Beam Shaping Devices Research Articles

Tailoring Second-Harmonic Emission from (111)-GaAs Nanoantennas.

Second-harmonic generation (SHG) in resonant dielectric Mie-scattering nanoparticles has been hailed as a powerful platform for nonlinear light sources. While bulk-SHG is suppressed in elemental semiconductors, for example, silicon and germanium due to their centrosymmetry, the group of zincblende III-V compound semiconductors, especially (100)-grown AlGaAs and GaAs, have recently been presented as promising alternatives. However, major obstacles to push the technology toward practical applications are the limited control over directionality of the SH emission and especially zero forward/backward radiation, resulting from the peculiar nature of the second-order nonlinear susceptibility of this otherwise highly promising group of semiconductors. Furthermore, the generated SH signal for (100)-GaAs nanoparticles depends strongly on the polarization of the pump. In this work, we provide both theoretically and experimentally a solution to these problems by presenting the first SHG nanoantennas made from (111)-GaAs embedded in a low index material. These nanoantennas show superior forward directionality compared to their (100)-counterparts. Most importantly, based on the special symmetry of the crystalline structure, it is possible to manipulate the SHG radiation pattern of the nanoantennas by changing the pump polarization without affecting the linear properties and the total nonlinear conversion efficiency, hence paving the way for efficient and flexible nonlinear beam-shaping devices.

Flat-top TIRF illumination boosts DNA-PAINT imaging and quantification

Super-resolution (SR) techniques have extended the optical resolution down to a few nanometers. However, quantitative treatment of SR data remains challenging due to its complex dependence on a manifold of experimental parameters. Among the different SR variants, DNA-PAINT is relatively straightforward to implement, since it achieves the necessary ‘blinking’ without the use of rather complex optical or chemical activation schemes. However, it still suffers from image and quantification artifacts caused by inhomogeneous optical excitation. Here we demonstrate that several experimental challenges can be alleviated by introducing a segment-wise analysis approach and ultimately overcome by implementing a flat-top illumination profile for TIRF microscopy using a commercially-available beam-shaping device. The improvements with regards to homogeneous spatial resolution and precise kinetic information over the whole field-of-view were quantitatively assayed using DNA origami and cell samples. Our findings open the door to high-throughput DNA-PAINT studies with thus far unprecedented accuracy for quantitative data interpretation.

Using the spatial light modulator as a binary optical element: application to spatial beam shaping for high-power lasers.

The liquid crystal spatial light modulator (SLM) is an effective active beam-shaping device through adjusting each pixel transmittance to improve the spatial beam quality of the output laser, which can also be used as a binary optical element (BOE) with each pixel transmittance 0 or 1 to realize spatial beam shaping for high-power lasers. We present and demonstrate an efficient shaping method of the SLM used as BOE based on diffraction principle. The method can be used to control the output nearfield actively by compensating the spatial nonuniformity of transmission and amplification in the high-power laser system. Results show the output nearfield beam quality improves significantly after shaping by using this method with the fluence contrast changing from 22% to 11.3% within only two shots in the single-shot operation laser.

Broadband Variable Meta-Axicons Based on Nano-Aperture Arrays in a Metallic Film

Metasurfaces are two-dimensional metamaterials composed of a carefully designed series of subwavelength meta-atom (antenna or aperture) arrays. These surfaces can manipulate the phase, amplitude and polarization of output light by changing the shapes and orientations of the meta-atoms on a subwavelength scale. Using these properties, we experimentally demonstrate variable meta-axicons composed of rectangular nano-apertures arranged in several concentric rings that can focus left circularly polarized (LCP) light into a real Bessel beam and defocus right circular polarized (RCP) light to form a virtual beam. A desired phase discontinuity in cross-polarized transmitted light is introduced along the interface by controlling the orientations of the nano-apertures. In addition, the meta-axicons can generate Bessel beams of arbitrary orders by suitable design of the phase profile along the surface. The meta-axicons demonstrate broadband optical properties that can switch the wavelength of the incident light from 690 nm to 1050 nm. These variable meta-axicons open a path towards the development of new applications using integrated beam shaping devices.

