Near-field Concentration Research Articles

Reduced-order modeling for parameterized large-eddy simulations of atmospheric pollutant dispersion

Mapping near-field pollutant concentration is essential to track plume dispersion in urban areas. By solving most of the turbulence spectrum, large-eddy simulations (LES) can accurately represent concentration spatial variability. Synthesizing this large amount of information to improve the accuracy of lower-fidelity operational models is particularly appealing. Due to the computational cost of LES, it becomes a challenge in multi-query contexts, to quantify how tracer dispersion changes with atmospheric and source parametric variations. We propose a data-driven reduced-order model (ROM) combining proper orthogonal decomposition (POD) and probabilistic Gaussian process regression (GPR) to efficiently predict tracer concentration field first- and second-order statistics. The novelty of this work lies in the original approach used to efficiently tune the GPR based on the available reduced-basis information and properties. In practice, the GPR hyperparameters are optimized through the hierarchy of POD modes thanks to Bayesian optimization informed by POD. The robustness of the approach is also addressed under small data constraint. A detailed analysis of our model performance is carried out in the case of a turbulent atmospheric boundary-layer flow over a surface-mounted obstacle for which the convective character of the system together with the uncertainty of the pollutant source location make it very challenging for ROM. The model is able to capture the wide range of spatial scales across the POD modes. The ROM accuracy remains acceptable for a minimal budget of about one hundred LES snapshots, providing a first estimation moving towards more realistic atmospheric dispersion applications.

Multi-scale bullseye antennas.

We design, simulate and experimentally characterize a multi-scale bullseye antenna for the broadband manipulation of microwaves. The device achieves far-field beam-forming via tailored diffraction at the interface between two concentric bullseye geometries, with near-field energy concentration resulting from the overlap of the diffracted beams. This article is part of the theme issue 'Wave generation and transmission in multi-scale complex media and structured metamaterials (part 1)'.

Synergy of Adsorption and Plasmonic Photocatalysis in the Au-CeO2 Nanosystem: Experimental Validation and Plasmonic Modeling.

Adsorption-mediated water treatment leaves adsorbents as secondary pollutants in the environment. However, photocatalysis aids in decomposing the contaminant into its nontoxic forms. In this context, we demonstrate an adsorption-photocatalysis pairing in Au-CeO2 nanocomposites for a total methylene blue (MB) removal from water. We synthesized Au-CeO2 through the citrate (cit) reduction method at different Au loading and studied its adsorption capacity with kinetics and thermodynamic models. We observe that the high adsorption capacity of Au-CeO2 is primarily because of the presence of Ce3+ states in CeO2 and citrate ligands on Au NPs. The Ce3+ states interact and transfer their electrons to supported Au NPs, rendering a negative charge over Au. The negatively charged Au surface and the carboxyl (-COO-) group of citrate ligands mediate an electrostatic interaction/adsorption of cationic MB. The total removal of MB is expedited under white light and lasers. A control experiment with Au NPs shows less adsorption-photocatalysis. The size of Au NPs and Au-CeO2 interfacial interaction is responsible for the surface plasmon resonance spectral position at 550-600 nm. Linear sweep voltammetry (LSV) and plasmonic field simulation show surface plasmon-driven photocatalysis in Au-CeO2. LSV shows a 3-fold higher photocurrent density in Au-CeO2 than colloidal Au NPs under white light. The simulated electric field intensity in Au-CeO2 is maximum at SPR excitation and the closest interfacial separation (d = 0 nm). The plasmon-driven photocatalysis in colloidal Au NPs is mainly due to the interaction of hot electrons with the adsorbed MB molecule. Notably, near-field light concentration, hot electrons, and interfacial charge separation are responsible for excellent MB removal in the Au-CeO2 nanosystem. The total MB removal through adsorption-photocatalysis pairing is 99.3% (Au-CeO2), 30.7% (Au NPs), and 13% (CeO2).

