Variable Spot Size Research Articles

AutoFocus: AI-driven alignment of nanofocusing X-ray mirror systems.

We describe the application of an AI-driven system to autonomously align complex x-ray-focusing mirror systems, including mirrors systems with variable focus spot sizes. The system has been developed and studied on a digital twin of nanofocusing X-ray beamlines, built using advanced optical simulation tools calibrated with wavefront sensing data collected at the beamline.We experimentally demonstrated that the system is reliably capable of positioning a focused beam on the sample, both by simulating the variation of a beamline with random perturbations due to typical changes in the light source and optical elements over time, and by conducting similar tests on an actual focusing mirror system.

A Novel Technique and Analytical Model for Laser Directed Energy Deposition with Variable Spot Sizes Based on Multiple Beams

A Novel Technique and Analytical Model for Laser Directed Energy Deposition with Variable Spot Sizes Based on Multiple Beams

The effect of a variable focal spot size on the contrast channels retrieved in edge-illumination X-ray phase contrast imaging

Multi-modal X-ray imaging allows the extraction of phase and dark-field (or “Ultra-small Angle Scatter”) images alongside conventional attenuation ones. Recently, scan-based systems using conventional sources that can simultaneously output the above three images on relatively large-size objects have been developed by various groups. One limitation is the need for some degree of spatial coherence, achieved either through the use of microfocal sources, or by placing an absorption grating in front of an extended source. Both these solutions limit the amount of flux available for imaging, with the latter also leading to a more complex setup with additional alignment requirements. Edge-illumination partly overcomes this as it was proven to work with focal spots of up to 100 micron. While high-flux, 100 micron focal spot sources do exist, their comparatively large footprint and high cost can be obstacles to widespread translation. A simple solution consists in placing a single slit in front of a large focal spot source. We used a tunable slit to study the system performance at various effective focal spot sizes, by extracting transmission, phase and dark-field images of the same specimens for a range of slit widths. We show that consistent, repeatable results are obtained for varying X-ray statistics and effective focal spot sizes. As the slit width is increased, the expected reduction in the raw differential phase peaks is observed, compensated for in the retrieval process by a broadened sensitivity function. This leads to the same values being correctly retrieved, but with a slightly larger error bar i.e. a reduction in phase sensitivity. Concurrently, a slight increase in the dark-field signal is also observed.

Effect of anionic surfactant on dispersibility and luminescence of silicon carbide nanotubes

We investigated the relationship between the dispersibility and optical properties of silicon carbide nanotubes (SiCNTs). By dissolving the aggregation of SiCNTs using sodium cholate as an anionic surfactant, the cluster size decreased and the dispersibility improved with the increasing surfactant concentration. The macroscopic and microscopic photoluminescence properties were evaluated using excitation lights with variable spot size. The emission intensity of SiCNTs increased with the decreasing cluster size, owing to the improvement of the dispersibility, and the emission intensity of the well-dispersed SiCNTs increased superlinearly with the excitation density. These results indicate the presence of SiCNTs that recombine non-radiatively in the cluster, and the effect of their NTs is suppressed as the cluster size is reduced.

Dynamic optical beam shaping system to generate Gaussian and top-hat laser beams of various sizes with circular and square footprint for Additive Manufacturing applications

The speed of production in laser-based additive manufacturing processes is rather low compared to conventional manufacturing methods and could be improved by using a top-hat laser beam intensity profile, a variable spot size or a combination of both, enabling wider melt pools during processing. This while retaining or possibly even improving part quality. In this work, the design and implementation of a dynamic optical beam shaping system is presented to enable different shapes and sizes of top-hat beams at the build platform of a Laser Powder Bed Fusion machine which is equipped with a fiber laser emitting at 1070nm. The system has been demonstrated by assembling custom-made beam shapers with commercially available beam expanders and by measuring the intensity profiles. Initial results indicate the possibilities for tuning of the spot size at the build platform by a magnification factor ranging from 1x to 4x. Parameter optimization for the Gaussian beam with 4x magnification was performed and a first 316L demonstrator part is further discussed in this work to illustrate a gain in productivity of 44%, while retaining >99% density.

