Incident Beam Research Articles

Observation of the Goos-Hänchen shift in monolayer WSe2 for an arbitrary linearly polarized incident light beam using weak measurement

We report, to our knowledge, the first experimental investigation of the spatial Goos-Hänchen (GH) shift at an absorbing material interface comprised of monolayer (ML) tungsten di-selenide (WSe2) on a SiO2/Si substrate under a total internal reflection (TIR) condition. The critical angle for this design is drastically shifted to 23.31°, compared to the glass-air interface, which was at 41.3°. Utilizing the weak value amplification (WVA) approach, the behavior of spatial GH shifts at this interface with various regulating parameters such as angle of incidence, polarization angle, and post-selection angle has systematically been studied. At critical incidence, the greatest shift of approximately 116 µm exceeds the maximum limit of beam shift w0/2, where w0 is the beam waist (180 µm). A generic theoretical model compatible with polarization-dependent studies is also established that has demonstrated excellent agreement with experimental results. Moreover, this work established three distinct features that allow us to readily tweak the value of spatial GH shifts. The observation of a controllable spatial GH shift at the ML WSe2-SiO2/Si configuration has potential implications for optical sensors, optical differential operation, and other photonic manipulations.

Interference effects in commercially available free-space silicon single-photon avalanche diodes

Single-photon avalanche diodes (SPADs) are essential for photon-based measurements and metrology, enabling measurement comparisons at the few-photon level and facilitating global traceability to the SI. A spatially uniform detector response is crucial for these applications. Here, we report on interference effects in commercially available silicon SPADs that are detrimental to their spatial uniformity. Contrasts as high as 18% are observed, posing problems for metrology and general applications that utilize coherent light and require stable detection efficiencies. We eliminate the device optical window as a contributing interface, isolating likely causes to anti-reflective coatings, the semiconductor surface, and the SPAD's internal structure. We also present results where we leverage this sub-optimal behavior by aligning an incident beam with the position of maximum constructive interference, yielding an effective detection efficiency of 51.1(1.7)% compared to the normal value of 44.3(1)% obtained with the interference suppressed. We anticipate that this work will significantly impact the continuing development of these devices, the methods for characterizing them, and their use in accurate measurements.

Measuring the OAM spectrum of a fractional helical beam in a single shot

We propose and experimentally demonstrate a new technique, to our knowledge, to precisely measure the orbital angular momentum (OAM) spectrum of the fractional helical beam in a single shot. This is realized using a single-path interferometer scheme combined with space division multiplexing and polarization phase-shifting. Such a combination enables the single-shot recording of multiple phase-shifted interferograms, which leads to extracting the phase profile of the incident fractional helical beam. Furthermore, by adopting an orthogonal projection method, this measured phase is utilized to evaluate the corresponding OAM spectrum. To test the efficacy, a set of simulations and experiments for different fractional helical beams is demonstrated. The proposed method shows enormous potential to characterize the OAM spectrum in real time.

Laser-induced graphene gas sensors for environmental monitoring.

Artemesia tridentata is a foundational plant taxon in western North America and an important medicinal plant threatened by climate change. Low-cost fabrication of sensors is critical for developing large-area sensor networks for understanding and monitoring a range of environmental conditions. However, the availability of materials and manufacturing processes is still in the early stages, limiting the capacity to develop cost-effective sensors at a large scale. In this study, we demonstrate the fabrication of low-cost flexible sensors using laser-induced graphene (LIG); a graphitic material synthesized using a 450-nm wavelength bench top laser patterned onto polyimide substrates. We demonstrate the effect of the intensity and focus of the incident beam on the morphology and electrical properties of the synthesized material. Raman analyses of the synthesized LIG show a defect-rich graphene with a crystallite size in the tens of nanometers. This shows that the high level of disorder within the LIG structure, along with the porous nature of the material provide a good surface for gas adsorption. The initial characterization of the material has shown an analyte response represented by a change in resistance of up to 5% in the presence of volatile organic compounds (VOCs) that are emitted and detected by Artemisia species. Bend testing up to 100 cycles provides evidence that these sensors will remain resilient when deployed across the landscapes to assess VOC signaling in plant communities. The versatile low-cost laser writing technique highlights the promise of low-cost and scalable fabrication of LIG sensors for gas sensor monitoring.