A Novel 3-D Printed Microwave Probe for ISM Band Ablation Systems of Breast Cancer Treatment Applications

In this article, a 3-D printed microwave (MW) probe is proposed to operate within 2.4–2.5-GHz industrial, scientific, and medical (ISM) band along with the numerical and experimental results for the local annihilation of contained tumor tissue that is the type of a human breast cancer cell line (MCF7) in MW ablation systems. The proposed MW ablation probe is fabricated by using an acrylonitrile butadiene styrene (ABS) material to construct a lightweight, cost effective, and easy to fabricate an MW applicator. The design principle of 3-D printed MW probe is based on the structural formation of coaxial transmission-line fed dielectric-loaded cone-shaped radiator for the near-field energy concentration in the close proximity of the cancerous tissue to be ablated at 2.45 GHz. Since conductive coating techniques have been improved recently, the inner surfaces of the designed MW applicator have been coated with the copper spray conveniently for the final fabricated probe prototype. The designed MW applicator has been numerically studied in CST MW Studio in an attempt to develop the near-field radiation performance, which can affect the ablation performance of MCF7 cells. The computation and experimental measurement results agree quite well with the indication of 3-D printed MW ablation probe to be utilized as an alternative medical device for cancer treatment.

Anapole-enabled RFID security against far-field attacks

Abstract Radio frequency identification (RFID) is a widely used wireless technology for contactless data exchange. Owing to international standardization and one-way security nature of the communication protocol, RFID tags, holding sensitive information, may be a subject to theft. One of the major security loopholes is the so-called far-field attack, where unauthorized interrogation is performed from a distance, bypassing the user’s verification. This loophole is a penalty of using a dipole-like RFID tag antenna, leaking wireless information to the far-field. Here we introduce a new concept of anapole-enabled security, prohibiting far-field attacks by utilizing fundamental laws of physics. Our design is based on radiationless electromagnetic states (anapoles), which have high near-field concentration and theoretically nulling far-field scattering. The first property enables performing data readout from several centimeters (near-field), while the second prevents attacks from a distance, regardless an eavesdropper’s radiated power and antenna gain. Our realization is based on a compact 3 cm high-index ceramic core–shell structure, functionalized with a thin metal wire and an integrated circuit to control the tag. Switching scheme was designed to provide a modulation between two radiation-less anapole states, blocking both up and down links for a far-field access. The anapole tag demonstrates more than 20 dB suppression of far-field interrogation distance in respect with a standard commercial tag, while keeping the near-field performance at the same level. The proposed concept might significantly enhance the RFID communication channel in cases, where information security prevails over cost constrains.

Smartphone-based surface plasmon resonance imaging for near-field concentration mapping

In this work, smartphone-based surface plasmon resonance imaging is used for the near-field mapping of a liquid sample’s concentration distribution. The principal design uses a special coupler based on a parallel incident light beam with p-polarization from a smartphone screen; this light is used to excite surface plasmons on a gold layer 50 nm thick. Using a bespoke application, surface plasmon resonance (SPR) intensity changes are detected in the SPR images captured with the front-facing camera. The SPR intensity is converted to the corresponding concentration using the three-layer Fresnel equation. Concentration mapping observations are presented for the dilution of ethanol dilution by water in a flow cell and an array of chlorine droplets on a gold surface. The high efficiency of this device means that it can be used for photonics research and advanced laboratory experiments.

Measurements and predictions of harmful releases of the gathering station over the mountainous terrain

Having a risk analysis of harmful releases over mountainous terrains through wind tunnel experiment is a frontier problem in China. In this paper, a straight-flow wind tunnel is applied to simulate the atmospheric boundary layer and research the motion of high-sulfur gas released to atmosphere when accidental releases occur in a gathering station over the mountainous terrain. After an analysis of hourly concentration in the field accident for eight wind directions, experimental results reveal that nearby concentration fields are dominated by wind and far-field concentration distribution is dominated by topography, which leads to complete levels of consequence impact for the personnel risk inside and around the gathering station. Based on CFD techniques, a three-dimensional modelling was established in comparison with the wind tunnel experiment, which suggests that CFD prediction had underestimated the near-field gas concentration and the performance could not precisely match actual risks the gathering station causes to the mountainous terrain, which leads to a modified equation for numerical prediction. Instead of proposing a lower personnel risk evaluation obtained through the use of CFD techniques, the wind tunnel experiment offers a new choice for the consequence impact analysis for the petrochemical industry in China.