Spatial frequency performance limitations of radiation dose optimization and beam positioning

The flexibility and sophistication of modern radiotherapy treatment planning and delivery methods have advanced techniques to improve the therapeutic ratio. Contemporary dose optimization and calculation algorithms facilitate radiotherapy plans which closely conform the three-dimensional dose distribution to the target, with beam shaping devices and image guided field targeting ensuring the fidelity and accuracy of treatment delivery. Ultimately, dose distribution conformity is limited by the maximum deliverable dose gradient; shallow dose gradients challenge techniques to deliver a tumoricidal radiation dose while minimizing dose to surrounding tissue. In this work, this ‘dose delivery resolution’ observation is rigorously formalized for a general dose delivery model based on the superposition of dose kernel primitives. It is proven that the spatial resolution of a delivered dose is bounded by the spatial frequency content of the underlying dose kernel, which in turn defines a lower bound in the minimization of a dose optimization objective function. In addition, it is shown that this optimization is penalized by a dose deposition strategy which enforces a constant relative phase (or constant spacing) between individual radiation beams. These results are further refined to provide a direct, analytic method to estimate the dose distribution arising from the minimization of such an optimization function. The efficacy of the overall framework is demonstrated on an image guided small animal microirradiator for a set of two-dimensional hypoxia guided dose prescriptions.

Propagation of a Vector Cosine-Gaussian Correlated Beam Through an Active GRIN Medium

This paper presents a convenient approach that uses the tensor method to study the propagation of a nonuniformly correlated electromagnetic Cosine-Gaussian Schell-model (ECGSM) beam through an active gradient-index (GRIN) medium. It is shown that the initial correlation structure has a significant influence on the intensity gain; an ECGSM beam ( n = 1) experiences more gain than a conventional electromagnetic Gaussian Schell-model (EGSM) beam ( n = 0). The inhomogeneous gain induced by the medium leads to a nonlinear modulation of beam parameters such as the spectral intensity, coherence, and polarization. Although the gain leads to a broadening of the beam width, the coherence gradually decreases, which yields a useful guideline for manipulating active GRIN materials to generate numerous partially coherent beams. It is found that the polarization ellipse in the center of a conventional EGSM beam gradually evolves into circular polarization. However, the state of polarization of a nonuniformly correlated ECGSM beam gradually changes into linear polarization. The ability to manipulate light beams using active GRIN materials opens an alternative avenue for the generation of complex vector beams, which promises important supports in the fields of waveguide amplifiers, beam shaping, optical sensors, and beam transformer devices.

High-performance geometric phase elements in silica glass

High-precision three-dimensional ultrafast laser direct nanostructuring of silica glass resulting in multi-layered space-variant dielectric metasurfaces embedded in volume is demonstrated. Continuous phase profiles of nearly any optical component are achieved solely by the means of geometric phase. Complex designs of half-wave retarders with 90% transmission at 532 nm and >95% transmission at >1 μm, including polarization gratings with efficiency nearing 90% and computer generated holograms with a phase gradient of ∼0.8π rad/μm, were fabricated. A vortex half-wave retarder generating a single beam optical vortex with a tunable orbital angular momentum of up to ±100ℏ is shown. The high damage threshold of silica elements enables the simultaneous optical manipulation of a large number of micro-objects using high-power laser beams. Thus, the continuous control of torque without altering the intensity distribution was implemented in optical trapping demonstration with a total of 5 W average power, which is otherwise impossible with alternate beam shaping devices. In principle, the direct-write technique can be extended to any transparent material that supports laser assisted nanostructuring and can be effectively exploited for the integration of printed optics into multi-functional optoelectronic systems.

