied in optical sensing, field-enhanced nonlinear process as well as biophotonic applications.

Modified plasmonic response of dimer nanoantennas with nonlocal effects: From near-field enhancement to optical force

The metallic nanoparticle dimer is a fundamental model system for enhancing and tuning localized surface plasmon resonances. In the past, it had been found that the far- and near-field optical properties of dimer antennas can be regulated by many parameters (e.g., gap, size, orientation, materials, and surrounding medium). In recent years, the quantum mechanical effects such as nonlocal screening and electron tunneling have been achieved when the gap distance in a dimer approaches 1 nm and subnanometer. In this communication, both the near-field enhancement and optical force in dimer are fully investigated and compared between classical and nonlocal models. Compared with classical theory, we found that both the resonant wavelength and peak intensity have smaller changes in nonlocal model when geometrical or material parameters changes. Besides, the extent of parameter-induced spectral changes is slightly different between near-field enhancement and optical force. These results make possible the quantitative analysis of nonlocal effects in surface-enhanced spectroscopy, nanoantennas, refractive-index sensing, surface-enhanced optical force, and quantum plasmonics.

Arbitrary control of the diffusion potential between a plasmonic metal and a semiconductor by an angstrom-thick interface dipole layer.

Localized surface plasmon resonances (LSPRs) are gaining considerable attention due to the unique far-field and near-field optical properties and applications. Additionally, the Fermi energy, which is the chemical potential, of plasmonic nanoparticles is one of the key properties to control hot-electron and -hole transfer at the interface between plasmonic nanoparticles and a semiconductor. In this article, we tried to control the diffusion potential of the plasmonic system by manipulating the interface dipole. We fabricated solid-state photoelectric conversion devices in which gold nanoparticles (Au-NPs) are located between strontium titanate (SrTiO3) as an electron transfer material and nickel oxide (NiO) as a hole transport material. Lanthanum aluminate as an interface dipole layer was deposited on the atomic layer scale at the three-phase interface of Au-NPs, SrTiO3, and NiO, and the effect was investigated by photoelectric measurements. Importantly, the diffusion potential between the plasmonic metal and a semiconductor can be arbitrarily controlled by the averaged thickness and direction of the interface dipole layer. The insertion of an only one unit cell (uc) interface dipole layer, whose thickness was less than 0.5 nm, dramatically controlled the diffusion potential formed between the plasmonic nanoparticles and surrounding media. This is a new methodology to control the plasmonic potential without applying external stimuli, such as an applied potential or photoirradiation, and without changing the base materials. In particular, it is very beneficial for plasmonic devices in that the interface dipole has the ability not only to decrease but also to increase the open-circuit voltage on the order of several hundreds of millivolts.

TiO2 Nanocolumn Arrays for More Efficient and Stable Perovskite Solar Cells.

Organic-inorganic hybrid perovskite solar cells have attracted much attention due to their high power conversion efficiency (>25%) and low-cost fabrication. Yet, improvements are still needed for more stable and higher-performing solar cells. In this work, a series of TiO2 nanocolumn photonic structures have been intentionally fabricated on half of the compact TiO2-coated fluorine-doped tin oxide substrate by glancing angle deposition with magnetron sputtering, a method particularly suitable for industrial applications due to its high reliability and reduced cost when coating large areas. These vertically aligned nanocolumn arrays were then applied as the electron transport layer into triple-cation lead halide perovskite solar cells based on Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3. By comparison to solar cells built onto the same substrate without nanocolumns, the use of TiO2 nanocolumns can significantly enhance the power conversion efficiency of the perovskite solar cells by 7% and prolong their shelf life. Here, detailed characterizations on the morphology and the spectroscopic aspects of the nanocolumns, their near-field and far-field optical properties, solar cells characteristics, as well as the charge transport properties provide mechanistic insights on how one-dimensional TiO2 nanocolumns affect the performance of perovskite halide solar cells in terms of charge transport, light harvesting, and stability, knowledge necessary for the future design of higher-performing and more stable perovskite solar cells.