Anapole mediated giant photothermal nonlinearity in nanostructured silicon

Featured with a plethora of electric and magnetic Mie resonances, high index dielectric nanostructures offer a versatile platform to concentrate light-matter interactions at the nanoscale. By integrating unique features of far-field scattering control and near-field concentration from radiationless anapole states, here, we demonstrate a giant photothermal nonlinearity in single subwavelength-sized silicon nanodisks. The nanoscale energy concentration and consequent near-field enhancements mediated by the anapole mode yield a reversible nonlinear scattering with a large modulation depth and a broad dynamic range, unveiling a record-high nonlinear index change up to 0.5 at mild incident light intensities on the order of MW/cm2. The observed photothermal nonlinearity showcases three orders of magnitude enhancement compared with that of unstructured bulk silicon, as well as nearly one order of magnitude higher than that through the radiative electric dipolar mode. Such nonlinear scattering can empower distinctive point spread functions in confocal reflectance imaging, offering the potential for far-field localization of nanostructured Si with an accuracy approaching 40 nm. Our findings shed new light on active silicon photonics based on optical anapoles.

Measurements of helium distributions in a scaled-down parking garage model for unintended releases from a fuel cell vehicle

Most previous studies of hydrogen fuel cell vehicle (HFCV) releases in garages have focused on residential garages. However, hydrogen releases in large underground parking garages can be completely different with more concerns about public safety. In the present work, unintended hydrogen releases were studied in a scaled-down underground parking garage model to fill the knowledge gap concerning hydrogen dispersion in a large confined space. Helium was used in place of hydrogen for safety reasons. Gas sensors were used to measure the helium concentrations along the ground and the ceiling. The nearfield concentration distributions were studied to show that the leak jet was strongly affected by the buoyancy, so the gas concentration was very low along the floor with a flammable gas mixture only appearing right under the leaking car. The gas plume driven by the buoyancy reached the ceiling and spread in all directions resulting in a flammable layer along the ceiling. The effects of neighboring cars and walls were also investigated to show that the obstacles affected the nearfield gas dispersion but did not significantly affect the farfield concentration distributions. The concentration increased linearly with the release flow rate for the flow rates in this study. The gas dispersion during the release showed self-similar characteristics, which could be used for leak detection. This database will be useful for model validation for modeling HFCV releases in large underground parking garages and also provides guidelines for gas sensor placement in large confined spaces.

(Invited) Macroscopically Aligned Carbon Nanotubes for Efficient Thermophotovoltaic Energy Conversion

