arXiv subjects

Joan Vila-Comamala

Publications and source records attributed to Joan Vila-Comamala.

13 recordsLinked to original sources

arXiv2026

A Neutron Microscope Using a Nested Wolter-I Condenser and a Bank of Diffractive-Refractive Achromatic Objectives

We propose a nested Wolter-I mirror design for a neutron condenser, which is based on established X-ray telescope technology. We demonstrate through simulations that it can increase the flux density at the ESS imaging instrument ODIN by up to two orders of magnitude. Experimental measurements of reflectivity and figure errors on a prototype mirror element confirm the technical feasibility of the approach. Then, we discuss design strategies for an imaging objective to fully exploit the condenser specifications while achieving spatial resolutions comparable to those of X-ray micro-CT instruments. Analytically, we show that for monochromatic beams suitable solutions exist employing arrays of hundreds of identical objectives, realized either as compound refractive lenses (CRLs) or Fresnel zone plates (FZPs). To mitigate the inherent chromatic aberration of these optics, each individual objective could be replaced by an achromatic FZP/CRL combination. Key optical properties of the resulting microscope are estimated. This novel full-field microscopy concept for highly divergent, polychromatic neutron beams has the potential to improve temporal and spatial resolution for large samples and sample environments and to enable the simultaneous acquisition of hundreds of projections in neutron tomography.

physics.ins-det↗

arXiv2026

Spin waves excited by hard x-ray transient gratings

Recent progress in ultrafast x-ray sources helped establish x-rays as an important tool for probing lattice and magnetic dynamics initiated by femtosecond optical pulses. Here, we explore the potential of ultrashort hard x-ray pulses for driving magnetic dynamics. We use a transient grating technique in which a spatially periodic x-ray excitation pattern gives rise to material excitations at a well-defined wave vector, whose dynamics are monitored via diffraction of an optical probe pulse. The excitation of a ferrimagnetic gadolinium bismuth iron garnet film placed in an external tilted magnetic field by x-rays at the Gd L3 edge results in both magnetic and non-magnetic transient gratings whose contributions to the diffracted signal are separated by polarization analysis. We observe the magnetization precession at both longitudinal acoustic and spin wave frequencies. An analysis with the Landau-Lifshitz-Gilbert equation indicates that the magnetization precession is driven by strain resulting from thermal expansion induced by absorbed x-rays. The results establish x-ray transient gratings as a tool for driving coherent phonons and magnons, with the potential of accessing wave vectors across the entire Brillouin zone.

physics.optics↗

arXiv2025

Soft x-rays with Orbital Angular Momentum for resonant scattering experiments at the SOLEIL synchrotron

The paper presents a comprehensive description of a new setup implemented and commissioned at the SEXTANTS beamline of the SOLEIL synchrotron for absorption and scattering experiments with x-ray beams carrying an orbital angular momentum, also known as twisted x-ray beams. Two alternative methods have been implemented, based on the use of either spiral zone plates or fork gratings devices, and we show how they can be used for both defining and assessing the orbital angular momentum of an x-ray beam. We show also how multiple devices can be used in sequence to define an integer arithmetic of the orbital angular momentum of the final x-ray beam. Finally, we report the results of the first resonant scattering pilot experiments in transmission and reflection mode, intended to assess the feasibility of future users measurements. The availability of twisted soft x-rays complements the range of experimental techniques in elastic, resonant and coherent scattering available at the SEXTANTS beamline of the SOLEIL synchrotron.

cond-mat.mtrl-sci↗

arXiv2025

Nanoscale Ultrafast Lattice Modulation with Hard X-ray Free Electron Laser

Understanding and controlling microscopic dynamics across spatial and temporal scales has driven major progress in science and technology over the past several decades. While ultrafast laser-based techniques have enabled probing nanoscale dynamics at their intrinsic temporal scales down to femto- and attoseconds, the long wavelengths of optical lasers have prevented the interrogation and manipulation of such dynamics with nanoscale spatial specificity. With advances in hard X-ray free electron lasers (FELs), significant progress has been made developing X-ray transient grating (XTG) spectroscopy, aiming at the coherent control of elementary excitations with nanoscale X-ray standing waves. So far, XTGs have been probed only at optical wavelengths, thus intrinsically limiting the achievable periodicities to several hundreds of nm. By achieving sub-femtosecond synchronization of two hard X-ray pulses at a controlled crossing angle, we demonstrate the generation of an XTG with spatial periods of 10 nm. The XTG excitation drives a thermal grating that drives coherent monochromatic longitudinal acoustic phonons in the cubic perovskite, SrTiO3 (STO). With a third X-ray pulse with the same photon energy, time-and-momentum resolved measurement of the XTG-induced scattering intensity modulation provides evidence of ballistic thermal transport at nanometer scale in STO. These results highlight the great potential of XTG for studying high-wave-vector excitations and nanoscale transport in condensed matter, and establish XTG as a powerful platform for the coherent control and study of nanoscale dynamics.

