Ideal Point Source Research Articles

Reducing penumbral blur in computed tomography by learning the inverse finite focal spot model.

Penumbral blur is one of the major limitations of the high spatial resolution micro-CT, due to a nonideal large focal spot. Penumbral blur hinders the ability to resolve small features that may only be a few pixels in size. Reducing the focal spot size by decreasing the x-ray tube power is a straightforward solution, but it leads to prolonged scan durations. In this paper, we propose to mitigate the penumbral blur by learning the inverse finite focal spot model. First, we derived the finite focal spot model that builds a relationship from the ideal point source projection to the finite focal spot projection. Based on the derived model, we numerically compute a paired projection dataset. Second, we utilized two neural networks-U-net, and convolution modulation-based U-net (CMU-net) -to learn the inverse finite focal spot model. The goal is to estimate the ideal point source projection from the actual finite focal spot projection. CMU-net, which introduces convolution modulation blocks into the contracting path of the U-net, is proposed to boost the robustness of the U-net. Finally, the standard filtered back-projection (FBP) is employed for reconstruction using the estimated ideal point projection. The experiments show that both U-net and CMU-net can effectively reduce the penumbral blur, whereas CMU-net demonstrates better performance on the real data. Experiments on real measured data demonstrate that CMU-net is more robust than U-net and can effectively resolve fine details. This method has great potential in improving the efficiency of micro-CT acquisition. It allows increasing the tube power since our method can computationally compensate for the blur caused by an increased focal spot size.

On the robustness of multilateration of ionoacoustic signals for localization of the Bragg peak at pre-clinical proton beam energies in water

Objectives. The energy deposited in a medium by a pulsed proton beam results in the emission of thermoacoustic waves, also called ionoacoustics (IA). The proton beam stopping position (Bragg peak) can be retrieved from a time-of-flight analysis (ToF) of IA signals acquired at different sensor locations (multilateration). This work aimed to assess the robustness of multilateration methods in proton beams at pre-clinical energies for the development of a small animal irradiator. Approach. The accuracy of multilateration performed using different algorithms; namely, time of arrival and time difference of arrival, was investigated in-silico for ideal point sources in the presence of realistic uncertainties on the ToF estimation and ionoacoustic signals generated by a 20 MeV pulsed proton beam stopped in a homogeneous water phantom. The localisation accuracy was further investigated experimentally based on two different measurements with pulsed monoenergetic proton beams at energies of 20 and 22 MeV. Main results. It was found that the localisation accuracy mainly depends on the position of the acoustic detectors relative to the proton beam due to spatial variation of the error on the ToF estimation. By optimally positioning the sensors to reduce the ToF error, the Bragg peak could be located in-silico with an accuracy better than 90 μm (2% error). Localisation errors going up to 1 mm were observed experimentally due to inaccurate knowledge of the sensor positions and noisy ionoacoustic signals. Significance. This study gives a first overview of the implementation of different multilateration methods for ionoacoustics-based Bragg peak localisation in two- and three-dimensions at pre-clinical energies. Different sources of uncertainty were investigated, and their impact on the localisation accuracy was quantified in-silico and experimentally.

Fundamental/harmonics beam control using 1-bit space time-modulated plasma DMA

This paper investigates a space-time modulated digital metamaterial array (DMA) based on reconfigurable plasma ionization. The DMA consists of 8 × 8 unit-cell elements with total dimensions of 120 × 120 × 3.2 mm3. Each unit-cell consists of a ring container filled with argon gas and is backed with a grounded dielectric substrate. The argon gas is ionized into a plasma state through metallic electrodes. The logic state of the unit cell is controlled via the changing of the plasma frequency, {omega }_{p}. The value of {omega }_{p}=6times {10}^{11}mathrm{ rad}/mathrm{sec} represents logic “0”, and {omega }_{p}=8times {10}^{11} mathrm{rad}/mathrm{sec} represents logic “bit 1”. The periodic time switching of the plasma ionization controls the radiation at the fundamental and harmonic frequencies. The on-time instants and on-time durations control the number of radiated beams, their directions, amplitudes, and side-lobe levels. Different time-switching sequences are investigated for beam steering, dual-sum beams, broadside beams, end-fire beams, multi-beams, and fan-shaped beams for wireless communications applications. The DMA was investigated under different switching sequences for phase-modulation and amplitude-modulation schemes. A full-wave simulation CST Microwave Studio simulator is used to analyze the proposed DMA and the results are compared with ideal point sources array excited with the same switching sequences.

