Spatial Nonuniformity Research Articles

Increasing Laser-Doping Depth of Al in 4H-SiC by Using Expanded-Pulse Excimer Laser

Al doping into 4H-SiC performed by irradiating pulse-width-expanded excimer laser to an Al film deposited on the 4H-SiC surface is investigated. An optical pulse stretcher was constructed to produce the laser pulse whose peak intensity was reduced as half as that of the original pulse and pulse width was expanded from 55 ns to 100 ns. The irradiation of the expanded pulses is found to reduce the ablation of the materials from the surface and enable irradiation of multiple shots. As the result, doping depth of Al is significantly increased. The multiple shots of the expanded pulses is also fund to decrease the sensitivity to spatial non-uniformity of laser intensity and increase the uniformity of doped region.

Investigation of speckle imagery spectral estimation challenges for modulation transfer function measurements

Efforts to extend speckle-based focal plane array modulation transfer function measurements beyond the detector Nyquist frequency have unearthed challenging spectral estimation issues. In an attempt to better understand the task of speckle imagery spectral estimation, we explore the nuances of various estimation techniques, making comparisons using both real speckle imagery and simulated data. Parameters and features of the techniques investigated include number of image realizations, the size of image realizations, and applications of windows to speckle imagery spectral estimation. Real-world testing considerations such as laser stability and the challenge of collecting significant numbers of independent image realizations are addressed in the analysis. Results from this research show the advantage increasing the number of realizations has on estimation variance, the robustness of smaller realization segments when battling speckle field imagery spatial nonuniformities, the benefits of windowing image segments with regard to power spectral density estimation accuracy, and the impact that the increasing aperture area has on system signal-to-noise ratio.

Paired cycle-GAN-based image correction for quantitative cone-beam computed tomography.

The incorporation of cone-beam computed tomography (CBCT) has allowed for enhanced image-guided radiation therapy. While CBCT allows for daily 3D imaging, images suffer from severe artifacts, limiting the clinical potential of CBCT. In this work, a deep learning-based method for generating high quality corrected CBCT (CCBCT) images is proposed. The proposed method integrates a residual block concept into a cycle-consistent adversarial network (cycle-GAN) framework, called res-cycle GAN, to learn a mapping between CBCT images and paired planning CT images. Compared with a GAN, a cycle-GAN includes an inverse transformation from CBCT to CT images, which constrains the model by forcing calculation of both a CCBCT and a synthetic CBCT. A fully convolution neural network with residual blocks is used in the generator to enable end-to-end CBCT-to-CT transformations. The proposed algorithm was evaluated using 24 sets of patient data in the brain and 20 sets of patient data in the pelvis. The mean absolute error (MAE), peak signal-to-noise ratio (PSNR), normalized cross-correlation (NCC) indices, and spatial non-uniformity (SNU) were used to quantify the correction accuracy of the proposed algorithm. The proposed method is compared to both a conventional scatter correction and another machine learning-based CBCT correction method. Overall, the MAE, PSNR, NCC, and SNU were 13.0HU, 37.5dB, 0.99, and 0.05 in the brain, 16.1HU, 30.7dB, 0.98, and 0.09 in the pelvis for the proposed method, improvements of 45%, 16%, 1%, and 93% in the brain, and 71%, 38%, 2%, and 65% in the pelvis, over the CBCT image. The proposed method showed superior image quality as compared to the scatter correction method, reducing noise and artifact severity. The proposed method produced images with less noise and artifacts than the comparison machine learning-based method. The authors have developed a novel deep learning-based method to generate high-quality corrected CBCT images. The proposed method increases onboard CBCT image quality, making it comparable to that of the planning CT. With further evaluation and clinical implementation, this method could lead to quantitative adaptive radiation therapy.

Molecular beam epitaxy growth of GaSb1-xBix without rotation

GaSb1-xBix thin film was grown on a 2 inch GaSb substrate by molecular beam epitaxy (MBE) without substrate rotation. Bi composition is found to vary from 2.76% to 3.98% across the wafer. The distribution of Bi content is mainly determined by spatial non-uniformity of Sb/Ga flux ratio, while Bi flux has slightly influence. Ostwald ripening process is confirmed to be reason for bigger Bi droplets via Bi surface diffusion. With the increase of Sb/Ga flux ratio, Ostwald ripening process is suppressed. At high Bi flux, excess Ga atoms accumulate on surface and form droplets.