Optical parameter design model of hemispherical laser retroreflector arrays for satellite laser ranging

A laser retroreflector array (LRA) with multiple retroreflectors is used to directionally reflect an incident laser beam and aims to enhance the received signal strength into laser ranging ground stations. A comprehensive mathematical equation of the received photons is established by modelling the LRA far field diffraction intensity (FFDI) and the satellite velocity aberration position. Based on this received photon model, a novel method on designing the dihedral angle offset and the aperture of retroreflectors is proposed, which maximizes the received photon under the minimal elevation angle of the ground station. Taking the hemispherical structure parameters of Chinese HY-2 satellite LRA and the specification parameters of the Shanghai Astronomical Observatory, the optimal aperture and the dihedral angle offset of the HY-2 laser retroreflectors are 17 mm and 1.6″, respectively. These two optimal optical parameters make the received photon under the elevation angle of 0° reach ∼0.9 count per laser shot. Until now, ∼851,400 successful observations have been successfully obtained by global ground stations since the first launch of HY-2 satellite series in 2011, which guarantees the high-precise orbit determination of HY-2 satellite series.

Enhancing steering angle in liquid crystal beam-steering devices with wideband performance.

Previous studies have demonstrated the refraction-type liquid crystal (LC)-based beam-steering devices within the visible wavelength range. However, these devices require substantial improvements in a steering angle for practical application. In this study, we first demonstrate that our device operates as a beam-steering device regardless of the wavelength of an incident laser beam. Furthermore, the steering angle can be improved by increasing the refractive index gradient and stacking four cells, increasing the steering angle by 10-fold.

Synchrotron CT dosimetry for wiggler operation at reduced magnetic field and spatial modulation with bow tie filters.

The Australian Synchrotron Imaging and Medical Beamline (IMBL) uses a superconducting multipole wiggler (SCMPW) source, dual crystal Laue monochromator and 135 m propagation distance to enable imaging and computed tomography (CT) studies of large samples with mono-energetic radiation. This study aimed to quantify two methods for CT dose reduction: wiggler source operation at reduced magnetic field strength, and beam modulation with spatial filters placed upstream from the sample. Transmission measurements with copper were used to indirectly quantify the influence of third harmonic radiation. Operation at lower wiggler magnetic field strength reduces dose rates by an order of magnitude, and suppresses the influence of harmonic radiation, which is of significance near 30 keV. Beam shaping filters modulate the incident beam profile for near constant transmitted signal, and offer protection to radio-sensitive surface organs: the eye lens, thyroid and female breast. Their effect is to reduce the peripheral dose and the dose to the scanned volume by about 10% for biological samples of 35-50 mm diameter and by 20-30% for samples of up to 160 mm diameter. CT dosimetry results are presented as in-air measurements that are specific to the IMBL, and as ratios to in-air measurements that may be applied to other beamlines. As CT dose calculators for small animals are yet to be developed, results presented here and in a previous study may be used to estimate absorbed dose to organs near the surface and the isocentre.

Monte Carlo simulation code for polarized inverse Compton scattering sources

Polarized x rays or gamma rays generated from inverse Compton scattering (ICS) sources are high-brightness, quasimonochromatic, and polarization-tunable and are widely applied in medical imaging, material science, and nuclear physics research. In this paper, to simulate ICS between electron beams and polarized laser beams, a Monte Carlo simulation code has been developed and implemented based on the 4 toolkit. This code considers the polarization as well as the bandwidth of incident laser beams. The code is benchmarked by comparing its simulation results with those from the well-known Monte Carlo tool called and experimental results. The comparisons show that this code accurately reproduces the scattered photons, demonstrating its potential for various ICS-based applications. Published by the American Physical Society 2024

