Degree Of Collimation Research Articles

Current-induced birefringent absorption and non-reciprocal plasmons in graphene

We present extensive calculations of the optical and plasmonic properties of a graphene sheet carrying a dc current. By calculating analytically the density–density response function of current-carrying states at finite temperature, we demonstrate that an applied dc current modifies the Pauli blocking mechanism and that absorption acquires a birefringent character with respect to the angle between the in-plane light polarization and current flow. Employing random phase approximation at finite temperature, we show that graphene plasmons display a degree of non-reciprocity and collimation that can be tuned with the applied current. We discuss the possibility to measure these effects.

Lissajous figure-based single-frame collimation technique

An accurate collimation technique based on a double grating system is proposed. Transversal displacement of the grating is not required and then, automatic single-frame processing can be performed. Talbot self-images are projected onto a mask composed by several shifted diffraction gratings. A Lissajous figure is obtained with the signals acquired by a CMOS camera where the mask is simulated by software. The collimation degree is determined by measuring the ellipticity of the Lissajous figure. Visual or automatic procedures for simple and accurate collimation of a light source are proposed. Experimental results are obtained which show a resolution of δϕ≈4.16μrad in the divergence of the beam when a lens with focal length f=25mm and diameter D=20mm is used for collimation.

Improving the performance of gold nanohole array biosensors by controlling the optical collimation conditions.

An experimental investigation on how the bulk and surface sensitivities of gold nanohole arrays fabricated by interference lithography affect the degree of white light beam collimation is presented. The optical transmission response of nanohole arrays has been recorded by focused and collimated beam transmission spectra. The results show that both the bulk and surface sensitivities for the collimated case are much larger than for the focused case. In particular, the shape of the spectra was dependent on the degree of beam collimation. The results showed that improved sensing performance (around 3.5 times) and higher figure of merit (around 4.4 times) can be obtained by simply adjusting the incident/collection experimental conditions in transmission measurements.

A novel backlight unit for volume-holographic optical elements-based time-multiplexed three-dimensional displays

In volume-holographic optical elements (VHOEs)-based time-sequential three-dimensional (3-D) displays, two reference beams generated from a backlight unit (BLU) illuminate VHOEs, and from which object beams satisfying the Brag condition are then diffracted. These beams form a pair of alternating light fields for stereo 3-D view. Since this system operates based on diffraction optics, its performance highly depends on the degrees of collimation and uniformity of the reference beams. Thus, a new BLU system to generate uniformly-collimated reference beams for the VHOEs-based 3-D display is proposed by combined use of a light-guide-plate (LGP) grooved with an array of angle-variant flat-top prisms, and two LED light sources attached with reflection-type beam collimators. Simulation results with LightTools 7.1 show that the average full-width at half maximum (FWHM), backshift ratio and intensity uniformity of the LGP output beam of the proposed system have been significantly reduced down to 2.8° and 0.4%, and increased up to 90.9%, respectively, from the values of 51.8°, 26.5% and 24.5% of the conventional system. They represent 18.5-, 66.3- and 3.7-fold improvements of those values in the proposed system, respectively. These successful results confirm the feasibility of the proposed system in the practical VHOEs-based 3-D display.

Radiation dose reduction in fluoroscopic procedures: left varicocele embolization as a model.

To investigate the effect of a radiation reduction program on total dose, fluoroscopy dose per second corrected for body habitus and degree of collimation in left varicocele embolizations (LVE). A radiation reduction program for LVE was implemented, consisting of a technique minimizing fluoroscopy time, using low-dose presets, virtual collimation, and virtual patient positioning. Height, weight, fluoroscopy time, kerma area product (KAP) and reference air kerma (Ka,r) were recorded for 100 consecutive cases satisfying the inclusion criteria. For each patient, a device specific dose correction factor, determined using a phantom, was used to standardize the KAP to that of the cylindrical diameter of the standard man and a collimation index was derived from the KAP and Ka,r. Median fluoroscopy time was 3 minutes (mean 4.5, range 1-23.8). Median KAP was 0.54 Gy/cm(2) (mean 0.82, range 0.12-6.52). There was a significant decrease in KAP/second corrected for cylindrical diameter (p < 0.001) and the collimation index (p < 0.001) over time. This study shows that a dedicated dose reduction program can achieve very low total radiation dose rates for LVE. The significant decrease in collimation index and standardized KAP per second during this study suggest a learning curve for collimation. • Left varicocele embolization can be performed with very low radiation doses. • The most effective real time radiation reduction measure is collimation. • There is a learning curve to achieving optimal collimation.