Interferometric Phase Transmitarray for Spatial Power Combining to Enhance EIRP of Millimeter-Wave Transmitters

An interferometric phase transmitarray is proposed for spatial power combining to enhance the equivalent isotropically radiated power (EIRP) of the millimeter-wave (mmW) transmitters. The study shows that the multiple beams emitting from the ideal point sources located at different positions are combined into one beam to radiate in the boresight direction of the transmitarray, namely, achieving spatial power combining. As an example, the prototyped transmitarray fed by two patch antennas operating over the bandwidth of 23–27 GHz shows that with the spatial power combining, the proposed antenna achieves the combined gain realized by two feeds of 19.6 dBi at boresight 2.5 dB higher than the realized gain of the single feed, so that the EIRP of the two-feed transmitarray is enhanced by 5.5 dB because of 2.5 dB from the gain enhancement of two-feed transmitarray and 3 dB by the two-way power combining.

Impact of the image alignment over frequency for the VLBI Global Observing System

Aims. The VLBI Global Observing System (VGOS), which is the next generation of geodetic very long baseline interferometry (VLBI), observes simultaneously in four frequency bands in the range 3.0-10.7 GHz (expected to be extended to 14 GHz). Because source structure changes with frequency, we aim to study the source position estimates from the observations of this new VLBI system. Methods. Based on an ideal point source model, simulations are run to determine the relation between the source positions as determined by VGOS observations and the locations of the radio emission in the four bands. Results. We obtained the source positions as determined by VGOS observations as a function of the source positions in the four frequency bands for both group and phase delays. The results reveal that if the location of the radio emission in one band is offset with respect to that in the other bands, the VGOS position estimates can be shifted by a larger amount (up to more than three times that offset), possibly in the opposite direction. Conclusions. The VGOS source positions will be highly variable with time and imprecise as to the locations of the radio emission in the four bands if the effects of source structure are not modeled. The image alignment over frequency is essential for modeling these effects, which is the only way to mitigate the impact of such frequency-dependent radio emission on the resulting VGOS source positions.

Laser driven coherent white emission of graphene bulb

The laser-driven white emission of graphene foam in a vacuum bulb was investigated. Thanks to high brightness of light emission attributed to multiphoton ionization, it was possible to construct a compact, spatially coherent, white light source. A new graphene foam bulb was designed to efficiently couple white light into a multimode fiber. The efficiency of coupling light to multimode fibers with varying core diameters was measured. It is concluded that the performance of the graphene foam bulb is close to that simulated for an ideal geometrical point source.

Source mask optimization using the covariance matrix adaptation evolution strategy.

Source mask optimization (SMO) is one of the indispensable resolution enhancement techniques to guarantee the image fidelity and process robustness for the 2Xnm technology node and beyond. The optimization capacity and convergence efficiency of SMO are important, especially for full-chip SMO. An SMO method using the covariance matrix adaptation evolution strategy (CMA-ES), together with a new source representation method, is proposed in this paper. Based on the forward vector imaging formulation, the encoding and decoding methods of the source and the mask, and the constructed merit function, the source and the mask are optimized using the CMA-ES algorithm. The solution search space and the search step size are adaptively updated during the optimization procedure. Considering the sparsity of the optimal source, the source is represented by a set of ideal point sources with unit intensity and adjustable positions. The advantageous spatial frequency components of the source for imaging performance improvement are identified through the aggregation of the point sources. Simulations and comparisons verify the superior optimization capacity and convergence efficiency of the proposed method.

Freeform lens design for a point source and far-field target.

The field of freeform illumination design has surged since the introduction of new fabrication techniques that allow for the production of non-axially symmetric surfaces. Freeform surfaces aim to efficiently control the redistribution of light from a particular source distribution to a target irradiance, but designing such surfaces is a challenging problem in the field of nonimaging optics. Optical design strategies have been developed in both academia and industry. In this paper, we consider the design of a single freeform lens that converts the light from an ideal (zero-étendue) point source into a far-field target. We present a mathematical approach and numerically solve the corresponding generalized Monge-Ampère equation of the optical system. We derive this equation using optimal transport theory and energy conservation. We use a generalized least-squares algorithm that can handle a non-quadratic cost function in the corresponding optimal transport problem. The algorithm first computes the optical map and subsequently constructs the optical surface. We demonstrate that the algorithm can generate a peanut-shaped lens for roadlighting purposes and a highly detailed lens that produces an image on a projection screen in the far field.