Employing refractive beam shaping in a Lloyd's interference lithography system for uniform periodic nanostructure formation

Uniform periodic structure formation over a large sample area has been challenging in laser interference lithography (LIL) mainly due to the Gaussian intensity distribution inherent to a laser beam. In this work, refractive beam shaping devices are applied in a Lloyd's interferometer to create a flat-top light field (2.8% intensity variation over an area of 20 × 20 cm2) for wafer-scale nanopatterning. Around 10−2 variation in fill factors are obtained for all the reported one dimensional and two dimensional periodic structures across a 2-in. wafer, which is 1 order of magnitude lower than the values obtained for the samples exposed to a Gaussian light field. The proposed LIL system also allows gradual light field transitions from the Gaussian, super-Gaussian, and flat-top to the inverse-Gaussian by simply adjusting the spot size of the laser incident to the beam shaper. The authors believe that the proposed LIL system can be applied for a variety of applications that benefit from the nature of periodic nanostructures.

Characterization and applications of spatial mode multiplexers based on Multi-Plane Light Conversion

Abstract Multi-Plane Light Conversion (MPLC) enables novel beam shaping devices, and in particular highly mode-selective spatial mode multiplexers. MPLC-based multiplexers can combine up to 10 spatial modes with cross-talk of −26 dB and insertion loss below 4 dB. These multiplexers are versatile and can be used in many applications, including long haul mode division multiplexing, short reach mode group division multiplexing, multi-mode optical amplification, and their mode conversion capabilities can be used for optimizing wavelength selective switches.

SU‐F‐T‐212: A Comparison of Treatment Strategies for Intracranial Stereotactic Radiosurgery

Purpose:Stereotactic radiosurgery is an effective and noninvasive treatment for intracranial lesions that uses highly focused radiation beams in a single treatment fraction. The purpose of this study is to investigate the dosimetric differences between the treatment brain metastasis with a proton beam vs. intensity modulated radiation therapy (IMRT).Methods:Ten separate brain metastasis targets where chosen and treatment plans were created for each, using three different strategies: custom proton beam shaping devices, standardized proton beam shaping devices, and IMRT. Each plan was required to satisfy set parameters for providing adequate coverage and minimizing risk to adjacent tissues. The effectiveness of each plan was calculated by comparing the homogeneity index, conformity index, and V12 for each target using a paired one tailed T‐test (α=0.05). Specific comparison of the conformity indices was also made using a subcategory containing targets with volume>1cc.Results:There was no significant difference between the homogeneity indices of the three plans (p>0.05), showing that each plan has the capability of adequately covering the targets. There was a statistically significant difference (p<0.01) between the conformity indices of the custom and the standard proton plan, as with the custom proton and IMRT (p<0.01), with custom proton showing stronger conformity to the target in both cases. There was also a statistical difference between the V12 of all three plans (Custom v. Standardized: p=0.02, Custom v. IMRT: p<0.01, Standardized v. IMRT: p<0.01) with custom proton supplying the lowest dose to surrounding tissues. For large targets (volume>1cc) there was no statistical difference between the proton plans and the IMRT treatment for the conformity index.Conclusion:A custom proton plan is the recommended treatment explored in this study as it is the most reliable way of effectively treating the target while sparing the maximum amount of normal tissue.

Adaptive optical beam shaping for compensating projection-induced focus deformation

Abstract Scanner-based applications are already widely used for the processing of surfaces, as they allow for highly dynamic deflection of the laser beam. Particularly, the processing of three-dimensional surfaces with laser radiation initiates the development of highly innovative manufacturing techniques. Unfortunately, the focused laser beam suffers from deformation caused by the involved projection mechanisms. The degree of deformation is field variant and depends on both the surface geometry and the working position of the laser beam. Depending on the process sensitivity, the deformation affects the process quality, which motivates a method of compensation. Current approaches are based on a local adaption of the laser power to maintain constant intensity within the interaction zone. For advanced manufacturing, this approach is insufficient, as the residual deformation of the initial circular laser spot is not taken into account. In this paper, an alternative approach is discussed. Additional beam-shaping devices are integrated between the laser source and the scanner, and allow for an in situ compensation to ensure a field-invariant circular focus spot within the interaction zone. Beyond the optical design, the approach is challenging with respect to the control theory’s point of view, as both the beam deflection and the compensation have to be synchronized.