Single-walled carbon nanotubes (SWCNTs) exhibit many unique properties arising from their quantum confinement. However, the lack of a controllable large-scale alignment procedure had limited the exploration of their unique properties. Our recent work has overcome this problem by demonstrating a vacuum-filtration based technique to align SWCNTs on wafer-scale. These aligned SWCNT films exhibit many interesting optical properties such as extreme optical anisotropy or hyperbolic dispersion. Besides, aligned SWCNTs are flexible, refractory (stable up to 1900 K), inexpensive, and based on earth-abundant carbon making them an excellent refractory infrared nanophotonic material platform. Such a refractory infrared material platform is exactly what is necessary to enable efficient thermophotovoltaic energy conversion.Thermophotovoltaics is a solid-state heat-to-electricity conversion technique. It relies upon photovoltaic conversion of thermal light emanating from a hot surface to electricity. The broadband nature of thermal light makes the thermophotovoltaic conversion inefficient. Optical resonators, filters, spectrally selective thermal emitters are a few methods proposed to increase the efficiency of thermophotovoltaics. Selective thermal emitters are very promising due to their ease of design and integration. Selective thermal emitters are hot surfaces that emit thermal radiation in a narrow spectral band above the bandgap of the photovoltaic cell. The suppression of thermal emission below the bandgap and enhancement above the bandgap are the key requirements for efficient thermophotovoltaic conversion. Generally, optical properties of materials at high temperatures limit the performance of such selective emitters and the current efficiency record is about 8%. But, the hyperbolic optical property of aligned SWCNTs together with their thermal stability make aligned SWCNTs ideal for efficient thermophotovoltaics.Here in this work, we study the infrared optical properties of aligned SWCNTs at high temperatures (1000 K) and demonstrate spectrally selective thermal emission from plain and nano-patterned thin films. Using deep sub-wavelength sized optical cavities, we show that aligned SWCNTs exhibit hyperbolic dispersion in almost of all of infrared and enhance thermal photon density by at least 100 times. Such a large enhancement in thermal photon density leads to a large suppression of unwanted sub-bandgap thermal photons and a large enhancement of above-bandgap photons. Our analysis shows that aligned SWCNTs enable 30% or higher thermophotovoltaic efficiency when operating at 1300 K with no concentration. With near-field concentration, aligned SWCNTs can allow conversion efficiencies over 50%.

Plasmon Controlled Shaping of Metal Nanoparticle Aggregates by Femtosecond Laser-Induced Melting.

In this work, we show how to control the morphology of femtosecond laser melted gold nanosphere aggregates. A careful choice of both laser fluence and wavelength makes it possible to selectively excite different aggregate substructures to produce larger spherical nanoparticles, nanorods, and nanoprisms or necklace-like 1D nanostructures in which the nanoparticles are interlinked by bridges. Finite integral technique calculations have been performed on the near-field concentration of light in the nanostructures which confirm the wavelength dependence of the light concentration and suggest that the resulting localized high intensities lead to nonthermal melting. We show that by tuning the wavelength of the melting light it is possible to choose the spatial extension of the ensembles of NPs heated thus allowing us to exhibit control over the morphology of the nanostructures formed by the melting process. By a proper combination of this method with self-assembly of chemically synthesized nanoparticles, one can envisage the development of an innovative high-throughput high-resolution nanofabrication technique.

Light Trapping with Silicon Light Funnel Arrays.

Silicon light funnels are three-dimensional subwavelength structures in the shape of inverted cones with respect to the incoming illumination. Light funnel (LF) arrays can serve as efficient absorbing layers on account of their light trapping capabilities, which are associated with the presence of high-density complex Mie modes. Specifically, light funnel arrays exhibit broadband absorption enhancement of the solar spectrum. In the current study, we numerically explore the optical coupling between surface light funnel arrays and the underlying substrates. We show that the absorption in the LF array-substrate complex is higher than the absorption in LF arrays of the same height (~10% increase). This, we suggest, implies that a LF array serves as an efficient surface element that imparts additional momentum components to the impinging illumination, and hence optically excites the substrate by near-field light concentration, excitation of traveling guided modes in the substrate, and mode hybridization.

Nanoscale Mapping and Spectroscopy of Nonradiative Hyperbolic Modes in Hexagonal Boron Nitride Nanostructures.

The inherent crystal anisotropy of hexagonal boron nitride (hBN) provides the ability to support hyperbolic phonon polaritons, that is, polaritons that can propagate with very large wave vectors within the material volume, thereby enabling optical confinement to exceedingly small dimensions. Indeed, previous research has shown that nanometer-scale truncated nanocone hBN cavities, with deep subdiffractional dimensions, support three-dimensionally confined optical modes in the mid-infrared. Because of optical selection rules, only a few of the many theoretically predicted modes have been observed experimentally via far-field reflection and scattering-type scanning near-field optical microscopy (s-SNOM). The photothermal induced resonance (PTIR) technique probes optical and vibrational resonances overcoming weak far-field emission by leveraging an atomic force microscope (AFM) probe to transduce local sample expansion caused by light absorption. Here we show that PTIR enables the direct observation of previously unobserved, dark hyperbolic modes of hBN nanostructures. Leveraging these optical modes and their wide range of angular and radial momenta could provide a new degree of control over the electromagnetic near-field concentration, polarization in nanophotonic applications.

