Elliptical Focusing Research Articles

Multi-frequency excitation of Rayleigh waves using an elliptical reflector focusing structure and its application for measurement of the acoustic properties of liquids

Abstract This study describes multi-frequency Rayleigh wave excitation below 10 MHz, which is impossible with a conventional piezoelectric-single-crystal surface acoustic wave (SAW) device. To overcome this limitation, we utilized a SAW device with an elliptical reflector focusing structure (ELIPS). The prototype ELIPS SAW device excited a Rayleigh wave on a duralumin surface at 1.1 MHz, 3.4 MHz, 5.6 MHz, and 7.9 MHz in the single device. The maximum vibration velocities were 0.38 m/s, 0.21 m/s, 0.070 m/s, and 0.034 m/s, respectively, which suggests the possibility of high-power mechatronics applications. To demonstrate the usefulness of the multi-frequency SAW below 10 MHz in a single device, we measured the frequency dependence of leaky SAW attenuation with a liquid droplet. The result showed that the acoustic properties of the liquid could be successfully estimated.

Shear Wave Ultrasound Inspection of Flaws in Silicon Wafers using Focused Transducers.

Silicon parts can contain micrometer-sized vertical cracks that are challenging to detect. Inspection using high-frequency focused ultrasound has shown promise for detecting defects of this size and geometry. However, implementing focused ultrasound to inspect anisotropic media can prove challenging, given the directional dependence of wave propagation and subsequent focusing behavior. In this work, back surface-breaking defects at various orientations within silicon wafers (0-, 15-, and 45-degrees relative to the [010] crystallographic axis) are experimentally inspected in an immersion tank setup. Using 100 MHz unfocused and focused shear waves, the impact of medium anisotropy on focusing and defect detection is evaluated. The scattering amplitude and defect detection sensitivity results demonstrate orientation-dependent patterns that strongly rely on the use of focused transducers. The defects along the 45-degree orientation reveal two-lobe scattering patterns with maximum amplitudes less than half that of the defects in the 0-degree orientation, which in contrast show a one-lobe scattering pattern. The experimental results are further explored using Finite Element (FE) modeling and ray tracing to visualize the impact of focusing on wave propagation within the silicon. Ray tracing results show that the focused beam profiles for the 45- and 0-degree orientations form a butterfly wing and elliptical focusing profile, respectively, which correspond directly to experimentally found scattering patterns from defects. Additionally, the FE scattering results from unfocused transducers reveal single lobe scattering for both 0- and 45-degree orientations, proving the varying scattering patterns to be driven by the anisotropic focusing behavior.

Upgrade of the OSIRIS primary spectrometer

A proposed upgrade is presented for the primary part of the indirect time-of-flight near-backscattering spectrometer OSIRIS at the ISIS Facility. This upgrade comprises a new supermirror guide with non-linear defocusing and focusing sections, a new wavelength bandwidth chopper system and a flexible slit system to adjust the divergence. The new neutron guide has an elliptical defocusing section, a curved section and an elliptical focusing section. The estimated gain factor in intensity is wavelength dependent between 4 and 7 at the sample position with a homogeneous divergence distribution on a smaller beam spot. The divergence will increase by up to a factor 3 compared to the present guide, but can be reduced by up to a factor 5 through the slit system. The increased guide cross section requires a system of two counter-rotating disc choppers with larger disc diameters than the present ones for the selection of the wavelength bandwidth. Estimates for the performance indicate that the proposed upgrade will keep the OSIRIS spectrometer competitive in the years to come.

High-Performance Cold Cathode X-ray Tubes Using a Carbon Nanotube Field Electron Emitter.