Atmospheric pressure plasma activation of water droplets

Low temperature plasma treatment of water is being investigated due to its use in pollution abatement, wound treatment and agriculture. Plasma produced reactive oxygen and nitrogen species (RONS) are formed in the gas phase and solvate into the liquid. Activation of the liquid is often limited by transport of these RONS to the liquid surface. Micrometer scale droplets immersed in the plasma have a large surface to volume ratio, which increases the interaction area for a given volume of water, and can increase the rates of transport from the gas to liquid. In this paper, results from 0 and 2D modeling of air-plasma activation of water micro-droplets are discussed. The solvation dynamics are sensitive to the Henry’s law constant (h) of each species, which describes its hydrophobicity (low h) or hydrophilicity (high h). The liquid densities of stable species with high h values (e.g. H2O2, HNOx) are sensitive to droplet diameter. For large droplets, hydrophilic species may deplete the gas-phase inventory of RONS before liquid-phase saturation is reached, limiting the total in-liquid density for species with high h. For smaller droplets, higher average in-droplet densities of these species can be produced. Liquid concentrations of stable species with low h (e.g. O3, N2O, H2) had a weak dependence on droplet size as droplets are quickly saturated and solvation does not deplete the gas phase. An analysis of this behavior is discussed using the well-stirred reactor (0D) approximation. Spatial non-uniformity of the plasma also has an impact on the solvation rates and kinetics. Gas phase depletion of high-h species leads to a decrease in solvation rates. Low-h species that saturate the surface of the droplets during plasma-on periods can quickly de-solvate in the afterglow.

Shading correction for volumetric CT using deep convolutional neural network and adaptive filter.

Shading artifact may lead to CT number inaccuracy, image contrast loss and spatial non-uniformity (SNU), which is considered as one of the fundamental limitations for volumetric CT (VCT) application. To correct the shading artifact, a novel approach is proposed using deep learning and an adaptive filter (AF). Firstly, we apply the deep convolutional neural network (DCNN) to train a human tissue segmentation model. The trained model is implemented to segment the tissue. According to the general knowledge that CT number of the same human tissue is approximately the same, a template image without shading artifact can be generated using segmentation and then each tissue is filled with the corresponding CT number of a specific tissue. By subtracting the template image from the uncorrected image, the residual image with image detail and shading artifact are generated. The shading artifact is mainly low-frequency signals while the image details are mainly high-frequency signals. Therefore, we proposed an adaptive filter to separate the shading artifact and image details accurately. Finally, the estimated shading artifacts are deleted from the raw image to generate the corrected image. On the Catphan©504 study, the error of CT number in the corrected image's region of interest (ROI) is reduced from 109 to 11 HU, and the image contrast is increased by a factor of 1.46 on average. On the patient pelvis study, the error of CT number in selected ROI is reduced from 198 to 10 HU. The SNU calculated from the ROIs decreases from 24% to 9% after correction. The proposed shading correction method using DCNN and AF may find a useful application in future clinical practice.

Evolution of averaged local premixed flame thickness in a turbulent flow

In the combustion literature, contradictory results on the influence of turbulence on the local thickness of a premixed flame can be found and the present paper aims at contributing to reconcile this issue. First, different measures of local flame thickness in a turbulent flow, e.g. area-weighted and unweighted surface-averaged values of (i) |∇c|, i.e., the absolute value of 3D gradient of the combustion progress variable c, or (ii) 1/|∇c|, are studied and analytical relationships/inequalities between them are obtained. Second, the evolution of the different flame thickness measures is explored by numerically evaluating them, as well as various terms in relevant evolution equations derived analytically. To do so, various measures and terms are extracted from DNS data obtained from (i) a highly turbulent, constant-density, dynamically passive, single-reaction wave, (ii) moderately and highly turbulent, single-step-chemistry flames, and (iii) moderately and highly turbulent, complex-chemistry lean methane-air flames. In all those cases, all studied flame thickness measures are reduced during an early stage of premixed turbulent flame development, followed by local flame re-broadening at later stages. Analysis of various terms in the aforementioned evolution equations shows that the initial local flame thinning is controlled by turbulent strain rates. The subsequent local flame re-broadening is controlled by (i) curvature contribution to the stretch rate, which counter-balances the strain rate, (ii) spatial non-uniformities of the normal diffusion contribution to the local displacement-speed vector Sdn, and (iii) dilatation, which plays an important role in moderately turbulent flames, but a minor role in highly turbulent flames. Moreover, the present study shows that differently defined measures of a local flame thickness can be substantially different. This difference should also be borne in mind when comparing data that indicate local flame thinning with data that indicate local flame broadening.