Design of illumination optics with extended sources based on wavefront tailoring

So far, illumination optics design for extended sources (like LEDs) has been mainly treated as perturbations of ideal point sources without any extent. Such approaches work well provided that this ideal case is approximately met. However, the demand for very compact luminaires equipped with modern high-brightness LEDs results in configurations where the actual size of the sources cannot be ignored. Here, we develop a “wavefront tailoring” method producing prescribed illumination patterns while fully encompassing the extended source size. We combine this technique with a wavefront-coupling design scheme, obtaining a powerful tool for creating optics for large Lambertian emitters and prescribed intensity emissions. As an example, we design a highly compact and efficient freeform lens delivering a constant illuminance pattern. Importantly, the presented design strategy can work with various methods for calculating the optical surfaces, such as direct methods or optimization routines.

Triple nitrogen-vacancy centre fabrication by C5N4Hn ion implantation

Quantum information processing requires quantum registers based on coherently interacting quantum bits. The dipolar couplings between nitrogen vacancy (NV) centres with nanometre separation makes them a potential platform for room-temperature quantum registers. The fabrication of quantum registers that consist of NV centre arrays has not advanced beyond NV pairs for several years. Further scaling up of coupled NV centres by using nitrogen implantation through nanoholes has been hampered because the shortening of the separation distance is limited by the nanohole size and ion straggling. Here, we demonstrate the implantation of C5N4Hn from an adenine ion source to achieve further scaling. Because the C5N4Hn ion may be regarded as an ideal point source, the separation distance is solely determined by straggling. We successfully demonstrate the fabrication of strongly coupled triple NV centres. Our method may be extended to fabricate small quantum registers that can perform quantum information processing at room temperature.

Particle level screening of scalar forces in 1+1 dimensions

We investigate how non-linear scalar field theories respond to point sources. Taking the symmetron as a specific example of such a theory, we solve the non-linear equation of motion in one spatial dimension for (i) an isolated point source and (ii) two identical point sources with arbitrary separation. We find that the mass of a single point source can be screened by the symmetron field, provided that its mass is above a critical value. We find that two point sources behave as independent, isolated sources when the separation between them is large, but, when their separation is smaller than the symmetron's Compton wavelength, they behave much like a single point source with the same total mass. Finally, we explore closely related behavior in a toy Higgs-Yukawa model, and find indications that the maximum fermion mass that can be generated consistently via a Yukawa coupling to the Higgs in 1+1 dimensions is roughly the mass of the Higgs itself, with potentially intriguing implications for the hierarchy problem.

Wideband circular polarized Fabry-Perot cavity antennas for V-band indoor point-to-point communications

ABSTRACTIn this paper, a 60 GHz planar Fabry-Perot cavity (FPC) antenna with a circularly polarized (CP) radiation characteristic is proposed as a good candidate for millimeter-wave wireless systems. A sparse array design is employed to enhance the gain bandwidth (GBW) product of the antenna. Complete formulations for circular polarized radiation are developed to serve as a general guideline scalable to any frequencies. The theoretical results using ideal point sources are validated by numerical analysis using the finite elements method (FEM). In the full-wave simulations, the ideal sources are replaced by four annular-ring slot antennas to excite the FPC.

Spatial resolution improvement for an optical transition radiation monitor by asymmetric light collection.

The applicability of optical transition radiation (OTR) for measurements of micron sized transverse electron beam profiles is limited not only by the optical system resolution which has a fundamental limit imposed by the uncertainty principle. In the case of OTR generation, a single electron crossing the boundary between vacuum and screen cannot be considered as a single emitting point with isotropic angular distribution. On the contrary, the radiation is emitted from an area with a transverse range that is defined by the radial extension of the electron's Lorentz contracted Coulomb field and is typically estimated as γλ (with γ the Lorentz factor and λ the wavelength of observation). The OTR angular distribution has a characteristic "funnel" shape. As a result the one-dimensional image of a single electron measured with an ideal thin lens has a double lobe shape, and the resolution of any OTR based imaging system is determined by this double lobe function which is also known as OTR Point Spread Function (PSF). As a consequence, the reconstruction of micron sized electron beam profiles is hampered not only due to the fundamental diffraction limit, but also due to the PSF lobe shape. In this paper we present two approaches to improve the spatial resolution of an OTR monitor based on asymmetric light collection using a traditional optical system which allows blocking of one of the lobes. With such a scheme, an OTR PSF can be achieved that is comparable to the one of an ideal point source (Airy distribution).