Synchrotron X-ray Scattering techniques for soft matter industrial research and development

Consumer products based on soft matter technology often exhibit macroscopic properties which are strongly dependent on their micro- and nano-structures extending over multiple size scales. Synchrotron scattering techniques are ideally suited for probing these multilevel structures and deliver complementary and in some cases unique information as compared to real space methods like confocal microscopy, cryo-electron microscopy or atomic force microscopy. The European Synchrotron Radiation Facility (ESRF) is a world-leading synchrotron light source which operates several state-of-the-art instruments for the investigation of soft materials and offers expertise to academic and industrial users. Fast and flexible access for proprietary experiments with a modular, fine-tuned service is guaranteed. A range of dedicated sample environments which mimic industrial processing conditions are available. This presentation will illustrate the state-of-the-art performance of the following synchrotron scattering techniques by recent examples of industrial relevance. Simultaneous small and wide angle X-ray scattering (SAXS/WAXS) is a powerful method to determine the microstructure and phase behavior of multi-component systems like detergents, food products, pharmaceutical components, polymer composites, etc. The high photon flux translates to high throughput measurements, while the high degree of collimation and resolution permit to elucidate a wide range of length scales from a few Angstroms up to micron scale. Scanning microbeam SAXS/WAXS and single micro-crystal/fiber diffraction (µXRD) allows elucidating the local nanostructure of very small objects like micro-specimens of composite organic/inorganic materials, teeth, bones, micromechanical parts, polymer fibers, micro fluidics, etc. with micro/nanometric real space resolution. X-ray reflectivity (XR) and grazing incidence diffraction/scattering (GID /GISAXS) can reveal the nanoscale structure and complexity of nano-structured complex fluids at interfaces, organic films, biological membranes, etc.

Collimation testing and calibration using a heterodyne Moiré method

This study proposes a simple, rapid, and highly accurate method to achieve the collimation testing and calibration of laser light beams. By applying a relative constant velocity to two gratings, every pixel of a CMOS camera can receive a series of heterodyne moiré signals. Based on the least squares sine fitting algorithm, the phase of the optimized sinusoidal wave can be obtained. Consequently, the phase slope along the direction perpendicular to the grating lines can be estimated and then used to judge the degree of collimation. Furthermore, by measuring only two phase slopes at two positions of the collimating lens, the calibration of the light beam collimation can also be achieved. The experiment validated the proposed method, and the positioning error of the light beam collimation was approximately 7μm.

COLLECTIVE OUTFLOW FROM A SMALL MULTIPLE STELLAR SYSTEM

The formation of high-mass stars is usually accompanied by powerful protostellar outflows. Such high-mass outflows are not simply scaled-up versions of their lower-mass counterparts, since observations suggest that the collimation degree degrades with stellar mass. Theoretically, the origins of massive outflows remain open to question because radiative feedback and fragmentation of the accretion flow around the most massive stars, with M > 15 M_Sun, may impede the driving of magnetic disk winds. We here present a three-dimensional simulation of the early stages of core fragmentation and massive star formation that includes a subgrid-scale model for protostellar outflows. We find that stars that form in a common accretion flow tend to have aligned outflow axes, so that the individual jets of multiple stars can combine to form a collective outflow. We compare our simulation to observations with synthetic H_2 and CO observations and find that the morphology and kinematics of such a collective outflow resembles some observed massive outflows, such as Cepheus A and DR 21. We finally compare physical quantities derived from simulated observations of our models to the actual values in the models to examine the reliability of standard methods for deriving physical quantities, demonstrating that those methods indeed recover the actual values to within a factor of 2-3.