A cold cathode X-ray tube was fabricated using a carbon nanotube (CNT) field electron emitter made by a free-standing CNT film which is composed of a highly packed CNT network. A lot of CNT bundles with a sharp tip are vertically aligned at the edge of the thin CNT film with a length of 10 mm and a thickness of 7 μm. The cold cathode X-ray tube using the CNT field emitter presents an extremely high tube current density of 152 A/cm2 (corresponding to tube current of 106.4 mA), the electron beam transmittance of 95.2% and a small focal spot size (FSS) of 0.5 mm. In addition, the cold cathode X-ray tube also shows stable lifetime during 100 000 shots. High emission current density of the cold cathode X-ray tube is mainly attributed to a lot of electron emission sites at an edge of the CNT film. The small FSS is caused by an ensemble of the CNT field electron emitter made by a free-standing thin CNT film and the optimized curve-shape elliptical focusing lens. Based on obtained results, the cold cathode X-ray tube can be widely used for various X-ray applications such as medical diagnosis systems and security check systems in the future.

A Multimodal Dielectric Waveguide-Based Monopulse Radar at 160 GHz

For highly integrated imaging systems above 100 GHz, the complexity and chip area increase significantly with an increasing number of channels. In addition, bulky dielectric lenses prevent applications in spatially restricted surroundings. The presented concept of an imaging monopulse radar with a mechanically flexible front end reduces the required chip area and allows the antenna to be placed in any desired position apart from the sensitive electronics. The radar system is based on a two-channel 160-GHz microwave monolithic integrated circuit (MMIC) feeding a flexible dielectric waveguide. Depending on the angle of the incidence signal, a sum mode (HE11 mode) and a difference mode (HE21 mode) are excited in the dielectric waveguide. MMIC and waveguide are connected by a self-aligning transition reducing the requirements for packaging accuracy. The required chip area of the transition is only $0.022\lambda ^{2}$ with a spacing between the on-chip antennas of $\lambda /4$ . The measured ambiguity-free region between −18° and 15° is defined by the modified elliptical lens antenna focusing in the $E$ -plane only. A mechanical bending of the flexible waveguide is possible down to a radius of at least 2 cm without affecting the angle estimation capability.

Neutron Ray-Tracing Simulations of a New Supermirror Guide for the Osiris Spectrometer

A new supermirror guide has been proposed to replace the current neutron guide of the indirect time-of-flight near-backscattering spectrometer OSIRIS at the ISIS facility. Here we present an extensive Monte Carlo simulation study for the design and optimisation of a new guide system. Among the several guide geometry assessed, a curved guide with elliptical defocusing and focusing sections is shown to perform best. The estimated gain in intensity is a factor of 5–6 at the sample position with a homogeneous distribution of the divergence. The elliptic geometry results in a smaller beam spot and smaller samples will particularly benefit from this upgrade. The proposed guide replacement will ensure that the OSIRIS spectrometer will remain competitive in the years to come.

The effect of elliptical focusing on the threshold of a nanosecond BBO crystal optical parametric oscillator

ABSTRACTIn this paper, we report the results of our numerical model developed to determine the optimum elliptical focusing parameters of the high repetition rate input UV (355 nm) pump beam for achieving reliable low threshold operation of a singly resonant nanosecond β-BaB2O4 (BBO) based optical parametric oscillator (OPO). The analysis predicts that it is possible to achieve a substantial reduction in the threshold of the OPO by optimizing the 355 nm pump beam in an elliptical focus. We have used our model calculations to find out the maximum size of the elliptical focus for which the threshold pulse energy of the Type-I BBO crystal based OPO is lowered to ∼0.7 mJ. This provides the opportunity for a multi-kHz repetition rate, nanosecond BBO based OPO, pumped with the third harmonic of a DPSSL system operating with a pulse energy of ∼2–3 mJ, while maintaining high average power.

Evolutionary algorithm-assisted design of a UV SHG cavity with elliptical focusing to avoid crystal degradation

Long-term laser operation in the UV by frequency doubling in BBO is often limited due to degradation effects. We theoretically investigate the impact of elliptical focusing on the fundamental peak intensity to reduce the risk of crystal damage. After choosing a suitable elliptical Gaussian mode allowing for significantly lower fundamental peak intensity without sacrificing second harmonic output power, we describe an optimisation algorithm to find a stable resonator with cylindrical mirrors to ensure an elliptical focus with the specific beam waists inside the crystal. Experimental results achieved with this cavity are discussed.