A new toroidal x-ray crystal spectrometer for the diagnosis of high energy density plasmas at the National Ignition Facility.

The here-described spectrometer was developed for the extended x-ray absorption fine structure spectroscopy of high-density plasmas at the National Ignition Facility. It employs as the Bragg reflecting element a new type of toroidally bent crystal with a constant and very large major radius R and a much smaller, locally varying, minor radius r. The focusing properties of this crystal and the experimental arrangement of the source and detector make it possible to (a) fulfill the conditions for a perfect imaging of an ideal point source for each wavelength, (b) obtain a high photon throughput, (c) obtain a high spectral resolution by eliminating the effects of source-size broadening, and (d) obtain a one-dimensional spatial resolution with a high magnification perpendicular to the main dispersion plane.

A comparison of three approaches for determining scalar illuminance from cubic illuminance data

A method for calculating scalar illuminance using cubic illuminance values in a light field has been proposed in the literature. This enables exact measurement of the illuminance vector direction and magnitude by a fixed device, as well as providing a useful basis for calculation. However, the method yields an inexact estimate of the scalar illuminance which in some cases may lead to errors. Two alternative approaches using the concept of mean spherical semi-cubic and cubic illuminances are proposed in this paper, to determine which of these approaches yields the highest accuracy, and to observe the effect of source orientation in various multiple point source configurations. Three types of test are introduced: the first involves two, three, and four identical point sources, separated by a varying angle θ; the second involves four identical point sources arranged symmetrically at varying azimuth angle ψ and incident angle α; the third involves 10,000 combinations of up to six point sources with random luminous intensities and in random positions. Comparisons between the three approaches show that the approach using mean spherical semi-cubic illuminances yields the least amount of error and thus the highest accuracy for scalar illuminance and vector/scalar illuminance ratio in the first and second test. In the third test, this approach also yields the highest accuracy, even though it tends to underestimate the scalar illuminance in scenes with more sources.

600-mm aperture simultaneous phase-shifting Fizeau interferometer

In this study, a 600-mm aperture simultaneous phase-shifting Fizeau interferometer is presented. This interferometer has a special light source configuration produced by an arrangement of four identical point sources diffracted by a phase grating. With this configuration, the phase shifts are introduced between each pair of coherent beams. A 2 × 2 image lens array is designed such that four conjugated interferograms with a phase step of π/2 can be captured in a single shot using one detector, thereby realizing dynamic measurement. The experiment was carried out using the proposed interferometer with a pair of 600 mm aperture transmission flat and reference flat placed on a platform with no vibration resistance facility. The results depict a peak-valley value of λ/10 and present a real-time phase variation caused by environmental factors, which indicates that the instantaneous measurement of the phase by the proposed large-aperture interferometer provides robust results despite the presence of environmental factors such as vibration and airflow.

Regularized Huygens' plasmonic wave field synthesis using a metal-clad plasmonic waveguide array.

Huygens' principle states that point sources are the basis of optical wave field generation, and an array of point sources with complex amplitudes that are separated by subwavelength distances can generate a desired optical field distribution. In field synthesis based on the Huygens' principle, the construction of ideal point sources has been overlooked when compared to other elements in optical field synthesis engineering, such as complex modulation. However, the construction of ideal point sources should be considered an important goal because the use of non-ideal point sources generates considerable optical noise in the background of the synthesized field distribution. In this Letter, we investigate Huygens' plasmonic wave field synthesis and its regularization by analyzing the noise features that arise during wave field synthesis based on non-ideal point sources and proposing a novel structure for regularized point source construction. It is shown that the quality of plasmonic wave field synthesis based on the Huygens' principle is greatly improved with the proposed design of a structure that generates a unit point source. Practical field synthesis examples involving plasmonic focusing and Airy beams are presented in support of the proposed design.