Mesoscale modeling of metal-based selective laser melting: evolution of the vapor capillary

In this work we apply a multiphysics approach to selective laser melting to model the evolution of the vapor capillary, taking into account the effects of variable laser intensity, scan speed, and spot size. Restricting the work to a single line scan with a continuous wave laser, we focused on the application of Ti-6Al-4V alloy. In addition to heat, momentum and mass transfer, the solid/liquid/vapor interface was simulated using a front-tracking, level-set method, and an imposed recoil pressure was used to model the evolving vapor capillary. Initial benchmark applications focusing on an evolving melting front served to validate the heat transfer/phase change numerical framework.

Observation of carrier lifetime distribution in 4H-SiC thick epilayers using microscopic time-resolved free carrier absorption system

The carrier lifetime is an important parameter for high voltage SiC bipolar devices because its distribution in drift layers affects the device performance. Observation techniques for carrier lifetime, along with the development of carrier lifetime control processes, are important to control carrier lifetime distribution. In this study, we developed a microscopic time-resolved free carrier absorption system that has a variable spot size of excitation light and two different probe light wavelengths (405 and 637 nm). By selecting a relatively small spot size of excitation light and the probe light of shorter wavelength (405 nm), the distribution of carrier lifetime was observed with a high spatial resolution of ∼3 μm. Additionally, by using a relatively large spot size of excitation light and the probe light that leads to stronger free carrier absorption (637 nm), an accurate measurement of carrier lifetime was obtained. The developed system enables the design and development of bipolar SiC devices with carrier lifetime distribution control.

Variable spot size optical system for a dual-wavelength laser therapy device

The majority of existing high-power laser therapeutic instruments employ a single wavelength for a single target; thus, they do not meet the requirements for clinical treatment. Therefore, this study designs an optical system for a dual-wavelength high-power laser therapeutic device with a variable spot size. The waist of the short arm of the optical cavity and the G1G2 parameter (G-parameter equivalent cavity method) is calculated using MATLAB software, the spot size and divergence angle on the lens are calculated using an ABCD matrix, and the distance between the treatment spot at different spot sizes and the transformation lens is calculated in order to design the treatment handpiece. Experiments are conducted to analyze the stability at an output power of 532 nm before beam combination and the power loss after beam combination. The results show that the output power stability of the 532-nm beam varies by less than 2% over 150 min, and the loss of both wavelengths is less than 20%, which meets the clinical requirements of the system. The safety performance can meet the requirements of national general standards for medical electrical safety. The proposed dual-wavelength laser therapy instrument has both visible wave and near-infrared wave characteristics; thus, it can accurately target both superficial vessels and vessels with a larger diameter and deeper position. This therapeutic device has the advantages of simple operation, stable and reliable laser output, high security and strong anti-interference ability, and meets the comprehensive clinical treatment demands of vascular diseases.

Accurate measurement of in-plane thermal conductivity of layered materials without metal film transducer using frequency domain thermoreflectance.

Measuring anisotropic thermal conductivity has always been a challenging task in thermal metrology. Although recent developments of pump-probe thermoreflectance techniques such as variable spot sizes, offset pump-probe beams, and elliptical beams have enabled the measurement of anisotropic thermal conductivity, a metal film transducer enabled for the absorption of the modulated pump laser beam and the detection of the thermoreflectance signal. However, the existence of the transducer would cause in-plane heat spreading, suppressing the measurement sensitivity to the in-plane thermal conductivity. In addition, the transducer film also adds complexity to data processing, since it requires careful calibration or fitting to determine extra parameters such as the film thickness and conductivity, and interface conductance between the transducer and the sample. In this work, we discussed the methodology for measuring in-plane thermal conductivity of layered semiconductors and semimetals without any transducer layer. We show that the removal of transducer results in the dominantly large sensitivity to in-plane thermal conductivity compared with other parameters, such as cross-plane thermal conductivity and the absorption depth of the laser beams. Transducerless frequency-domain thermoreflectance (FDTR) measurements are performed on three reference layered-materials, highly ordered pyrolytic graphite, molybdenum disulfide (MoS2), and bismuth selenide (Bi2Se3) and demonstrated using the analytical thermal model that the measured in-plane thermal conductivity showed much-improved accuracy compared with conventional FDTR measurement with a transducer.