Non-degrading CW UV generation in β-barium borate at 257 nm using an elliptical focusing enhancement cavity

With a combination of an elevated temperature for the β-barium borate (BBO) crystal and an elliptical focusing cavity we succeeded in eliminating the degradation in BBO during second harmonic generation into the ultraviolet (UV). We were capable of generating over 600 mW continuous-wave UV light at 257 nm with an off-the-shelf crystal for over 8 h without any sign of degradation. Our results indicate that the degradation is caused by color centers generated by two-photon absorption (TPA) strongly increasing the absorption of the fundamental radiation. While the higher crystal temperature reduces the TPA coefficient and the life time of the color centers, elliptical focusing strongly reduces the peak irradiance of the fundamental and thus its absorption.

Improvement of ultrasonic heat transfer enhancement using acoustical focusing and resonance properties

This paper is aiming to improve the ultrasonic heat transfer enhancement by controlling the sound intensity distribution and constructing an acoustic focusing field, which can reduce energy dissipation and dramatically enhance the local sound intensity of heat transfer surface. Two design criteria are proposed to construct the acoustic focusing field by utilizing acoustic interference and acoustic resonance. Correspondingly, an elliptical acoustic focusing tank is designed and fabricated. Two-dimensional simulation and experimental tests of sound intensity distribution in the elliptical acoustic focusing tank are performed to verify the theoretical conception and design. In addition, the heat transfer coefficients are measured at different locations in the elliptical tank under natural convection, using a square tank as a control. The experimental results show that the elliptical acoustic focusing tank can promote the intensity of the whole sound field, especially the focus region compared with the square tank. The heat transfer coefficient and heat transfer enhancement rate anywhere in the elliptical tank is higher than that in the square tank. The maximum heat transfer enhancement rate of the elliptical tank is 55.3% in natural convection, which is 47.1% higher than that of the square tank under the same condition.

Calculations of second harmonic generation with radially polarized excitations by elliptical mirror focusing.

Second harmonic generation (SHG) polarization intensity distribution illuminated with radially polarized beams by lens focusing appears two peaks, when the nonlinear optical coefficients dominate that is relevant to the transverse electric field components. Such two peaks pattern may result in ghosting and the decrement of imaging resolution. In this paper, an elliptical mirror based system is proposed in the case of radially polarized beams illumination for SHG. The calculated predictions and numerical simulations demonstrate that for radially polarized beams, the proportion of transverse field components at the focal plane under the condition of elliptical mirror focusing is 2.6 times smaller than that with lens focusing when its numerical aperture (NA) is 1. Furthermore, the full width at half maximum (FWHM) of the total field intensity profile is approximately 81% of that in a lens focusing system. Due to the enhancement of longitudinal components of incident field, the distribution of SHG polarization presents a single-peak pattern, in which two peaks can be observed with lens focusing. The SHG polarizations in collagen fiber, KTiOPO4 , and LiNbO3 have been numerical simulated and discussed in detail to verify the validity of the proposed method. LAY DESCRIPTION: A novel focusing mode for second harmonic generation (SHG) microscopy is described in this paper. In the case of radially polarized excitations, the SHG polarization distributions of some specimens appear two peaks pattern with traditional lens focusing. Such two peaks pattern may result in ghosting and the decrement of imaging resolution. By introducing elliptical mirror, the modulation of electric field at the focal plane can be realized. Thus, the distribution of SHG polarization is converted into a single-peak pattern which eliminates the ghosting and improves the spatial resolution on SHG images. This proposed method can be used for SHG in various materials and the full width at half maximum (FWHM) of SHG polarization will compress in different degrees. Besides, there is a significant improvement on signal-to-noise ratio in SHG imaging when an annular aperture illumination is applied. The focusing mode with elliptical mirror can also be utilized in other nonlinear optics areas such as polarized third harmonic generation (THG) measurements and two-photon fluorescence (TPF) microscopy.

Design and performance of AERHA, a high acceptance high resolution soft x-ray spectrometer.