Spectral line broadening of the Raman scattered waves in laser plasmas

Parametric instabilities are of a great concern especially in the context of laser fusion. Electromagnetic waves scattered by these instabilities can reach high amplitudes, thereby diverting a significant portion of the energy from the incident laser beam, which is essential for compressing and heating the fuel capsule. In addition, the emerging electrostatic wave can trap and accelerate electrons, which, upon impact, can preheat the target and thus prevent its effective compression. Here, we focus on the stimulated Raman scattering and the associated processes. Spectral broadening of the backward Raman scattering daughter wave is theoretically described process, which is caused by so-called trapped particle instability. The existence of trapped and freely moving electrons in plasma influenced by high amplitude electron plasma wave leads also to the anomalous dispersion of this wave. It is reflected in the shifts in the electrostatic spectrum which, on the other hand, must be visible also in the electromagnetic spectrum via the feed-back loop of the instability. In the present paper, we discuss scattered wave modes spectral broadening using results of numerical simulations.

Crossed-cell-tile multiplexed 1st-order gratings, for three-dimensional beam-splitter applications

Oblique angle of incidence two-way and three-way beam splitters were designed and fabricated. The devices feature two first-order diffraction gratings, arrayed crossed in alternating adjacent tiles, resulting in conical diffraction spot separation of two 1 st -orders in orthogonal planes while overlapping the 0 th -order. The two-way beam splitter was designed for 0 th −order suppression. The three-way beam splitter was designed to distribute light equally between the 1 st and 0 th − order spots. Testing of the devices yielded efficiencies of 2%:46%:46% for the two-way beam splitter at 604 nm, and 32%:32%:32% for the three-way beam splitter at 633 nm wavelength. The polarization state of the incident beam was preserved after diffraction through the devices.

Non-uniform phase distribution of a tightly focused elliptically polarized vortex beam

Abstract Tight focusing of elliptically polarized vortex beams has been previously studied for optical manipulation, optical information encoding, and so on. Still, there is a lack of research on the status of the phase distribution on the focal plane. In this study, we found that the phase distribution of a tightly focused elliptically polarized vortex beam is non-uniform, i.e., the phase distribution exhibits flatter and steeper regions due to the elliptical polarization of the input vortex beam. It is mentioned that the phase non-uniformity was related to the ellipticity of the polarization of the incident beam. Furthermore, we analyzed the intensity and phase distribution of a tightly focused elliptically polarized vortex beam. We found that the spin angular momentum was converted to the orbital angular momentum because the topological charge of the output beam was greater than that of the input beam. The non-uniform phase distribution of a tightly focused elliptically polarized vortex beam enables control over light–matter interaction, leading to advancements in optical tweezers, quantum information processing, and super-resolution microscopy.

Vectorial spatial differentiation of optical beams with metal–dielectric multilayers enabled by spin Hall effect of light and resonant reflection zero

We theoretically describe and numerically investigate the operation of “vectorial” optical differentiation of a three-dimensional light beam, which consists in simultaneous computation of two partial derivatives of the incident beam profile with respect to two spatial coordinates in different transverse electric field components. It is implemented upon reflection of the beam from a layered structure by simultaneously utilizing the effect of optical resonance and the spin Hall effect of light. As an example of a layered structure performing this operation, we propose a three-layer metal–dielectric–metal (MDM) structure. We show that by choosing the parameters of the MDM structure, it is possible to achieve the so-called isotropic vectorial differentiation, for which the intensity of the reflected optical beam (squared electric field magnitude) is proportional to the squared absolute value of the gradient of the incident linearly polarized beam. The presented numerical simulation results demonstrate high-quality vectorial differentiation and confirm the developed theoretical description.