Temperature-controlled synthesis of hollow, porous gold nanoparticles with wide range light absorption

An easy synthesis method of hollow, porous gold nanoparticles (AuHP NPs) with controlled diameter and pores sizes and with a wide range of light absorbance (continuous between 500 and 900 nm) is presented together with the explanation of the nanoparticle formation mechanism. The NPs were investigated using transmission electron microscopy (TEM) combined with the selected area electron diffraction patterns, X-ray diffraction and ultraviolet–visible spectroscopy. TEM images showed that changing the synthesis temperature allows to obtain AuHP NPs with sizes from 35 ± 4 nm at 60 °C to 76 ± 8 nm at 90 °C. The effects of nanoscale porosity on the far- and near-field optical properties of the nanoparticles, as well as on effective conversion of electromagnetic energy into thermal energy, were applied in simulated photothermal cancer therapy. The latter one was simulated by irradiation of two cancer cell lines SW480 and SW620 with lasers operating at 650 nm and 808 nm wavelengths. The mortality of cells after using the synthesized AuHP NPs as photosensitizers is between 20 and 50% and increases with the decrease in the diameter of the AuHP NPs. All these attractive properties of the AuHP NPs make them find application in many biomedical studies.

Diffractive dipolar coupling in non-Bravais plasmonic lattices.

Honeycomb plasmonic lattices are paradigmatic examples of non-Bravais lattices. We experimentally measure surface lattice resonances in effectively free-standing honeycomb lattices composed of silver nanospheres. By combining numerical simulations with analytical methods, we analyze the dispersion relation and the near-field properties of these modes along high symmetry trajectories. We find that our results can be interpreted in terms of dipole-only interactions between the two non-equivalent triangular sublattices, which naturally lead to an asymmetric near-field distribution around the nanospheres. We generalize the interaction between the two sublattices to the case of variable adjacent interparticle distance within the unit cell, highlighting symmetry changes and diffraction degeneracy lifting associated to the transition between Bravais and non-Bravais lattices.

置于不同基底表面的单一金纳米粒子的近场特性

置于不同基底表面的单一金纳米粒子的近场特性

Studies of flight-velocity effects on near-field and intermittent properties of a subsonic jet

The flight velocity effect on a cold jet flow is numerically predicted by the improved delayed detached eddy simulation (IDDES) with a modification to the grid length scale, mainly focusing on the near-field flows and intermittency properties in the jet centerline region. First, the modification in IDDES is validated using three meshes and available measurements. This method can reduce the modeled eddy viscosity effectively and improve the performance of original IDDES. Then, the flight velocity effects on the mean flow, turbulence, and intermittency are studied. From the results, the fluctuation intermittency near the end of potential core is caused by the intrusion of turbulent shear-layer structures into the jet core. When the flight velocity increases, the intermittency factor in the jet centerline region is reduced, and its peak moves downstream. The skewness and kurtosis factors perform in a similar manner.

Optical properties of Au–Ag nanosphere dimer: Influence of interparticle spacing

In this paper, the extinction and near-field properties of Au–Au, Ag–Ag nanosphere homo-dimer and Au–Ag nanosphere hetero-dimer were studied by using the discrete dipole approximation method. The relation between extinction and interparticle spacing was investigated under y- polarization (perpendicular to dimer axis) and z-polarization (parallel to dimer axis) incidence. The results show that the both extinction peak position and enhancement factor of electric field are sensitive to interparticle spacing when the dimer is illuminated by z-polarization incidence. For Au–Ag hetero-dimer, double same-intensity extinction peaks in visible band were found by regulating interparticle space under z-polarization incidence, and the experience forum was obtained for realizing the double same-intensity peaks.

Photonic nanojet beam shaping by illumination polarization engineering

Photonic Nanojets (PNJs) have attracted considerable research attention in the fields of super-resolution optical microscopy, nano-photolithography, and single molecule sensors because of their sub-wavelength near-field focusing properties. In this paper, we thoroughly studied how the polarizations and amplitude profiles of the incident beams affect the shape, size, and location of the PNJs generated from the illuminated microspheres. Numerical results showed that the PNJs generated by microspheres were strongly modulated by the polarizations and amplitude profiles of the illumination beams. Therefore, PNJs can be engineered according to the requirements of a specific application by designing the polarizations and amplitude profiles of the illumination light. Various fascinating properties of the PNJs generated with different illumination schemes were demonstrated and their implications for potential applications were discussed as well.