A soft x-ray spectrometer based on the use of an elliptical focusing mirror and a plane varied line spacing grating is described. It achieves both high resolution and high overall efficiency while remaining relatively compact. The instrument is dedicated to resonant inelastic x-ray scattering studies. We set out how this optical arrangement was judged best able to guarantee performance for the 50 - 1000 eV range within achievable fabrication targets. The AERHA (adjustable energy resolution high acceptance) spectrometer operates with an effective angular acceptance between 100 and 250 μsr (energy dependent) and a resolving power well in excess of 5000 according to the Rayleigh criterion. The high angular acceptance is obtained by means of a collecting pre-mirror. Three scattering geometries are available to enable momentum dependent measurements with 135°, 90°, and 50° scattering angles. The instrument operates on the Synchrotron SOLEIL SEXTANTS beamline which serves as a high photon flux 2 × 200 μm(2) focal spot source with full polarization control.

Generation of 266 nm continuous-wave with elliptical Gaussian beams

The 266 nm continuous-wave coherent radiation is generated by the second harmonic generation of a β-BBO crystal placed inside an external enhancement cavity. Its fundamental beam is a 532 nm laser from commercial production of Coherent Company. Impact of focusing shape of the crystal on the conversion efficiency is simulated detailedly. Theoretical and experimental results show that the elliptical focusing can decrease the walk-off effect and improve conversion efficiency of the second harmonic generation (SHG). We obtain experimentally the 266 nm radiation output of 180 mW with elliptical focusing in a BBO crystal using bowtie cavity when the fundamental wave input 1 W . The SHG conversion efficiency reaches 18%.

The IMAGINE instrument: first neutron protein structure and new capabilities for neutron macromolecular crystallography

The first high-resolution neutron protein structure of perdeuterated rubredoxin from Pyrococcus furiosus (PfRd) determined using the new IMAGINE macromolecular neutron crystallography instrument at the Oak Ridge National Laboratory is reported. Neutron diffraction data extending to 1.65 Å resolution were collected from a relatively small 0.7 mm(3) PfRd crystal using 2.5 d (60 h) of beam time. The refined structure contains 371 out of 391, or 95%, of the D atoms of the protein and 58 solvent molecules. The IMAGINE instrument is designed to provide neutron data at or near atomic resolution (1.5 Å) from crystals with volume <1.0 mm(3) and with unit-cell edges <100 Å. Beamline features include novel elliptical focusing mirrors that deliver neutrons into a 2.0 × 3.2 mm focal spot at the sample position with full-width vertical and horizontal divergences of 0.5 and 0.6°, respectively. Variable short- and long-wavelength cutoff optics provide automated exchange between multiple-wavelength configurations (λmin = 2.0, 2.8, 3.3 Å to λmax = 3.0, 4.0, 4.5, ∼20 Å). These optics produce a more than 20-fold increase in the flux density at the sample and should help to enable more routine collection of high-resolution data from submillimetre-cubed crystals. Notably, the crystal used to collect these PfRd data was 5-10 times smaller than those previously reported.

Numerical Simulation of a Beam Divergence Correction for NRSE Spectrometer using Polygonal 2D-focusing Supermirrors

By inserting two-dimensional (2D) elliptical focusing mirrors between each pair of RSFs in neutron resonance spin echo spectrometer, the flight path lengths between the pair of RSFs can be made equal even for any divergent beam. We investigate numerically the feasibility of using polygonal 2D elliptical focusing mirrors as beam correction devices, and we show the relation of the energy resolution and the neutron intensity including effect of gravity.