An empirical model for calculation of the collimator contamination dose in therapeutic proton beams

Collimators are used as lateral beam shaping devices in proton therapy with passive scattering beam lines. The dose contamination due to collimator scattering can be as high as 10% of the maximum dose and influences calculation of the output factor or monitor units (MU). To date, commercial treatment planning systems generally use a zero-thickness collimator approximation ignoring edge scattering in the aperture collimator and few analytical models have been proposed to take scattering effects into account, mainly limited to the inner collimator face component. The aim of this study was to characterize and model aperture contamination by means of a fast and accurate analytical model. The entrance face collimator scatter distribution was modeled as a 3D secondary dose source. Predicted dose contaminations were compared to measurements and Monte Carlo simulations. Measurements were performed on two different proton beam lines (a fixed horizontal beam line and a gantry beam line) with divergent apertures and for several field sizes and energies. Discrepancies between analytical algorithm dose prediction and measurements were decreased from 10% to 2% using the proposed model. Gamma-index (2%/1 mm) was respected for more than 90% of pixels. The proposed analytical algorithm increases the accuracy of analytical dose calculations with reasonable computation times.

Impact of Spot Size and Beam-Shaping Devices on the Treatment Plan Quality for Pencil Beam Scanning Proton Therapy

This study aimed to assess the clinical impact of spot size and the addition of apertures and range compensators on the treatment quality of pencil beam scanning (PBS) proton therapy and to define when PBS could improve on passive scattering proton therapy (PSPT). The patient cohort included 14 pediatric patients treated with PSPT. Six PBS plans were created and optimized for each patient using 3 spot sizes (∼12-, 5.4-, and 2.5-mm median sigma at isocenter for 90- to 230-MeV range) and adding apertures and compensators to plans with the 2 larger spots. Conformity and homogeneity indices, dose-volume histogram parameters, equivalent uniform dose (EUD), normal tissue complication probability (NTCP), and integral dose were quantified and compared with the respective PSPT plans. The results clearly indicated that PBS with the largest spots does not necessarily offer a dosimetric or clinical advantage over PSPT. With comparable target coverage, the mean dose (Dmean) to healthy organs was on average 6.3% larger than PSPT when using this spot size. However, adding apertures to plans with large spots improved the treatment quality by decreasing the average Dmean and EUD by up to 8.6% and 3.2% of the prescribed dose, respectively. Decreasing the spot size further improved all plans, lowering the average Dmean and EUD by up to 11.6% and 10.9% compared with PSPT, respectively, and eliminated the need for beam-shaping devices. The NTCP decreased with spot size and addition of apertures, with maximum reduction of 5.4% relative to PSPT. The added benefit of using PBS strongly depends on the delivery configurations. Facilities limited to large spot sizes (>∼8 mm median sigma at isocenter) are recommended to use apertures to reduce treatment-related toxicities, at least for complex and/or small tumors.

Nonlinear Beam Shaping with Plasmonic Metasurfaces

We present here a method for generating second-harmonic beams with tailored beam profiles using nonlinear metasurfaces based on split ring resonators. By manipulating both the phase and the amplitude of the quadratic nonlinear coefficient locally, at the single inclusion level, the emitted second-harmonic wavefront is perfectly controlled. These concepts are demonstrated experimentally by the far-field generation of second-harmonic Airy and vortex beams from nonlinear binary phase computer-generated holograms and the perfect near-field generation of a Hermite–Gauss beam by precise amplitude and phase construction. We believe that these demonstrations open the door to use nonlinear metasurfaces for a variety of integrated nonlinear beam shaping devices.