General methods for the characterization of the nonuniformity of a radiometric calibration source.

The spatial nonuniformity of a radiometric calibration source is key to its performance, uncertainty budget, and utility. Multiple disparate approaches are currently used to assess nonuniformity; thus, it is critical to understand how nonuniformity is calculated when comparing radiometric calibration sources. Two factors are key in assessing nonuniformity: (1) the methodology chosen to calculate the nonuniformity and (2) how the intensity of the light is sampled. Here, we present and compare different methodologies of calculating spatial nonuniformity. In general, we find that these methods can be categorized into two groups: those in which the equations assume a Gaussian population distribution and those in which the equations can be applied to any shape of distribution. Additionally, we compare different methods of sampling the light intensity within the region of interest. We show that peak deviation nonuniformity analysis, in which the pixels of the image of the illumination source are resampled into 69 equally sized bins encompassing the region of interest, can be used to effectively assess the nonuniformity of any radiance distribution. Furthermore, the nonuniformity determined by this method allows for an accurate comparison to industry standard nonuniformity metrics.

The Role of Oxygen Atoms on Excitons at the Edges of Monolayer WS2.

We clarify that the chemisorption of oxygen atoms at the edges is a key contributor to the frequently observed edge enhancement and spatial non-uniformities of photoluminescence (PL) in WS2 monolayers. Here we have investigated with momentum- and real-space nanoimaging of the chemical and electronic density inhomogeneity of WS2 flakes. Our finding from a large panoply of techniques together with density functional theory calculation confirms that the oxygen chemisorption leads to the electron accumulation at the edges. This facilitates the trion dominance of PL at the edges of WS2 flakes. Our results highlight and unravel the significance of chemisorbed oxygen at the edges in the PL emission and electronic structure of WS2, providing a viable path to enhance the performance of transition-metal-dichalcogenide-based devices.

Realization of red shift of absorption spectra using optical near-field effect

In many applications such as CO2 reduction and water splitting, high-energy photons in the ultraviolet region are required to complete the chemical reactions. However, to realize sustainable development, the photon energies utilized must be lower than the absorption edge of the materials including the metal complex for CO2 reduction, the electrodes for water splitting, because of the huge amount of lower energy than the visible region received from the sun. In the previous works, we had demonstrated that optical near-fields (ONFs) could realize chemical reactions, by utilizing photon energies much lower than the absorption edge because of the spatial non-uniformity of the electric field. In this paper, we demonstrate that an ONF can realize the red shift of the absorption spectra of the metal-complex material for photocatalytic reduction. By attaching the metal complex to ZnO nano-crystalline aggregates with nano-scale protrusions, the absorption spectra by using diffuse reflection of the metal complex can be shifted to a longer wavelength by 10.6 nm. The results of computational studies based on a first-principles computational program including the ONF effect provide proof of the increase in the absorption of the metal complex at lower photon energies. Since the near-field assisted field increase improves the carrier excitation in the metal-complex materials, this effect may be universal and it could applicable to CO2 reduction using the other metal-complex materials, as well as to the other photo excitation process including water splitting.

Scatter correction for a clinical cone-beam CT system using an optimized stationary beam blocker in a single scan.