Su1616 Prospective Evaluation of Predictors of Increased Patient Radiation During ERCP: the Stanford ERCP Radiation Safety (Sers) Study

The use of fluoroscopy during ERCP exposes patients to ionizing radiation with the potential for deleterious health consequences. Therapeutic ERCP is associated with a 2-5 fold greater patient radiation exposure, than diagnostic ERCP, with radiation exposure levels approaching those of CT scans of the abdomen. Endoscopists now have the ability to control modifiable factors on the fluoroscopy table, including the fluoroscopy frame rate, degree of magnification, collimation and imaging geometry, all of which can modulate the radiation dose received, over the same duration of Fluoroscopy Time.

Reconstruction of High Dynamic Range Images: Simulations of LBT Observations of a Stellar Jet, a Pathfinder Study for Future AO-Assisted Giant Telescopes

We present simulated Large Binocular Telescope (LBT) infrared narrow-band observations of a star-jet system, in conjunction with improved and optimized deconvolution and image reconstruction algorithms, considering two cases of interest: single-dish direct imaging with an AO-assisted camera and imaging through a Fizeau interferometer that combines the beams of the two mirrors of LBT. We aim at understanding what accuracy can be obtained with the use of present AO-assisted large telescopes (such as LBT) and what improvements an interferometric instrument (such as LINC-NIRVANA) will be able to provide. The proposed deconvolution method is based on the target decomposition as a sum of a point source (the star) and an extended source (the jet). By assuming Poisson noise we add to the negative logarithm of the likelihood a regularization term enforcing smoothness of the jet component. Finally, we use a Richardson-Lucy-like method for the minimization of this function. This approach is an improvement of a method proposed by Lucy in 1994 for accurate photometric restoration of HST images and called two channel photometric restoration. We denote the new method as the multi-component Richardson-Lucy (MC-RL) method. The analysis of the reconstructed objects shows that the MC-RL method applied to the interferometric observations allows us to evaluate the width and the spatial intensity profile of the jet down to 20 mas with an accuracy better than about 20% in the best case of a central star fainter than 10 mag. These limits allow us to obtain a very good reconstruction of the jet acceleration region very close to the exciting source, which would provide fundamental scientific information on the jet collimation degree and eventually on its launching mechanism. As concerns the proposed MC-RL method, it demonstrates a good performance in the reconstruction of images with a very high dynamic range. It can be improved in several directions, by increasing both its efficiency, thanks to recently proposed acceleration techniques, and its accuracy by means of more sophisticated regularization terms. We are also planning to apply the method to simulated observations of upcoming super giant earth-based telescopes.

FORMATION AND COLLIMATION OF RELATIVISTIC MHD JETS — SIMULATIONS AND RADIO MAPS

We present results of magnetohydrodynamic (MHD) simulations of jet formation and propagation, discussing a variety of astrophysical setups. In the first approach we consider simulations of relativistic MHD jet formation, considering jets launched from the surface of a Keplerian disk, demonstrating numerically — for the first time — the self-collimating ability of relativistic MHD jets. We obtain Lorentz factors up to ≃ 10 while acquiring a high degree of collimation of about 1 degree. We then present synchrotron maps calculated from the intrinsic jet structure derived from the MHD jet formation simulation. We finally present (non-relativistic) MHD simulations of jet lauching, treating the transition between accretion and ejection. These setups include a physical magnetic diffusivity which is essential for loading the accretion material onto the outflow. We find relatively high mass fluxes in the outflow, of the order of 20-40% of the accretion rate.

Laser-driven generation of collimated ultra-relativistic positron beams

We report on recent experimental results concerning the generation of collimated (divergence of the order of a few mrad) ultra-relativistic positron beams using a fully optical system. The positron beams are generated exploiting a quantum-electrodynamic cascade initiated by the propagation of a laser-accelerated, ultra-relativistic electron beam through high-Z solid targets. As long as the target thickness is comparable to or smaller than the radiation length of the material, the divergence of the escaping positron beam is of the order of the inverse of its Lorentz factor. For thicker solid targets the divergence is seen to gradually increase, due to the increased number of fundamental steps in the cascade, but it is still kept of the order of few tens of mrad, depending on the spectral components in the beam. This high degree of collimation will be fundamental for further injection into plasma-wakefield afterburners.