Indoor Direct Comparison between High-Accuracy Pyrheliometer and Room Temperature Absolute Radiometer

Abstract This article proposes a practicable indoor method by directly comparing the pyrheliometer with the room temperature absolute radiometer (RTAR) to overcome the inconvenience of outdoor pyrheliometer calibration caused by the limitation of weather conditions. The RTAR serves as the primary standard for radiant power, irradiance, illuminance, and candela at the Center for Measurement Standards/ITRI. Given that the RTAR and pyrheliometer exhibit different sensitivity curves, the key to a successful comparison lies in designing an incident beam that enables both the RTAR and pyrheliometer to operate within their respective uniform sensitivity regions. While the indoor incident beam’s irradiance is considerably lower than solar radiation, the comparison results demonstrate a quite well measurement consistence between the RTAR and the pyrheliometer. As the indoor method is insusceptible to weather, the complex data acquisition procedures for the outdoor method can be simplified due to the stable incident beam indoors. Despite current documentary standards recommending outdoor calibration and comparison for pyrheliometer, the indoor method presented in this article can serve as a supplementary approach for assessing the accuracy of pyrheliometers during the intervals between two calibration or comparison events.

Measurement of transcranial Doppler insonation angles from three-dimensional reconstructions of CT angiography scans.

Blood velocities measured by Transcranial Doppler (TCD) are dependent on the angle between the incident ultrasound beam and the direction of blood flow (known as the Doppler angle). However, when TCD examinations are performed without imaging the Doppler angle for each vessel segment is not known. We have measured Doppler angles in the basal cerebral arteries examined with TCD using three-dimensional (3D) vessel models generated from computed tomography angiography (CTA) scans. This approach produces angle statistics that are not accessible during non-imaging TCD studies. We created 3D models of the basal cerebral arteries for 24 vasospasm patients. Standard acoustic windows were mapped to the specific anatomy of each patient. Virtual ultrasound transmit beams were generated that originated from the acoustic window and intersected the centerline of each arterial segment. Doppler angle measurements were calculated and compiled for each vessel segment. Doppler angles were smallest for the middle cerebral artery M1 segment (median 24.6°) and ophthalmic artery (median 25.0°), and largest for the anterior cerebral artery A2 segment (median 76.4°) and posterior cerebral artery P2 segment (median 75.8°). The ophthalmic artery had the highest proportion of Doppler angles that were less than 60° (99%) while the anterior cerebral artery A2 segment had the lowest proportion of Doppler angles that were less than 60° (10%). These angle measurements indicate the expected deviation between measured and true velocities in the cerebral arteries, highlighting specific segments that may be prone to underestimation of velocity.

Talbot-effect-based multiplication of Laguerre–Gaussian beams with non-zero radial indices: From theory to experimental realization

We present a Talbot self-imaging inspired method for multiplying Laguerre-Gaussian (LG) beams of non-zero radial indices. We explore theoretically and experimentally the LG beam diffraction from two-dimensional (2D) periodic structures. In particular, we determine near-field LG diffraction patterns from a 2D binary grating of a small opening ratio (OR). We demonstrate that incident LG beam intensity patterns are reproduced as images of individual square apertures of the grating in certain Talbot planes. We also investigate the impact of the number of fractional-Talbot and Talbot planes on the quality of the generated 2D array of multiplexed LG modes. Our theoretical predictions are in good agreement with our experiments. Our method can find applications in optical communication, optical tweezing, multi-particle trapping, screening, and micro-manipulation.

Alternative and simple method to determine the refractive index of human dentin.

Determining a substance's refractive index (RI) is of great importance in a wide variety of disciplines, such as chemistry, biomedicine, engineering, and optics, to name just a few. This work utilizes critical-angle reflectometry to measure the refractive index of dentin, a biological tissue characterized by its high RI and complex structure. We fabricated a hollow semicircular prism and filled it with bromonaphthalene (1-B), allowing us to measure RIs up to 1.65. We used 5 tooth samples with exposed dentin and a laser diode beam (λ = 650nm) incident on interface 1-B/Dentine; the reflectance curve was measured from 60° to 75°. The critical angle is determined, and dentin RI is calculated. The real part of the calculated dentine's RI coincided well with those determined by other more expensive and specialized techniques with high technical support.