Scatter contamination in the cone-beam CT (CBCT) leads to CT number inaccuracy, spatial nonuniformity, and loss of image contrast. In our previous work, we proposed a single scan scatter correction approach using a stationary partial beam blocker. Although the previous method works effectively on a tabletop CBCT system, it fails to achieve high image quality on a clinical CBCT system mainly due to the wobble of the LINAC gantry during scan acquisition. Due to the mechanical deformation of CBCT gantry, the wobbling effect is observed in the clinical CBCT scan, and more missing data present using the previous blocker with the uniformly distributed lead strips. An optimal blocker distribution is proposed to minimize the missing data. In the objective function of the missing data, the motion of the beam blocker in each projection is estimated using the segmentation due to its high contrast in the blocked area. The scatter signals from the blocker are also estimated using an air scan with the inserted blocker. The final image is generated using the forward projection to compensate for the missing data. On the Catphan©504 phantom, our approach reduces the average CT number error from 86 Hounsfield unit (HU) to 9HU and improves the image contrast by a factor of 1.45 in the high-contrast rods. On a head patient, the CT number error is reduced from 97 HU to 6 HU in the soft-tissue region and the image spatial nonuniformity is decreased from 27% to 5%. The results suggest that the proposed method is promising for clinical applications.

Destriping Algorithms Based on Statistics and Spatial Filtering for Visible-to-Thermal Infrared Pushbroom Hyperspectral Imagery

Following calibration, hyperspectral images remain affected by spatial dimension nonuniformity, i.e., stripe noise, due to stray light interference, slit contamination, and instrument instability. The full spectrum airborne hyperspectral imager (FAHI) is a Chinese next-generation pushbroom sensor with a spectral range covering the visible near-infrared, shortwave-infrared (SWIR), and thermal infrared regions with spectral sampling intervals of 2.34, 3, and 32 nm, respectively. However, the residual stripe noise remains in FAHI images after relative radiometric correction based on the laboratory calibration, especially in low signal-to-noise ratio bands. To solve this problem, a new technique combining image statistics and spatial filtering algorithms has been developed for FAHI image correction. In this method, image statistics are obtained to calculate the gain and offset of each pixel for image nonuniformity correction. Then, a spatial filter removes the residual stripes. This paper presents the principles of this method along with details of validation experiments and results. To validate the effectiveness of the proposed method, comparison with two destriping methods and quantitative analyses are carried out. Moreover, the method is applied to an SWIR hyperspectral image from the TianGong-1 spacecraft, yielding good results. The experimental results suggest that the proposed method is convenient and practical for improving the relative radiometric accuracy of airborne/spaceborne hyperspectral images.

Laminar dispersion at low and high Peclet numbers in a sinusoidal microtube: Point-size versus finite-size particles

This paper adopts Brenner’s homogenization theory to investigate dispersion properties, over a wide range of Peclet values, of point-size and finite-size particles in sinusoidal cylindrical microchannels in the presence of a pressure-driven Stokes flow field. The periodic alternation of entropic barriers/traps can unexpectedly increase the effective finite-size particle velocity as well as decrease the effective dispersion coefficient for both point-size and finite-size particles, for large values of the radial Peclet number. While this phenomenon has a simple explanation for tracer particles, its understanding for finite-size particles is not trivial and goes through the analysis of the localization feature of the equilibrium unit-cell particle density w0(x) and how this spatial nonuniformity impacts upon the effective particle velocity and on the solution of the so-called b field, controlling the large scale axial dispersion coefficient. Unfortunately, dispersion reduction cannot be exploited for the sake of the separation of particles having different radii because the separation performance of a hydrodynamic sinusoidal column turns out to be worse than that of a standard straight column for experimentally feasible Peclet values. Interesting analytical results for long-wavelength sinusoidal channels are obtained by a long-wave asymptotic expansion. Both zero-order and first-order terms for the asymptotic expansion of the w0(x) measure and of the b field are obtained, thus exploring a wide range of Peclet values and deriving an analytical expression for the Taylor dispersion coefficient.