Equations for beam waist measurement of high peak power lasers

We have derived a set of equations for beam waist at lens focus as a function of variable spot size and demonstrated their physical validity. The derived equations are useful for the estimation of the beam waist size of high peak power lasers focused with high numerical aperture lenses and for spot size measurement of long-range collimated Gaussian beams for several meters. We have described an indirect method to estimate the high peak power focused laser beam waist. The same treatment is extended to the Gaussian vortex beam for estimating its parameters at the beam waist. These results can be useful in the focused Gaussian beam and Gaussian vortex beam applications.

Impact of varying planning parameters on proton pencil beam scanning dose distributions in four commercial treatment planning systems.

In pencil beam scanning proton therapy, target coverage is achieved by scanning the pencil beam laterally in the x- and y-directions and delivering spots of dose to positions at a given radiological depth (layer). Dose is delivered to the spots on different layers by pencil beams of different energy until the entire volume has been irradiated. The aim of this study is to investigate the implementation of proton planning parameters (spot spacing, layer spacing and margins) in four commercial proton treatment planning systems (TPSs): Eclipse, Pinnacle3 , RayStation and XiO. Using identical beam data in each TPS, plans were created on uniform material synthetic phantoms with cubic targets. The following parameters were systematically varied in each TPS to observe their different implementations: spot spacing, layer spacing and margin. Additionally, plans were created in Eclipse to investigate the impact of these parameters on plan delivery and optimal values are suggested. It was found that all systems except Eclipse use a variable layer spacing per beam, based on the Bragg peak width of each energy layer. It is recommended that if this cannot be used, then a constant value of 5mm will ensure good dose homogeneity. Only RayStation varies the spot spacing according to the variable spot size with depth. If a constant spot spacing is to be used, a value of 5mm is recommended as a good compromise between dose homogeneity, plan robustness and planning time. It was found that both Pinnacle3 and RayStation position spots outside of the defined volume (target plus margin). All four systems are capable of delivering uniform dose distributions to simple targets, but their implementation of the various planning parameters is different. In this paper comparisons are made between the four systems and recommendations are made as to the values that will provide the best compromise in dose homogeneity and planning time.

Using refractive lenses to provide a variable spot size for Kirkpatrick-Baez mirrors.

For many X-ray microprobe experiments it is desirable to be able to vary the beam size: using large beams for survey scans and a small beam for the final measurements. Beryllium refractive lenses were found to be a simple and controllable method for enlarging the focus in a Kirkpatrick-Baez-based microprobe. They can provide variable spot size, can be quickly inserted or removed and do not move the beam center on the sample.

Variable-spot-size laser-flash Raman method to measure in-plane and interfacial thermal properties of 2D van der Waals heterostructures

Stacked layers of different atomically thin 2D materials is called the van der Waals (vdW) heterostructure, which has become a rapidly developing research field due to its extraordinary and tunable properties. In this paper, we develop a variable-spot-size laser-flash Raman method to in-situ measure the thermal properties as well as the laser absorption in the supported 2D vdW heterostructure with arbitrary layers. The extracted thermal properties include the in-plane thermal conductivity and diffusivity of each layer, and interfacial thermal conductance between every two adjacent layers. A three-dimensional transient heat conduction model is developed and analytically solved to describe the process of pulsed Gaussian laser heating supported n-layer heterostructure. The temperature of each atomic layer can be simultaneously non-contact detected from their distinct Raman peaks whose positions are temperature dependent. The laser spot sizes and pulse durations are varied to generate multiple temperature curves. The multiple thermal properties as well as the laser absorption can be extracted by simultaneously fitting these temperature curves into the analytical solutions at multiple spot sizes or/and pulse durations. We also establish the approach of sensitivity and uncertainty analysis for the multi-response multi-parameter least-square fitting in our proposed measurement methods. Case studies show that the transient temperature curves are generally more sensitive to the thermal properties than the steady-state temperatures at variable spot sizes. All the unknown thermal properties and laser absorption can be extracted with sufficiently high accuracy if multiple transient temperature curves at multiple spot sizes are simultaneously fitted into the analytical solution. The measurement method and uncertainty analysis approach presented here are useful for investigating the thermal transport in the emerging 2D materials and vdW heterostructures.

Time-domain thermoreflectance (TDTR) measurements of anisotropic thermal conductivity using a variable spot size approach.