Modulation for focusing properties of vector beams in imaging systems

Vectorial properties of light have significant effects on the focusing properties, especially for the case of tightly focusing, where the requirements on the ratio of the longitudinal field to the traverse fields (RLT) are different for various applications, such as particle acceleration, micronano-fabrication, fluorescent imaging, determination of three dimensional orientation of individual molecules and so on. Here, utilizing inhomogeneous polarization modulation (IPM) method, we demonstrate how to adjust the numbers of focal spots, the RLT, and the depth of focus (DOF) in terms of requirements and create successfully single or multiple focal spots with dominant transverse or longitudinal fields and small size. In terms of this method, not only the asymmetry of the focal spot for linearly polarized illumination with IPM can be obviously decreased, but also its size is the same as that of the short axis of the elliptical focusing spot for the case without IPM. Moreover, it can not only produce the focal spot with dominant transverse field, low sidelobe, and size slightly smaller than that of the radial polarization, but also can realize the identical resolutions in the transverse direction. In addition, for a focal spot with dominant transverse or longitudinal field, both the subdiffraction sizes (0.4λ or 0.5λ) and the DOF of 6.92λ are also achieved.

Rigorous theory on elliptical mirror focusing for point scanning microscopy

A rigorous elliptical mirror focusing formula based on spherical wave transformation is derived as a kind of imaging technique with high NA for potential applications in molecule imaging, spectroscopy and industrial artifact microscopy. An apodization factor is given and used to compare the energy conversation rules in lens transmission and parabolic and elliptical mirror reflections. Simulation results indicate that the axial HFWHM of elliptical and parabolic mirrors is about 80% of the corresponding HFWHM of lens in case of NA = 1 and φs = 0, and the side lobe noise is also slightly lower than that of lens, but the transverse HFWHM of mirrors is comparatively wider despite the width of main lobe is still smaller. In comparison with parabolic mirror based system, an elliptical mirror based system is potentially promising in aberration control of incident beam when the aperture of mirror is enlarged to adapt a large stage or specimen container at a small beam shading ratio.

Improved fourth harmonic generation in β-BaB2O4 by tight elliptical focusing perpendicular to walk-off plane

We present a scheme using beta-barium borate (BBO) crystal to maximize fourth-harmonic ultraviolet (UV) generation from high-power, kHz-range repetition rate Nd lasers by reducing thermal dephasing during the phase matching process. Thermal dephasing due to nonlinear absorption (NLA) induced by two-photon absorption of high peak power fourth-harmonic UV radiation is reduced by minimizing second- and fourth-harmonic beam overlap through tight elliptical focusing perpendicular to the walk-off plane, leading to multi-watt, kHz-range repetition rate UV generation in BBO crystal. With such beam shaping, nearly 5W at 10kHz of 262-nm UV radiation is generated using an 8-mm long BBO crystal. The results of our investigation show an efficient way of achieving multi-watt UV generation at kHz-range repetition rate.

A beam-shaping system for TIMEX beamline

FERMI@Elettra is a Free Electron Laser (FEL) user facility currently under construction at Sincrotrone Trieste in Italy. It will provide a spatially coherent and transform-limited photon beam in the sub-ps regime, covering the VUV/Soft X-ray range (from 100 down to 1.33 nm). Thanks to its high fluence this 4th generation light source will be able to create and probe warm dense matter (WDM) inside the TIMEX end-station. Since the WDM state has a short lifetime (a few ps), measurement of basic physical quantities, such as temperature and density, is a challenge and new approaches are needed. For this reason a new method has been proposed for measuring temperature using a slowly responding pyrometric probe (Principi et al., 2010 [1]). However, the technique does require the spatial photon beam profile to be properly shaped at the sample. This can be done using an active optic (i.e. a deformable plane mirror) placed before the elliptical focusing mirror. Ray-tracing simulations and metrology measurements on a prototype have been performed and the results are presented here.

Fabrication of Ultraprecision Millimeter-Thick Elliptical Neutron Focusing Mirror Substrate by Local Wet Etching

Numerically controlled local wet etching (NC-LWE) is a novel technique to fabricate the ultraprecision optical components and/or finishing the functional materials. In this technique, a figuring is performed by controlling the dwelling time of the combination nozzle, which consists of a supply and a suction part of an etchant, on the workpiece. In this paper, we proposed fabrication process of millimeter-thick elliptical neutron focusing mirror substrate by applying NC-LWE figuring involving CeO2 slurry polishing. We fabricated a millimeter-thick elliptical neutron focusing mirror substrate with a figure error of less than 0.2 μm and obtained a surface roughness of less than 0.15 nm rms.