A Comparative analysis of nanoparticle type variants for Plasmonic light trapping enhancement in thin film hydrogenated amorphous silicon solar cells

The new possibilities for the photoelectric conversion enhancement of solar cells are explored by the phenomenon of plasmonics with nanoparticles having higher charge carrier concentration deposited on to the substrate with the intention of meeting the energy demands by renewable sources over the fossil fuels. A systematic simulative analysis is performed to understand the role of different types of nanoparticles on the thin film hydrogenated amorphous silicon (a-Si:H) solar cells upon deposited on the front surface exposed to the incident light radiations. The three-dimensional simulations with finite difference time domain approach are performed to monitor enhanced optical absorption induced by localized surface plasmon, quantified by the near-field concentration and the modelled scattering and absorption cross-sections of nanoparticles in the interesting range of wavelength. The paper investigates the plasmonic coupling of light into thin film a-Si:H structures in terms of dielectric properties providing an option to narrow down to cost-effective nanoparticles for the enhanced performance across the visible range of the spectrum in the interest of large-scale manufacturability. The search for new functional NP materials covering noble metals and few nitrides and their contribution to light trapping enhancement is analysed for obtaining the highest device absorption with low NP parasitic absorption. For the optimal design of thin film a-Si:H solar cell, it’s demonstrated by the obtained results that the NP type is a choice between the dominant scattering over the undesired reflection and parasitic losses.

"Bright" plasmons of triangle or square cluster of nanowires made of noble metals

Plasmon resonances of the "bright" plasmons of triangle or square cluster of coupled nanowires made of noble metal are theoretically investigated. The effect of the spatial configuration of silver and gold nanowires on the sensitivity of hybrid plasmons to changes in the properties of the external medium and on the possibility of near-field concentration is studied.

Interzonal airflow rates for use in near-field far-field workplace concentration modeling

ABSTRACTInterzonal air flow rates (β) for a workspace above a table were measured in 12 indoor air spaces using an experimental apparatus simulating a vapor release into an occupied near zone. The near field was modeled as a 0.32 m3 rectangular cube volume 0.60 m high above the 0.60 m × 0.90 m table. A total of 74 experimental measurements of β were made. The apparatus consisted of photoionization detectors measuring concentrations of acetone around an evaporating liquid surface with a robot arm simulating worker motion in the near field. The vapor release rate and the resulting concentrations were used in a near-field far-field (NF-FF) model to calculate β. Measures of mixing within the near-field supported the assumption of the NF-FF model that the near field is well-mixed. Measured values of β ranged from 0.4–19 m3/min with an average of 4.8 m3/min. This corresponds to 1.2–59 air changes per minute in the near field and an average of 15 air changes per minute. The values of β were log normally distributed with a geometric mean of 3.4 m3/min and a geometric standard deviation of 2.3. The 95% confidence interval on the geometric mean of β was 2.8–4.2 m3/min. The product of the random air speed in the room and one half of the near-field free surface area was shown to be a good method of determining β. There was a slight correlation seen between room volume and β, but the effect size was small. Room air change rate was not found to be correlated with β. The observed distribution of β will be helpful in selecting values for interzonal airflow rate in NF-FF modeling of worker exposures.

Near-Field Optical Drilling of Sub-λ Pits in Thin Polymer Films.

Under UV illumination, polymer films can undergo chain scission and contract. Using this effect, tightly focused laser light is shown to develop runaway near-field concentration that drills sub-100 nm pits through a thin film. This subwavelength photolithography can be controlled in real time by monitoring laser scatter from the evolving holes, allowing systematic control of the void diameter. Our model shows how interference between the substrate and film together with near-field focusing by the evolving crevice directs this formation and predicts minimum pit sizes in films of 100 nm thickness on gold substrates. The smallest features so far are 60 nm diameter pits using 447 nm light focused onto polystyrene through a ×100 objective (NA = 0.8). Such arrays of pits can be easily used as masks for fabricating more complex nanostructures, such as plasmonic nanostructures and biomicrofluidic devices. This demonstration shows the potential for harnessing near-field feedback in optical direct-writing for nanofabrication.