STED-SPIM made simple.

We report the development of a stimulated emission depletion (STED) selective plane illumination (SPIM) microscope based on a single diode-pumped solid state (DPSS) laser that simultaneously delivers nanosecond-pulses at two wavelengths. The two wavelengths, 355 nm and 532 nm, are generated by harmonic conversion and they are used to induce respectively excitation and stimulated emission depletion. This source should allow a low-cost, compact, very efficient and simplified STED scheme since the two beams are intrinsically aligned and synchronized. Using a chromatic beam shaping device which leaves the excitation beam unaffected and produces a donut-shaped STED beam, we demonstrate a 300% reduction of the light sheet thickness, together with an enhancement of the sheet uniformity over larger field of view, at low STED power, in Coumarin dye solution.

A radiation oncologist's story: high tech meets high touch.

I grew up hearing stories about my grandmother, a missionary and an American-trained surgeon, who brought her doctoring skills to Korea at the turn of the twentieth century. She spent her first few months there learning the language and customs of this ancient kingdom where she’d spend the rest of her life. Working in a preantibiotic era, supportive care was an important part of her practice. Keeping her patients comfortable was sometimes the best she could offer (see Fig. 1). The personal side of medicine, often known as high touch, was as important as her surgical technique. Thus began my appreciation of the interplay between high tech and high touch. Not surprisingly, as an undergraduate I majored in anthropology and East Asian studies. When later accepted to medical school, I saw myself as a family practitioner. A chance conversation with a radiation oncologist on a shared car ride and a subsequent elective in the field changed my career course. Radiation oncology seemed the perfect way to merge some of my major life influences. I would be able to enlist my math and physics skills, courtesy of my math teacher father, into the service of hands-on doctoring like my grandmother, or so I thought. Radiation oncology has long been on the leading edge of technological advances in treatment planning and delivery systems. During my residency, computerized tomography (CT) imaging became more readily available. Today, CT-based volumetric planning with the fusion of magnetic resonance imaging(MRI)andpositron-emitting tomography(PET)scans, automated beam-shaping devices, and daily field imaging are common in practice. In plain language, with their current tools, radiation oncologists can customize treatment to accurately target the tumor and better spare surrounding normal tissue. During my residency, rigorous attention was given to mastery of the technical aspects of radiation oncology, but supportive care, though deemed important, was never given the same intense focus. You learned how to address treatmentrelated side effects on a case-by-case basis. Instruction and feedback on communication skills were not formalized and rarely mentioned, except when a major faux pas occurred

Theoretical modeling of the divergence of a flat-topped beam from a two-stage beam shaper into a conical intensity profile after propagation in free space

In this article, a two-stage beam shaping device based on a plastic fiber-bundle prism duct coupled waveguide is theoretically and experimentally described. ZEMAX software is used to simulate and investigate the divergence phenomenon on the beam shape outputted and the radiance profiles in both position and angle space. The effect of prism's geometrical structure on the beam divergence is also investigated and the optimum geometric conditions are reported. According to the theoretical results, due to an asymmetrical divergence effect the beam's cross section is increased with distance by a variable aspect ratio. The results also show that propagation in free space transformed a square beam with a flat-top intensity distribution into a rectangular beam with a conical intensity distribution. For experimental study, an imaging technique is applied to investigate the beam's output images and intensity profiles. A source light is used to illuminate the optical beam shaping elements of the reported design. Digital photographs of the beam's output for different axial distances are taken by a camera and the image data is converted into a response curve for comparison with the simulated image profiles. The obtained experimental results are strongly in agreement with the theoretical ones.