physics.optics↗

arXiv2025

Dichroism of coupled multipolar plasmonic modes in twisted triskelion stacks

We present a systematic investigation of the optical response to circularly polarized illumination in twisted stacked plasmonic nanostructures. The system consissts in two identical, parallel gold triskelia centrally aligned and rotated at a central angle relative to each other. Sample fabrication was acomplished through a double electron beam lithograpy process. This stack holds two plasmonic modes of multipolar character in the near-infrared range, showing a strong dependence of their excitation intensities on the handedness of the circularly polarized incident light. This translates in a large circular dicrhoism which can be modulated by adjusting the twist angle of the stack. Fourier-transform infrared spectroscopy and numerical simulations were employed to characterize the spectral features of the modes. Remarkable, in contrast to previous results in other stacked nanostructures, the system's response exhibits a behavious analogous to that of two interacting dipoles only at small angles. As the angle approaches 15 degrees, where the maximum dichroism is observed, more complex modes of the stack emerge. These modes evolve towards two in-phase multipolar excitations of the two triskelia as the angle increases uo to 60 degrees. Finally, simulations for a triangular array of such stacked elements show a sharp mode arising from the hybridization of a surface lattice resonance with the low-energy mode of the stack.

physics.optics↗

arXiv2024

Directly observing atomic-scale relaxations of a glass forming liquid using femtosecond X-ray photon correlation spectroscopy

Glass forming liquids exhibit structural relaxation behaviors, reflecting underlying atomic rearrangements on a wide range of timescales. These behaviors play a crucial role in determining many material properties. However, the relaxation processes on the atomic scale are not well understood due to the experimental difficulties in directly characterizing the evolving correlations of atomic order in disordered systems. Here, taking the model system Ge15Te85, we demonstrate an experimental approach that probes the relaxation dynamics by scattering the coherent X-ray pulses with femtosecond duration produced by X-ray free electron lasers (XFELs). By collecting the summed speckle patterns from two rapidly successive, nearly identical X-ray pulses generated using a split-delay system, we can extract the contrast decay of speckle patterns originating from sample dynamics and observe the full decorrelation of local order on the sub-picosecond timescale. This provides the direct atomic-level evidence of fragile liquid behavior of Ge15Te85. Our results demonstrate the strategy for XFEL-based X-ray photon correlation spectroscopy (XPCS), attaining femtosecond temporal and atomic-scale spatial resolutions. This twelve orders of magnitude extension from the millisecond regime of synchrotron-based XPCS opens a new avenue of experimental studies of relaxation dynamics in liquids, glasses, and other highly disordered systems.

cond-mat.mtrl-sci↗

arXiv2022

Generation of highly mutually coherent hard x-ray pulse pairs with an amplitude-splitting delay line

Beam splitters and delay lines are among the key building blocks of modern-day optical laser technologies. Progress in x-ray free electron laser source development and applications over the past decade is calling for their counter part operating in the Angstrom wavelength regime. Recent efforts in x-ray optics development have demonstrated relatively stable delay lines that most often adopted the division of wavefront approach for the beam splitting and recombination configuration. However, the two recombined beams have yet to achieve sufficient mutual coherence to enable applications such as interferometry, correlation spectroscopy, and nonlinear spectroscopy. We present the first experimental realization of the generation of highly mutually coherent pulse pairs using an amplitude-split delay line design based on transmission grating beam splitters and channel-cut crystal optic delay lines. The performance of the prototype system was analyzed in the context of x-ray coherent scattering and correlation spectroscopy, where we obtained nearly identical high-contrast speckle patterns from both branches. We show in addition the high level of dynamical stability during continuous delay scans, a capability essential for high sensitivity ultra-fast measurements.

physics.optics↗

arXiv2018

High aspect ratio silicon structures by Displacement Talbot lithography and Bosch etching