Performance enhancement of polymer solar cells by incorporating Ag nanoparticles at an indium tin oxide/MoO3 buffer layer interface

In the present study, the plasmonic effects of silver nanoparticles (Ag NPs) have been utilized to improve the performance of polymer solar cells. Plasmonic polymer solar cells were fabricated by incorporating Ag NPs at the indium tin oxide (ITO)/MoO3 buffer layer interface. The size of the Ag NPs has been optimized to improve the device performance. The optimized device with 75 nm sized Ag NPs shows 26% enhancement in the power conversion efficiency (PCE) compared with the reference device. The short circuit current density mainly contributes to the efficiency enhancement compared with other device parameters. The diffuse reflectance measurements indicate that there is significant enhancement in the optical absorption of active layers when Ag NPs are incorporated into the devices. The scattering cross section of Ag NPs and electric field intensity distribution around the nanoparticles were modeled by finite-difference-time-domain simulations. The experimental results and theoretical simulations revealed that both far-field scattering and near-field concentration of light contributed significantly to the PCE enhancement of the devices.

Trajectory Measurements of a Horizontally Oriented Buoyant Jet in a Coflow Using Filtered Rayleigh Scattering

AbstractThe filtered Rayleigh scattering technique was implemented to discern the mechanisms associated with trajectory and mixing characteristics of buoyant jets in the presence of a coflow. A continuous wave laser in conjunction with a high-speed camera and a molecular filter constituted the equipment needed to obtain near-field concentration measurements of a carbon dioxide jet within a coflow of air. The arrangement enabled carbon dioxide concentration measurements with and without the coflow at a sampling rate of 400 Hz. The time-averaged results demonstrate the significance that adding the coflow has on increasing the mixing rate, thus reducing the impact of the buoyant jet by flattening the jet trajectory. The effects of various flow parameters such as the jet velocity, the jet to coflow velocity ratio, the relative velocity between the jet and the coflow, and the Froude number on the jet trajectory are studied. The downward trajectory of the carbon dioxide jet, absent a coflow, was well correlated...

Defect Engineering in Plasmonic Metal Oxide Nanocrystals.

Defects may tend to make crystals interesting but they do not always improve performance. In doped metal oxide nanocrystals with localized surface plasmon resonance (LSPR), aliovalent dopants and oxygen vacancies act as centers for ionized impurity scattering of electrons. Such electronic damping leads to lossy, broadband LSPR with low quality factors, limiting applications that require near-field concentration of light. However, the appropriate dopant can mitigate ionized impurity scattering. Herein, we report the synthesis and characterization of a novel doped metal oxide nanocrystal material, cerium-doped indium oxide (Ce:In2O3). Ce:In2O3 nanocrystals display tunable mid-infrared LSPR with exceptionally narrow line widths and the highest quality factors observed for nanocrystals in this spectral region. Drude model fits to the spectra indicate that a drastic reduction in ionized impurity scattering is responsible for the enhanced quality factors, and high electronic mobilities reaching 33 cm(2)V(-1) s(-1) are measured optically, well above the optical mobility for tin-doped indium oxide (ITO) nanocrystals. We investigate the microscopic mechanisms underlying this enhanced mobility with density functional theory calculations, which suggest that scattering is reduced because cerium orbitals do not hybridize with the In orbitals that dominate the bottom of the conduction band. Ce doping may also reduce the equilibrium oxygen vacancy concentration, further enhancing mobility. From the absorption spectra of single Ce:In2O3 nanocrystals, we determine the dielectric function and by simulation predict strong near-field enhancement of mid-IR light, especially around the vertices of our synthesized nanocubes.