It is challenging to characterize thermal conductivity of materials with strong anisotropy. In this work, we extend the time-domain thermoreflectance (TDTR) method with a variable spot size approach to simultaneously measure the in-plane (Kr) and the through-plane (Kz) thermal conductivity of materials with strong anisotropy. We first determine Kz from the measurement using a larger spot size, when the heat flow is mainly one-dimensional along the through-plane direction, and the measured signals are only sensitive to Kz. We then extract the in-plane thermal conductivity Kr from a second measurement using the same modulation frequency but with a smaller spot size, when the heat flow becomes three-dimensional, and the signal is sensitive to both Kr and Kz. By choosing the same modulation frequency for the two sets of measurements, we can avoid potential artifacts introduced by the frequency-dependent Kz, which we have found to be non-negligible, especially for some two-dimensional layered materials like MoS2. After careful evaluation of the sensitivity of a series of hypothetical samples, we provided guidelines on choosing the most appropriate laser spot size and modulation frequency that yield the smallest uncertainty, and established a criterion for the range of thermal conductivity that can be measured reliably using our proposed variable spot size TDTR approach. We have demonstrated this variable spot size TDTR approach on samples with a wide range of in-plane thermal conductivity, including fused silica, rutile titania (TiO2 [001]), zinc oxide (ZnO [0001]), molybdenum disulfide (MoS2), hexagonal boron nitride (h-BN), and highly ordered pyrolytic graphite.

High speed, wide velocity dynamic range Doppler optical coherence tomography (Part V): Optimal utilization of multi-beam scanning for Doppler and speckle variance microvascular imaging.

In this paper, a multi-beam scanning technique is proposed to optimize the microvascular images of human skin obtained with Doppler effect based methods and speckle variance processing. Flow phantom experiments were performed to investigate the suitability for combining multi-beam data to achieve enhanced microvascular imaging. To our surprise, the highly variable spot sizes (ranging from 13 to 77 μm) encountered in high numerical aperture multi-beam OCT system imaging the same target provided reasonably uniform Doppler variance and speckle variance responses as functions of flow velocity, which formed the basis for combining them to obtain better microvascular imaging without scanning penalty. In vivo 2D and 3D imaging of human skin was then performed to further demonstrate the benefit of combining multi-beam scanning to obtain improved signal-to-noise ratio (SNR) in microvascular imaging. Such SNR improvement can be as high as 10 dB. To our knowledge, this is the first demonstration of combining different spot size, staggered multiple optical foci scanning, to achieve enhanced SNR for blood flow OCT imaging.

Simulation of field emission from volcano-gated tips for scanning probe lithography

Throughput and resolution in serial lithographic methods are naturally two competing goals - improving one leads to a deterioration of the other. This is also valid for field emission based scanning probe lithography FE-SPL), wherein low-energetic electrons (<100eV) are applied for direct exposure of ultra-thin resist films (<20nm). In this work a novel variable beam spot size concept for FE-SPL based on volcano-gated nanoprobes is shown. Here, a ring electrode surrounding the FE-SPL nanotip is working as an electrostatic lens. As a result the Fowler-Nordheim field emitted electron beam becomes focusable and defocusable in order to enhance the resolution or the throughput capability, respectively. In this paper the basic concept is demonstrated and proven by simulation of the electric field, the electron trajectories and the electron current density distribution on the sample surface. Based on simulations, the underlying physical effects are revealed and pathways for optimization of the volcano-type gated probe design and lithographic operation parameters are given.

Anisotropic heat conduction in silicon nanowire network revealed by Raman scattering

Anisotropic nanomaterials possess interesting thermal transport properties because they allow orientation of heat fluxes along preferential directions due to a high ratio (up to three orders of magnitude) between their in-plane and cross-plane thermal conductivities. Among different techniques allowing thermal conductivity evaluation, micro-Raman scattering is known to be one of the most efficient contactless measurement approaches. In this letter, an experimental approach based on Raman scattering measurements with variable laser spot sizes is reported. Correlation between experimental and calculated thermal resistances of one-dimensional nanocrystalline solids allows a simultaneous estimation of their in-plane and cross-plane thermal conductivities. In particular, our measurement approach is illustrated to be applied for anisotropic thermal conductivity evaluation of silicon nanowire arrays.