Self-collimated axial jet seeds from thin accretion disks

We show how an appropriate stationary crystalline structure of the magnetic field can induce a partial fragmentation of the accretion disk, generating an axial jet seed composed of hot plasma twisted in a funnel-like structure due to the rotation of the system. The most important feature we outline is the high degree of collimation, naturally following from the basic assumptions underlying the crystalline structure. The presence of nonzero dissipative effects allows the plasma ejection throughout the axial jet seed and the predicted values of the accretion rate are in agreement with observations.

BIPOLAR JETS LAUNCHED FROM MAGNETICALLY DIFFUSIVE ACCRETION DISKS. I. EJECTION EFFICIENCY VERSUS FIELD STRENGTH AND DIFFUSIVITY

We investigate the launching of jets and outflows from magnetically diffusive accretion disks. Using the PLUTO code we solve the time-dependent resistive MHD equations taking into account the disk and jet evolution simultaneously. The main question we address is which kind of disks do launch jets and which kind of disks do not? In particular, we study how the magnitude and distribution of the (turbulent) magnetic diffusivity affect mass loading and jet acceleration. We have applied a turbulent magnetic diffusivity based on \alpha-prescription, but have also investigate examples where the scale height of diffusivity is larger than that of the disk gas pressure. We further investigate how the ejection efficiency is governed by the magnetic field strength. Our simulations last for up to 5000 dynamical time scales corresponding to 900 orbital periods of the inner disk. As a general result we observe a continuous and robust outflow launched from the inner part of the disk, expanding into a collimated jet of super fast magneto-sonic speed. For long time scales the disk internal dynamics changes, as due to outflow ejection and disk accretion the disk mass decreases. For magneto-centrifugally driven jets we find that for i) less diffusive disks, ii) a stronger magnetic field, iii) a low poloidal diffusivity, or a iv) lower numerical diffusivity (resolution), the mass loading of the outflow is increased - resulting in more powerful jets with high mass flux. For weak magnetization the (weak) outflow is driven by the magnetic pressure gradient. We further investigate the jet asymptotic velocity and the jet rotational velocity in respect of the different launching scenarios. We find a lower degree of jet collimation than previous studies, most probably due to our revised outflow boundary condition.

Formation and propagation of MHD jets - relativistic jets, radiation pressure, and shock-induced rotation

We present recent results of magnetohydrodynamic (MHD) simulations of jet formation and propagation, discussing a variety of astrophysical setups. In the first approach the role of the disk magnetic flux profile and disk mass flux profile concerning the jet collimation degree is investigated. Our results suggest (and quantify) that in general magnetized outflows launched from a compact region close to the inner disk radius tend to be less collimated. The second setup considers simulation of relativistic MHD jet formation, considering jet launched from the surface of a Keplerian disks, demonstrating - for the first time - the self-collimating ability of relativistic MHD jets. We obtain Lorentz factors up to ≃ 10 while acquiring a high degree of collimation of about 1 degree. We then present MHD jet formation simulations taking into account radiation pressure of a central luminous source. We investigate radiative effects on jet collimation and propagation - an environment which is interesting for outflows from massive young stars and active galactic nuclei. Finally, we present a model which explains a possible jet rotation by shock compression of an intrinsic helical magnetic field.

Steering and collimating ballistic electrons with amphoteric refraction

We show that amphoteric refraction of ballistic electrons, i.e., positive or negative refraction depending on the incidence angle, occurs at an interface between an isotropic and an anisotropic medium and can be employed to steer and collimate electron beams. The steering angle is determined by the materials’ parameters, but the degree of collimation can be tuned in a significant range by changing the energy of ballistic electrons.