Testing the accuracy of low-beam-energy electron-excited X-ray microanalysis with energy-dispersive spectrometry

The accuracy of electron-excited X-ray microanalysis with energy-dispersive spectrometry (EDS) has been tested in the low beam energy range, specifically at an incident beam energy of 5 keV, which is the lowest beam energy for which a useful characteristic X-ray peak can be excited for all elements of the periodic table, excepting H and He. Elemental analysis results are reported for certified reference materials (CRM), stoichiometric compounds, minerals, and metal alloys of independently known or measured composition which had microscopic homogeneity suitable for microanalysis. Two-hundred sixty-three concentration measurements for 39 elements in 113 materials were determined following the k-ratio protocol and using the EDS analytical software NIST DTSA-II. The accuracy of the results, as characterized by the relative deviation from expected value (RDEV) metric, was such that more than 98% of the results were found to be captured within a range of ±5% RDEV, while 82% of the results fell in the range -2% to 2% RDEV.

Beam tests of SNSPDs with 120 GeV protons

We report the test results for a 120GeV proton beam incident on superconducting nanowire particle detectors of various wire sizes. NbN devices with the same sensitive area were fabricated with different wire widths and tested at a temperature of 2.82K. The relative detection efficiency was extracted from bias current scans for each device. The results show that the wire width is a critical factor in determining the detection efficiency and larger wire widths than 400nm leads to inefficiencies at low bias currents. These results are particularly relevant for novel applications at accelerator facilities, such as the Electron-Ion Collider, where cryogenic cooling is readily available.

Spin Photonics‐Based on a Twinning Hyperbolic Metamaterial

AbstractRouting and multiplexing of the optical surface waves are critical for the ultra‐compact nano‐photonic systems. Given this background, traditional metal‐based systems, despite of the ability of supporting the surface plasmon polariton propagation, are constrained on manipulations due to the optical isotropy, and artificially designed structures are always required at the input or output to help attain the additional selectivity. Here, a spin‐dependent multi‐channel scheme is proposed based on the modulation ability of the hyperbolic metamaterial's anisotropy on Dyakonov Surface Waves. The input coupling, propagation direction, as well as the output coupling of the optical single are well determined by the certain spin feature carried by the light and the orientation of the interface's anisotropy because of the spin‐momentum locking. Specially, by constructing a twinning hyperbolic metamaterial, which contains the structural symmetry and materials symmetry, a photonic chip with 4 routing channels are demonstrated experimentally, where the spin of an incident beam can be maintained during the propagation on‐chip and then delivered back into the free space, offering a new planar scheme for metamaterial‐based spin‐controlled nano‐photonic applications.

Multislit gamma camera for external beam radiotherapy assistance: Experimental proof of concept

The orthogonal computed tomography (OrthoCT) concept, based on orthogonal ray imaging, is a low-dose imaging technique currently under investigation to potentially aid in external-beam radiation therapy treatments. This technique involves detecting radiation scattered within the patient and emitted at approximately 90° from the direction of the incoming beam. This scattered radiation can be collected by a 1D-detector system with a multisliced collimator positioned perpendicular to the incident beam axis. Such a system holds promise for on-board imaging with the patient positioned and prepared for treatment, as well as for real-time treatment monitoring. In this study, a multi-slice OrthoCT detector prototype was developed and tested under in-beam irradiation. The system utilizes gadolinium orthosilicate scintillator crystals coupled to photomultiplier tubes and a collimator made of lead slices. Experimental measurements were conducted using a heterogeneous phantom made of acrylic with an air cavity inside. The phantom was irradiated with a TrueBeam linac operating at 6 MV in the flattening-filter-free mode. The findings of this study indicate that this innovative imaging technique is capable of providing morphological images of the phantom. This accomplishment is achieved without the need to rotate the X-ray source around the object to be irradiated, demonstrating the feasibility of such a system.