Wavefront correction for spatial nonuniformity of the liquid crystal on silicon based spatial light modulator

Liquid crystal on silicon (LCoS) based spatial light modulator (SLM) is widely used in many research areas, such as holography display, optical measurement, and optical tweezers, due to its programmable capability of phase modulation. However, a certain aberration term to the incident wavefront is introduced due to the spatial nonuniformity of the LCoS backplane, and the correction of this phase aberration is important to improve the accuracy of phase modulation by the SLM, which will then influence the SLM applications, e.g., the image quality in holography display, or the beam shaping for optical tweezers and free-space communications. Fortunately, the LCoS based SLM has the programmable capability of phase modulation; therefore, the spatial nonuniformity can be compensated by a wavefront correction. Conventionally, this aberration is corrected by adjusting the Zernike coefficients of the generated wavefront manually. However, this procedure is time-consuming and subjective, since there are dozens of coefficients to be adjusted, and the adjustment is evaluated manually by monitoring the interferogram. In this paper, we propose to use several phase retrieval techniques to measure the corresponding wavefront directly and employ the obtained wavefront to correct the aberration introduced by the spatial nonuniformity. A Twyman-Green interferometry is used, and an interferogram acquisition technique based on the self wavefront modulation of SLM is proposed. Besides, a method for determining the corresponding active area of the LCoS backplane and parameters of 2-D projective transformation is proposed to transform the active area to the fitted resolution of the SLM. RMS value of the aberration to approximately λ/32 can be obtained by the proposed method. Consequently, more accurate phase modulation by SLM for applications such as holography display and beam shaping is achievable. Effectiveness of the proposed method is validated by experiment, and the correction results by different phase retrieval techniques are evaluated and compared.

Application of Al-Cu-W-Ta graded density impactors in dynamic ramp compression experiments

Graded density impactors (GDIs) are used to dynamically compress materials to extreme conditions. Two modifications to a previously developed Mg-Cu-W GDI are made in this work before using it in a dynamic compression experiment: Mg is replaced with Al and a Ta disk is glued to the back. The Mg phase is replaced by Al because FCC Al remains solid to higher pressure along its Hugoniot compared to Mg. The addition of the Ta disk creates a constant particle velocity regime and facilitates a definition of peak pressure states. Microstructure analysis, profilometry, and ultrasonic C-scans of the Al-Cu-W GDI all confirm excellent uniformity. We evaluated signal variation in the radial direction of a dynamically compressed Al-LiF bilayer target to evaluate the contribution of spatial nonuniformity to errors. Velocity traces from five photon Doppler velocimetry (PDV) probes located at different radial distances from the center of the target varied at most by 1.1% with a root mean square of 0.3% during the compression ramp, demonstrating low PDV measurement error over a relatively large experimental area. The experimental PDV data also agrees well with 1D simulations that use inputs from predictive characterization models developed for the material properties resulting from tape casting, laminating, and powder consolidation processes. Low measurement error during quasi-isentropic compression, leading to better precision, ensures a robust platform to reach extreme compression and low-temperature recovery states and facilitates discovery via synthesis, quenching, and preservation of new high-pressure phases.

Calibration of 2‐D soil zymography for correct analysis of enzyme distribution

Soil zymography is a new technique developed to visualize two‐dimensional distributions of enzyme activities. The method consists of incubating a membrane saturated with an enzyme‐specific fluorogenic substrate on a surface of the soil sample, followed by recording the membrane image generated by a fluorescent product (e.g. MUF: methylumbelliferone) in ultraviolet light. Despite its relative ease of use, performing zymography involves multiple user‐made decisions that might affect the accuracy of enzyme activity estimates. Therefore, unification of the zymography methodology is required for correct estimations and comparisons of various studies. We evaluated the following methodological aspects of the implementation of zymography: (a) camera settings and image processing, (b) effects of evaporation and (c) calibration procedures. Camera settings (shutter speeds or exposure time) affected the intensity of background fluorescence and signal‐to‐noise ratios (SNR). However, because their combined effects varied depending on MUF concentrations, light and camera setting need to be optimized for the expected range of MUF concentrations prior to zymography. Evaporation of MUF solution from the membrane had no effect on fluorescence. Relations between MUF concentration and intensity of fluorescence during calibrations demonstrated a saturated pattern and were strongly affected by image noise outside the optimal range (e.g. 8–14 μm MUF pixel−1). We developed a new calibration approach that is based on a piecewise linear regression. The new approach accounted for specific ranges of MUF concentration and uses nonuniformly saturated membranes, reflecting the real distribution of enzyme activities in soil. The new calibration algorithm eliminated biases of the standard calibration and resulted in greater accuracy in predicting MUF concentrations.Highlights We developed a new approach to calibration for 2‐D soil zymography. The approach accounted for spatial nonuniformity of soil zymograms. Standard calibration resulted in systematic underestimation of enzyme activity. Soil zymography requires pixel‐based calibration with nonuniformly saturated membranes.