Focal spot imaging based on zero lag cross‐correlation amplitude fields: Application to dense array data at the San Jacinto fault zone

AbstractWe image the subsurface below a dense seismic array straddling the Clark branch of the San Jacinto fault zone in Southern California. The analysis is based on focal spots of surface waves associated with the zero lag amplitudes of noise cross‐correlations computed between all stations of the dense array. Local medium properties are inferred from the spatially variable focal spot size and shape based on the first zero crossing of amplitude versus distance distributions. The method provides simultaneous estimates of wave speed, apparent attenuation, and anisotropy without solving a tomographic inverse problem. The obtained images of the frequency dependent seismic velocity distributions are consistent with independent estimates from a far‐field Rayleigh wave tomography. We observe an anticorrelation between our apparent attenuation coefficient and seismic velocity, and a fault‐parallel alignment of fast propagation directions with greater structural complexity to the southwest of the fault. The results imply a complex fault zone structure including a waveguide to the northeast of the fault that is continuous across the observed depth range and a low‐velocity structure to the southwest associated with a shallow sedimentary basin.

SU‐F‐BRD‐12: When Does Pencil Beam Scanning Become Superior to Passive Scattered Proton Therapy for Pediatric Head and Neck Cancers?

Purpose:To investigate the dosimetric benefits of pencil beam scanning (PBS) compared with passive scattered (PS) proton therapy for treatment of pediatric head&amp;neck patients as a function of the PBS spot size and explore the advantages of using apertures in PBS.Methods:Ten pediatric patients with head&amp;neck cancers treated by PS proton therapy at our institution were retrospectively selected. The histologies included rhabdomyosarcoma, ependymoma, astrocytoma, craniopharyngioma and germinoma. The prescribed dose ranged from 36 to 54 Gy(RBE). Five PBS plans were created for each patient using variable spot size (average sigma at isocenter) and choice of beam specific apertures: (1) 10mm spots, (2) 10mm spots with apertures, (3) 6mm spots, (4) 6mm spots with apertures, and (5) 3mm spots. The plans were optimized for intensity modulated proton therapy (IMPT) with no single beam uniformity constraints. Dose volume indices as well as equivalent uniform dose (EUD) were compared between PS and PBS plans.Results:Although target coverage was clinically adequate for all cases, the plans with largest (10mm) spots provide inferior quality compared with PS in terms of dose to organs‐at‐risk (OAR). However, adding apertures to these plans ensured lower OAR dose than PS. The average EUD difference between PBS and PS plans over all patients and organs at risk were (1) 2.5%, (2) −5.1%, (3) ‐5%, (4) −7.8%, and (5) −9.5%. As the spot size decreased, more conformal plans were achieved that offered similar target coverage but lower dose to the neighboring healthy organs, while alleviating the need for using apertures.Conclusion:The application of PBS does not always translate to better plan qualities compared to PS depending on the available beam spot size. We recommend that institutions with spot size larger than ∼6mm at isocenter consider using apertures to guarantee clinically comparable or superior dosimetric efficacy to PS treatments.

A comparative fellow-eye prospective study evaluating the efficacy of wavefront-guided VSS VISX CustomVue STAR S4 IR versus the wavefront-optimized WaveLight Allegretto in correction of compound myopic astigmatism

Aim The aim of the present study was to compare the differences in visual outcomes, higher-order aberrations and corneal asphericity, as measured by Q values, in patients undergoing laser in-situ keratomileusis using wavefront-guided variable-spot size VISX CustomVue and wavefront-optimized small-spot size WaveLight Allegretto platforms. Type of study This study was designed as a randomized, prospective, single-blinded, double-armed, fellow-eye comparative study. Patients and methods A total of 100 eyes of 50 patients undergoing customized laser in-situ keratomileusis for compound myopic astigmatism were enrolled and randomly divided into two groups. Each patient underwent full ophthalmological examination; aberrometry was performed on either eye by using VISX CustomVue aberrometer and Allegretto WaveLight aberrometer. Each patient had one eye examined using the Allegretto Wave eye Q system (small-spot scanning), and the fellow eye by using VISX Star CustomVue S4 system (variable-spot scanning). All patients were given postoperative steroids, antibiotic drops and artificial tears. First day postoperative visit was scheduled to check for flap stability, and to exclude complications. Clinical examination and best corrected visual acuity, corneal imaging and aberrometry were scheduled at 1 month and 1 year later. Results Final best corrected visual acuity showed insignificant difference between the two groups ( P = 0.712). The final differences between the groups were insignificant in all other parameters. Conclusion The two ablation profiles are highly comparable in treating compound myopic astigmatism.