Spin-orbit coupling controlled near-field propagation and focusing of Bloch surface wave.

Bloch surface wave (BSW) can be considered as the dielectric analogue of surface plasmon polariton (SPP) with less loss since it is sustained at the surface of a truncated dielectric multilayer. As dielectric materials show nearly no ohmic loss, BSW can propagates much farther compared to SPP, and thus is beneficial for planar optical devices. In this paper, we study the spin-orbital interaction between incident beam and BSW. We demonstrate that due to the spin-orbital coupling, the near-field properties of generated BSW can be controlled with a meta-antenna structure. The meta-antenna is composed of two gold nano-antennas oriented at 45° and 135° as a near-field coupler. By careful design of the meta-antenna, the generated BSW can be guided and focused depending on the chirality of the incident beam. Three examples of meta-antennas are demonstrated for chiral sensitive focusing, directional switching and asymmetric focusing. The proposed method can be applied as a design method for low-loss on-chip photonic devices.

Practical Implementation of Accurate Finite-Element Calculations for Electromagnetic Scattering by Nanoparticles

The finite-element method (FEM) is increasingly used as a numerical tool to support experimental and theoretical studies of the optical properties of nanoparticles, in contexts such as surface-enhanced spectroscopy, molecular plasmonics, metamaterials, and optical trapping. Here, we investigate the validity of such calculations, focusing in particular on numerically challenging cases involving strong optical (plasmon) resonances and solutions with large electric field gradients and intensities. These are exemplified by elongated metallic nanoparticles and two closely spaced metallic spheres (dimer), where highly localized regions of intense electric field enhancements occur at the tip or in the gap, so-called electromagnetic hot-spots. We assess the accuracy of the FEM solutions by comparing the result to exact analytic solutions based on the T-matrix method for an elongated particle and generalized Mie theory for a dimer. Particular attention is given to the electromagnetic properties that have seldom been studied in this context, notably near-field properties such as surface-field enhancement factors and far-field radiation profiles. We also demonstrate explicitly how the accuracy of the FEM predictions can be inferred from the solution of two problems with different mesh and bounding box parameters. Such a numerical check is crucial in practice as no exact solutions are in general available to compare with. While we chose the commercial software COMSOL to illustrate our results, the methods and conclusions are equally applicable to other FEM implementations. We provide for convenience full details of how to set up these calculations in COMSOL, which we hope will allow readers to easily reproduce them and seamlessly adapt them to their modeling needs. We expect this work will cement the FEM as a reliable method for routine calculation of electromagnetic scattering by nanoparticles.

Theory of plasmonic properties of hyper-doped silicon nanostructures

The presence of a Localized Surface Plasmon Resonance in doped semiconductor nanostructures opens a new field for plasmonics and metasurfaces. Semiconductor nanostructures can be easily processed, have weak dissipation losses, and the plasmon resonance can be tuned from the mid- to the near-infrared spectral range by changing the dopant concentration (in complement to the constituent material, the size and shape of the nanostructure). We present in this paper an extension of the Green Dyadic Method applied to the case of doped silicon nanostructures of arbitrary shape on a planar silica substrate. The method is used to compute both far- and near-field optical properties, such as the extinction efficiency and the electromagnetic near-field intensity inside and around any doped silicon nanostructure, respectively. This theoretical approach provides an important tool for active dopant characterization in doped semiconductor nanostructures, for near-field imaging of complex nanoantennas produced by electron beam lithography, and for the definition of doped semiconductor-based metasurfaces.