Neutron radiography with sub-15 μm resolution through event centroiding

Conversion of thermal and cold neutrons into a strong ∼1ns electron pulse with an absolute neutron detection efficiency as high as 50–70% makes detectors with 10B-doped Microchannel Plates (MCPs) very attractive for neutron radiography and microtomography applications. The subsequent signal amplification preserves the location of the event within the MCP pore (typically 6–10μm in diameter), providing the possibility to perform neutron counting with high spatial resolution. Different event centroiding techniques of the charge landing on a patterned anode enable accurate reconstruction of the neutron position, provided the charge footprints do not overlap within the time required for event processing. The new fast 2×2 Timepix readout with >1.2kHz frame rates provides the unique possibility to detect neutrons with sub-15μm resolution at several MHz/cm2 counting rates. The results of high resolution neutron radiography experiments presented in this paper, demonstrate the sub-15μm resolution capability of our detection system. The high degree of collimation and cold spectrum of ICON and BOA beamlines combined with the high spatial resolution and detection efficiency of MCP-Timepix detectors are crucial for high contrast neutron radiography and microtomography with high spatial resolution. The next generation of Timepix electronics with sparsified readout should enable counting rates in excess of 107n/cm2/s taking full advantage of high beam intensity of present brightest neutron imaging facilities.

Digital holographic microscopy characterization of superdirective beam by metamaterial

Digital holographic microscopy (DHM) has been successfully applied for the first time to characterize the radiative out-of-plane emission properties of a superdirective device. Complementarily to near-field microscopy, DHM allows us to reconstruct the beam in the far-field region. The angular dispersion of the light beam radiated from a grating composed of air and anti-air metamaterial has been determined, and the proposed technique has highlighted a collimation degree higher than 0.04°, as already evaluated in a previous work. Further considerations on the retrieved phase map of the beam in the acquisition plane are presented.

Modeling and design of micromachined optical Söller collimators for lensless CCD-based fluorometry

To address the needs of medical diagnostics in resource-poor settings, it is necessary to develop low cost, simple and portable Point of Care detectors for integrated medical diagnostics. Previously, we have described a simple lensless fluorometer with sensitivity in the range of current ELISA plate readers. The key to the lensfree fluorometer is the uniform spatial distribution of light, which we achieved using a simple optical collimator based on a "stack of pinholes" (a stack of black PMMA plates with arrays of pinholes machined via laser) enabling the light to be collimated from the LED light source through the necessary wavelength filters and the assay's microfluidics directly onto the CCD without a lens. In this paper, we describe the optical principle for designing these Söller collimators for lensfree CCD-based fluorometry. The illuminating surface was modeled as a collection of differential areas emitting uniformly and spherically, and the intensity contribution of each emitting area was summed over the detector surface. To compute the final light intensity distribution from such a differential model we derived an integral equation to sum the individual intensity contributions from the two-dimensional emitting surface. The equation is for a single-hole collimator. Light intensity measurements were taken by placing a collimator with a particular aspect ratio (the ratio of hole length to diameter (L/d)) over the CCD image sensor and capturing an image. The resulting image is the 2D light intensity profile generated by the collimator. As the aspect ratio is increased the slope of the light intensity profile increases, corresponding to an increased degree of collimation. To test the model, the measured maximum and mean light intensities were compared with the theoretical predictions generated from the model. There was an agreement between the variation of the mean (R(2) = 0.990) and maximum (R(2) = 0.938) values of light intensities with aspect ratios based modeling. These profile measurements suggest an excellent agreement with the theoretical predictions. The integral equation presented here can be used to perfect the design of the optical Söller collimator. These results may lead to the development of more effective Söller collimators for lensfree CCD-based fluorometry for use in simple low cost lensfree optical detectors with the potential to enhance the accessibility and the quality of health care for underserved populations.

Simultaneous imaging of K-α radiation and coherent transition radiation from relativistic-intensity laser-irradiated solid target plasmas

Laser acceleration of hot electrons and their transport through 12–32 μm thick Ti foils was explored experimentally using two complementary diagnostics, a bent crystal imaging the Ti K α emission and optical imaging of the coherent transition radiation (CTR) produced by the exit of the hot electrons from the foil. The spatial extent of the hot electron production measured by these two diagnostics is dramatically different. Electrons producing CTR emerge in a spot of less than 7 μm and appear to maintain a high degree of collimation during transport through the foil while electrons that produce K α emission appear to diverge to sizes of 50–100 μm as viewed from the back surface of the foil. These results indicate that there is a large difference in the transport of the highest energy electrons contributing to CTR signal as compared with the bulk of the hot electron population generating K α signal.