Utilization of spatial non-uniformity of silicon photodetectors for vibration monitoring

Silicon photodetectors are extensively used for a variety of optical applications because of their unique advantages. It is presumed that their responsivities are uniform in active sensing area, which is not true in actual scenario. In such scenario, the spatial non-uniformity, if any, can introduce error in measurements. Accurate determination of the spatial responsivity of these photodetectors is highly essential for any application. In this paper, the spatial non-uniformities are mapped for two types of photodetectors. Subsequently, this paper demonstrates with corroborating experimental results that photodetectors with spatial non-uniformities can also be potentially utilized for vibration monitoring applications.

Dynamic Compression Garments for Sensory Processing Disorder Treatment Using Integrated Active Materials

Many medical conditions, including sensory processing disorder (SPD), employ compression therapy as a form of treatment. SPD patients often wear weighted or elastic vests to produce compression on the body, which have been shown to have a calming effect on the wearer. Recent advances in compression garment technology incorporate active materials to produce dynamic, low bulk compression garments that can be remotely controlled. In this study, an active compression vest using shape memory alloy (SMA) spring actuators was developed to produce up to 52.5 mmHg compression on a child's torso for SPD applications. The vest prototype incorporated 16 SMA spring actuators (1.25 mm diameter, spring index = 3) that constrict when heated, producing large forces and displacements that can be controlled via an applied current. When power was applied (up to 43.8 W), the prototype vest generated increasing magnitudes of pressure (up to 37.6 mmHg, spatially averaged across the front of the torso) on a representative child-sized form. The average pressure generated was measured up to 71.6% of the modeled pressure, and spatial pressure nonuniformities were observed that can be traced to specific garment architectural features. Although there is no consistent standard in magnitude or distribution of applied force in compression therapy garments, it is clear from comparative benchmarks that the compression produced by this garment exceeds the demands of the target application. This study demonstrates the viability of SMA-based compression garments as an enabling technology for enhancing SPD (and other compression-based) treatment.

Sun Simulator for Indoor Performance assessment of Solar Photovoltaic Cells

This paper presents systematic design procedure and features of a sun simulator developed for testing low concentrating linearly focusing solar photovoltaic concentrators. The designed solar simulator comprises of a xenon short arc lamp and paraboloidal reflector for uniform radiative flux distribution on focal plane at desired radiation intensity. Initial inputs to the design process include reflector geometry (shape and size), surface properties of the reflector surface (reflectivity), distance of the reflector aperture from the illuminated area, rated lamp power, lamp geometry and orientation. These specifications were inputted in a ray tracing analyzer using COMSOL Multiphysics. The angular distribution of radiative intensity from the lamp was also accounted for. The proposed design is able to deliver a radiative intensity of 500 W/m2 to test photovoltaic concentrators with aperture of up to 140 mm x 50 mm with a spatial non-uniformity of 4.5%.

Three-dimensional CFD simulation of a circular OWC with a nonlinear power-takeoff: Model validation and a discussion on resonant sloshing inside the pneumatic chamber

This study presents a three-dimensional numerical simulation of a circular bottom-sitting OWC device for wave energy extraction. The numerical model is based on Reynolds-Averaged Navier-Stokes equations with a modified k-ω turbulence model and a Volume-of-Fluid method to track the air-water surface. The numerical model and setup are validated against a set of wave-flume experimental results. The numerical simulation provides detailed flow-field information, which allows for an analysis of the spatial non-uniformity inside the OWC chamber and the measurement error associated with it. New numerical results show that vortex shedding at the sharp edge of the OWC chamber enhances the spatial non-uniformity inside the OWC chamber through a resonant sloshing mechanism. The enhanced spatial non-uniformity can affect the accuracy of the experimentally-determined quantities such as the extraction efficiency of the OWC chamber and the characteristics of the power take-off device; Suggestions are also provided to improve the accuracy of the experimentally-determined quantities that may be affected by the spatial non-uniformity inside the OWC chamber.