Reduced Secondary Neutrons and Occupational Exposure With Optimization of Beam Shaping Device Material in Proton Therapy

In proton therapy, beam shaping is achieved by patient-specific apertures (blocks), spot scanning, or a multi-leaf collimator (MLC). The primary collimators and apertures are the main sources of neutrons in scanning proton beams. Spot scanning is currently in limited clinical use, and brass apertures are used in most proton centers, while tungsten MLCs have been implement in one center. Neither brass nor tungsten are optimal materials for beam shaping due to activation and generation of secondary neutrons.

TU‐A‐108‐10: Pencil Beam Scanning Vs. Passive Scattered Proton Therapy for Pediatric Thoracic/pelvic Cancers

Purpose: To investigate if and under what conditions pencil beam scanning (PBS) will offer therapeutic benefits compared to passive scattered proton therapy for pediatric patients with tumors in the thorax/pelvis. Methods: A cohort of six pediatric patients with thorax/pelvic solid tumors treated with passive scattered protons was selected. The tumor types included prostate/bladder rhabdomyosarcoma and spinal/pelvic ewing sarcoma. Five PBS plans were created and optimized for each patient with variable beam spot sizes and utility of beam shaping devices namely apertures and range compensators. These included 12, 8 and 3 mm spot sigma at the isocenter, 12 mm spot with the use of apertures and 12 mm spot with both aperture and compensator. The conformity index for all plans was calculated to ensure sufficient tumor coverage. Dose volume histograms were analyzed and equivalent uniform dose (EUD) for each organ was computed. PBS and PSPT were compared by obtaining the % differences between the two methods for individual organs and for each patient and scanning parameter setting. Results: The results clearly indicate that PBS with 12 mm sigma spots size does not offer dosimetric or clinical advantage over PSPT. However, adding apertures or both apertures and compensators could ensure equivalent or better treatment quality. On average the EUD difference was 0.2% of the prescribed dose for the larger spot size and −3.9% when adding both beam shaping components. Decreasing the spot size further improved all plans, providing better dosimetric quality than PSPT, while alleviating the need for beam shaping devices. Conclusion: The added benefit of using PBS largely depends on the delivery configurations. We recommend the use of beam shaping components in facilities limited to larger spot size (>10mm sigma), in order to sharpen the dose penumbra and reduce the organ at risk toxicity in children with thoracic/pelvic cancers.

SU‐E‐T‐235: A Comparison of Small Field Output Factors for Three SRS/SRBT Systems

Purpose: Dosimetry measurement for small fields used in SRS/SBRT is challenging and the results can vary between institutions especially when different detectors are used. There are several reports in recent years that serious errors occurred in SRS/SRBT treatments due to incorrect output measurements in the initial commissioning processes. The present study compares small field output factors measured using a same detector for 6 MV photon beams in three SRS/SRBT systems: Cyberknife (cone), Brainlab (cone), and Varian TrueBEAM (Jaw), and determines if they are close enough to be used for reference purposes in commissioning similar SRS/SRBT systems. Methods: Relative output factors of small fields were measured by the author using a SRS diode (PTW 60012) in water for 6 MV photons with Cyberknife cones (5 to 60 mm), Brainlab cones on Varian TrueBEAM (5 to 30 mm), and TrueBEAM (Jaw: 10 × 10 mm2 to 40 × 40 mm2). The results are compared after being corrected for differences in reference field sizes, measurement distances and depths as defined in the three computer planning systems. Results: The corrected relative output factors of small fields measured for three systems are within +−2% from the average for collimator sizes larger than or equal to 20 mm, and within +−3% for smaller fields. Conclusion: The Results suggests that it may be feasible to establish a reference dataset of small field output factors for various SRS/SRBT systems and beam shaping devices (cone and MLC), similar to the RPC standard dataset for IMRT fields. Such a reference dataset is not to replace data collections during the institutional commissioning processes, but rather, is used as a secandary check to alert possible errors when large deviations from the reference data are observed.