Controlling the end-to-end assembly of gold nanorods to enhance the plasmonic response in near infrared

In the current paper, we report an efficient approach to produce in a controllable manner electromagnetic hot-spots localized in linear assemblies of gold nanorods (AuNRs) that can be further exploited as highly active surface-enhanced Raman scattering (SERS) sites. The process of the AuNRs self-assembling was initiated in an aqueous solution at pH = 3.1 in the presence of L-cysteine (Cys) molecules and was monitored in real-time for minutes up to hours. The recorded sequential extinction spectra featuring a well-defined isosbestic point at 834 nm is consistent with the first-order like reaction between individual and end-to-end assembled AuNRs as final products. Moreover, the dynamic of the self-assembling process was examined by correlating the extinction spectra with the average number of AuNRs in the chains as provided from the transmission electron microscopy (TEM) pictures. Noteworthy, the average number of connected AuNRs can be fixed at any moment in solution, depending of the desired plasmonic response, by blocking the assembling process with a negative layer of poly (sodium-p-styrenesulfonate) (PSS) polyelectrolyte. The high electric field in the hot-spots in between the linked nanoparticles was proved by SERS experiments using as target analyte para-aminothiophenol (p-ATP) and conducted to clear the demonstration of a higher enhancement factor relative to the individual AuNRs. Finally, all experimental findings and, especially, the far and near-field optical properties of such fabricated plasmonic nanoassemblies were supported by finite-difference time-domain (FDTD) simulations.

Precision non-contact displacement sensor based on the near-field characteristics of fiber specklegrams

In this paper, we present a precision non-contact displacement sensor based on the near-field property of specklegrams, achieving a precision of 10 nm and a detection range of 3–4 μm. The compact device envelope enables the application of the new fiber-optic sensor in many space-limited applications, e.g., probe-based nanofabrication and micro-/mesoscale positioning systems. In the sensor system, the laser is first coupled into a 50/50 fiber coupler; the flexible fiber transmits the laser beam to the remote sensor head that is a few micrometers away from the sample surface; the reflected light is collected by the sensor head. The interference among the different eigenmodes propagating in the multimode fiber form unique specklegram images for different sample positions in space. By recording the varying specklegrams along the scanning path, highly precise and repeatable non-contact displacement detection can be realized by comparing the recorded specklegrams with the calibrated specklegram database through imaging processing techniques. In the experiments, we demonstrate the fiber-optic sensor can perform one-axis displacement sensing with a precision of 10 nm over a range of 3–4 μm at 100 Hz, where the specklegram acquisition rate is limited only by the speed of the camera. The new fiber-optic sensor is flexible, precise, and low-cost, which may find important applications in optical metrology and nanomanufacturing.

Near field spray measurements for fixed spray plate sprinklers

The spatially resolved spray scanning system (4S) is a new technology providing high-fidelity near-field spray characteristics generated by sprinklers, capturing all the physical parameters necessary to completely describe the sprinkler’s spray. In this study, a fixed spray plate sprinkler (FSPS) using three different deflector plates and two orifice sizes has been successfully characterized using the 4S. The current research demonstrates the potential of 4S technology to support irrigation sprinkler design optimization. The influence of the deflector plate design on spray characteristics can be clearly observed in the 4S measurements. It has been shown that deflector plate design (smooth, grooved, flat or two-level) generates deep changes in the important near-field spray properties. The baseline smooth-flat deflector plate generates a homogenous spray mainly concentrated at a polar angle of 90°. While the grooved-flat deflector plate presents a fluctuating spatial distribution, with similar volume flux and reference velocity, and larger drop sizes (~ 15%) as compared with baseline, two-stage deflector plates tend to redirect the main water flow from a polar angle of 90° to 110°, doubling the volume flux and resulting in an important increase in drop size (> 20%) and reference velocities (~ 7%) as compared with the baseline. In addition, measurements show frame arms induce local decreases in volume flux and drop velocity by as much as 60% compared to average values. While the ability of the 4S to provide high-fidelity near-spray descriptions of FSPS has been demonstrated, application to other sprinklers types such as impact, spinners and off-center wobbling could be limited and will be explored in future research.

Spectral properties of chiral electromagnetic near fields created by chiral plasmonic nanostructures

The surface-enhanced chiral-optical spectroscopy is based on the interaction of chiral molecules with chiral electromagnetic near field localized on a chiral plasmonic nanostructure. It is of fundamental importance to reveal the spectral characteristics of chiral near fields for maximizing the chiral interaction. Here we investigate relations between near field and far field polarization characteristics of the chiral plasmonic nanostructures, using electromagnetic simulations. We found that spectral features of chiral near fields created by the nanostructures intercorrelate with those of far field optical rotation. This finding may provide us a method to characterize and design the chiral near field.