Despite the fact that the resolution of conventional contact/proximity lithography can reach feature sizes down to ~0.5-0.6 micrometers, the accurate control of the linewidth and uniformity becomes already very challenging for gratings with periods in the range of 1-2 μm. This is particularly relevant for the exposure of large areas and wafers thinner than 300μm. If the wafer or mask surface is not fully flat due to any kind of defects, such as bowing/warpage or remaining topography of the surface in case of overlay exposures, noticeable linewidth variations or complete failure of lithography step will occur. We utilized the newly developed Displacement Talbot lithography to pattern gratings with equal lines and spaces and periods in the range of 1.0 to 2.4 μm. The exposures in this lithography process do not require contact between the mask and the wafer, which makes it essentially insensitive to surface planarity and enables exposures with very high linewidth uniformity on thin and even slightly deformed wafers. We demonstrated pattern transfer of such exposures into Si substrates by reactive ion etching using the Bosch process. An etching depth of 30 μm or more for the whole range of periods was achieved, which corresponds to very high aspect ratios up to 60:1. The application of the fabricated gratings in phase contrast x-ray imaging is presented.

physics.app-ph↗

arXiv2018

Hot embossing of Au- and Pb- based alloys for X ray grating fabrication

Grating-based X-ray phase-contrast interferometry has a high application impact in material science and medicine for imaging of weakly absorbing (low Z) materials and soft tissues. For the absorbing gratings, casting of highly x-ray absorbing metals, such as Au and Pb alloys, has proven to be a viable way to generate large area periodic high aspect ratio microstructures. In this paper, we review the grating fabrication strategy with a special focus on a novel approach of casting low temperature melting alloys (Au-Sn and Pbbased alloy) into Si grating templates using hot embossing. The process, similar to nanoimprint lithography, requires particular adjusting efforts of process parameters as a function of the metal alloy and the grating feature size. The transition between solid and liquid state depends on the alloy phase diagram, the applied pressure can damage the high aspect ratio Si lamellas and the microstructure of the solid metal can affect the grating structure. We demonstrate that metal casting by hot embossing can be used to fabricate gratings on large area (up to 70x70 mm2) with aspect ratio up to 50:1 and pitch in the range of 1-20 μm.

physics.app-ph↗

arXiv2018

Towards Sub-micrometer High Aspect Ratio X-ray Gratings by Atomic Layer Deposition of Iridium

X-ray grating interferometry is an excellent technique for X-ray phase contrast imaging and X-ray wavefront sensing with applications in materials science, biology and medical diagnosis. Among other requirements, the method depends on the availability of highly X-ray absorbing metallic gratings. Here, we report on the fabrication and characterization of high aspect ratio iridium gratings with a period of one micrometer and a depth of 30 micrometer combining deep reactive ion etching of silicon and atomic layer deposition of iridium. The implementation of such structures can greatly enhance the sensitivity of grating-based X-ray phase contrast imaging and thus, expand further its broad range of applications.

physics.app-ph↗

arXiv2017

High aspect ratio metal microcasting by hot embossing for X-ray optics fabrication

Metal microstructured optical elements for grating-based X-ray phase-contrast interferometry were fabricated by using an innovative approach of microcasting: hot embossing technology with low melting temperature (280°C) metal alloy foils and silicon etched templates. A gold-tin alloy (80w%Au / 20w%Sn) was used to cast micro-gratings with pitch sizes in the range of 2 to 20 micrometers and depth of the structures up to 80 micrometers. The metal filling of the silicon template strongly depends on the wetting properties of the liquid metal on the groove surface. A thin metal wetting layer (20 nm of Ir or Au) was deposited before the casting in order to turn the template surface into hydrophilic with respect of the melted metal alloy. Temperature and pressure of the hot embossing process were optimized for a complete filling of the cavities in a low viscosity regime of the liquid metal, and for minimizing the shear force that might damage the silicon structures for small pitch grating. The new method has relevant advantages, such as being a low cost technique, fast and easily scalable to large area fabrication.

physics.app-ph↗

arXiv2017

Systematic efficiency study of line-doubled zone plates

Line-doubled Fresnel zone plates provide nanoscale, high aspect ratio structures required for efficient high resolution imaging in the multi-keV x-ray range. For the fabrication of such optics a high aspect ratio HSQ resist template is produced by electron-beam lithography and then covered with Ir by atomic layer deposition (ALD).

cond-mat.mtrl-sci↗

arXiv2014

Ion beam lithography for Fresnel zone plates in X-ray microscopy

Fresnel Zone Plates (FZP) are to date very successful focusing optics for X-rays. Established methods of fabrication are rather complex and based on electron beam lithography (EBL). Here, we show that ion beam lithography (IBL) may advantageously simplify their preparation. A FZP operable from the extreme UV to the limit of the hard X-ray was prepared and tested from 450 eV to 1500 eV. The trapezoidal profile of the FZP favorably activates its 2nd order focus. The FZP with an outermost zone width of 100 nm allows the visualization of features down to 61, 31 and 21 nm in the 1st, 2nd and 3rd order focus respectively. Measured efficiencies in the 1st and 2nd order of diffraction reach the theoretical predictions.

physics.optics↗