Active Cloaking for Finite Clusters of Pins in Kirchhoff Plates

This paper considers active cloaking of a square array of evenly spaced pins in a Kirchhoff plate in the presence of flexural waves. Active sources, modeled as ideal point sources, are represented by the nonsingular Green's function for the two-dimensional biharmonic operator and have an arbitrary complex amplitude. These sources are distributed exterior to the cluster, and their complex amplitudes are found by solving an algebraic system of equations. This procedure ensures that selected multipole orders of the scattered field are successfully annulled. For frequencies in the zero-frequency stop band, we find that a small number of active sources located on a grid is sufficient for cloaking. For higher frequencies, we achieve efficient cloaking with the active sources positioned on a circle surrounding the cluster. We demonstrate the cloaking efficiency with several numerical illustrations, considering key frequencies from band diagrams and dispersion surfaces for a Kirchhoff plate pinned in a doubly periodic fashion.

A novel methodology to determine the divergence of a neutron beam

This work posits a novel approach to characterize the divergence of a neutron beam emerging from a reactor port. Unlike the usual inverse of the L/D ratio, the term divergence as employed here refers to the deviation from an ideal parallel beam emitted from a surface source. Within this concept, an ideal point source in spite of its conical beam would not exhibit any divergence. Hence, the beam divergence of a surface source is more adequately characterized adopting the notion of Rocking Curve - RC, a term borrowed from the X-ray diffraction field. After this idea, every point of the surface source emits neutrons in all directions but with different intensities following a bell-shaped profile. Once the RC semi-width is determined, it is possible to assess its effect upon the quality of an acquired neutron radiograph, since it incorporates degrading agents such as geometrical unsharpness, neutron scattering, noise and statistical dispersion. In this work an inverse procedure is applied, i.e., to use an actual neutron radiograph to find the RC semi-width. To accomplish this task, synthetic images - generated with defined RC semi-widths and object-detector gaps - are compared with experimental ones acquired with the same gaps in order to find the most resemblance between them. The angular semi-width of the best synthetic image is assigned to that of the experimental one, defining thus the aimed beam divergence, which has been compared with a different method with a fair agreement. An equivalent procedure embedded in the algorithm has been employed to evaluate the L/D using the same radiographic images. The outcome fairly agrees with the value inferred from the neutron flux ratio at different locations. Both approaches RC semi-width and L/D ratio yielded consistent results with other utterly different methods. Yet, the rocking curve approach forecasts more precisely the neutron pattern hitting the detector and does not need a precisely machined test-object as required by the L/D conventional technique, but only a shielding foil provided with a straight edge.

Microcarrier-Guided Nanopore Dielectrophoresis for Selective Nucleic Acid Detection

Dielectrophoresis (DEP) is the motion of a polarizable particle in a non-uniform electric field due to an unbalanced electrostatic force on the particle's induced dipole. The DEP mechanism has been extensively utilized for manipulation of biological particles, from cancer cells and viruses to biomolecules such as DNAs and proteins, for their concentration, separation, sorting, and transport. However, current DEP approaches to molecular manipulation are not selective, as DEP is not sensitive enough to discriminate among the induced dipoles of different molecules. Here we explore a novel single-molecule DEP mechanism, carrier-guided nanopore dielectrophoresis, for selective nucleic acid sequence detection. Rather than rely on a target's native polarizability, we designed a polycationic carrier to impart a tunable synthetic dipole to the target nucleic acid molecule; carrier sensitivity and selectivity are both programmable. Such synthetic dipoles can be captured in an engineered nanopore, which acts as an ideal foul-free point source to generate an extremely high field gradient (ΔE∼107 V·m-1 per nanometer or ∼1016 V·m-2) for molecular dipole manipulation. Non-target nucleic acids do not bind the carriers and hence are repelled from the pore by electrophoresis. To elucidate the mechanism of the dipole capture by the nanopore, we took an all-atom molecular dynamics (MD) simulation approach to observe the movements of and forces on the dipole. Simulation results predicted significantly increased force attracting the probe into the engineered nanopore as opposed to the wild type, consistent with the increased capture rates observed in experiment. Most strikingly of all, we find that a carrier with only a few positive charges can drive any length of DNA or RNA with equal capture efficiency. Development of nanopore dielectrophoretic detection thus offers ready medical applications of nanopore technology.