arXiv subjects

S. Shwartz

Publications and source records attributed to S. Shwartz.

At least 19 recordsLinked to original sources

arXiv2026

Direct volumetric reconstruction for highly compressive x-ray fluorescence ghost tomography

X-ray fluorescence (XRF) enables element-specific, nondestructive imaging, but conventional raster scanning scales poorly with sample size, particularly for tomography, because measurements must be repeated at every projection angle and spatial position. We demonstrate direct volumetric XRF ghost tomography, which replaces point-by-point acquisition with compressive structured illumination and multiplexed fluorescence detection. Rather than reconstructing projections at each angle and then applying standard tomographic reconstruction, we recover the three-dimensional elemental distribution by solving a single inverse problem that jointly incorporates measurements from all angles. For a volume of 2.8 million voxels, we reconstruct the elemental distribution from only 400 measurements per angle, achieving a 43X reduction relative to raster scanning while maintaining spatial resolution and contrast. By exploiting sparsity directly in the volumetric domain, this approach enables scalable, multi-element XRF tomography of large and heterogeneous samples under stringent acquisition time constraints.

physics.optics↗

arXiv2026

Two-Dimensional Far-Field Correlations of X-ray Photon Pairs

We directly observe far-field correlations of x-ray photon pairs generated by spontaneous parametric down-conversion (SPDC). Using an energy-resolved, two-dimensional photon counting detector we record the full ring-shaped emission of both photons across a broad bandwidth and extract pair correlations directly from raw events without imposing angular constraints. The ring radii scale with photon energy, in quantitative agreement with transverse phase matching, providing a stringent momentum-space validation of x-ray SPDC. These observations open a route to leveraging quantum correlations in x-ray imaging and metrology, including correlation-enhanced magnification and reduced blurring.

quant-ph↗

arXiv2026

Phase-Sensitive Nonlinear X-Ray Response in a Charge-Density-Wave Quantum Material

We report a phase-sensitive nonlinear x-ray response in the charge-density-wave material 1T-TaS2, revealed through x-ray parametric down-conversion into the ultraviolet. Extending nonlinear x-ray wave mixing beyond conventional crystalline systems to a correlated quantum material, we employ reciprocal-lattice phase matching to isolate distinct Fourier components of the nonlinear susceptibility. By selecting a fundamental reciprocal-lattice vector and a stacking-sensitive half-integer reciprocal-lattice vector, we probe the response across the nearly commensurate and incommensurate charge-density-wave phases. Tuning the ultraviolet photon energy through Ta O-shell resonances uncovers pronounced Fourier-component and phase-dependent resonant structure, indicating that the stacking-related nonlinear susceptibility couples differently to Ta-centered resonant states than the average lattice response. Remarkably, the nonlinear signal is strongly enhanced in the nearly commensurate phase despite weaker Bragg diffraction, demonstrating that the nonlinear susceptibility provides information inaccessible to linear probes. These results establish nonlinear x-ray spectroscopy as a phase-sensitive and orbital-selective probe of electronic reconstruction in quantum materials.

physics.optics↗

arXiv2026

Human-organ-scale x-ray fluorescence ghost imaging for radioisotope-free diagnostics

A wide range of diagnostic information in medicine is currently obtained using radioactive tracers. While central to nuclear medicine, these methods are inherently constrained: radiation dose limits repeat examinations, short tracer half-lives and complex logistics restrict access and raise costs, and their relatively poor spatial resolution often necessitates complementary CT or MRI. Here we present a first proof-of-concept demonstration of a non-radioactive alternative based on x-ray fluorescence (XRF) computational ghost imaging (CGI) at the human-organ scale. Using a thyroid phantom filled with iodine solution as a model system, we show that structured illuminations combined with fluorescence detection reconstruct the iodine distribution with high fidelity. This approach eliminates the need for radioactive tracers while preserving image quality, and in principle can reach spatial resolution comparable to CT. Beyond this demonstration, XRF-CGI establishes a generalizable framework for non-radioactive tracer imaging, opening a route toward safer, repeatable, and more accessible diagnostics.

physics.med-ph↗

arXiv2026

Controlling X-ray emission with dispersion-engineered surface plasmon polaritons

We propose controlling the angular and spectral distribution of hard x-ray emission by entangling x-ray photons with ultraviolet surface plasmon polaritons (SPPs) whose dispersion is engineered by a metal-dielectric multilayer on a nonlinear crystal. Spontaneous parametric down-conversion of an x-ray pump produces a hard x-ray signal photon correlated with an ultraviolet SPP mode near its resonance. Engineering the SPPs dispersion reshapes the phase-matching landscape and imprints tunable angular-spectral structure on the emitted x-ray photons. The scheme enables compact, designable control of x-ray emission and extends surface-plasmon-assisted nonlinear and quantum x-ray optics.

physics.optics↗

arXiv2025

Surface Plasmon-Enhanced X-ray Ultraviolet Nonlinear Interactions

X ray matter interactions are intrinsically weak, and the high energy and momentum of X rays pose significant challenges to applying strong light matter coupling techniques that are highly effective at longer wavelengths for controlling and manipulating radiation. Techniques such as enhanced coupling between light and electrons at a metal dielectric interface or within nanostructures, as well as the Purcell effect where spontaneous emission is amplified near a metallic surface are not applicable to X rays due to their fundamentally different energy and momentum scales. Here we present a novel approach for coupling X rays to surface plasmon polaritons by entangling X ray photons with SPPs in the ultraviolet range through X ray to UV spontaneous parametric down conversion in aluminum. The distinct characteristics of the SPPs are imprinted onto the angular and energy dependence of the detected X ray photons, as demonstrated in this work. Our results highlight the potential to control X rays using SPPs, unlocking exciting opportunities to enhance X ray matter interactions and explore plasmonic phenomena with atomic scale resolution a capability uniquely enabled by X rays.

physics.optics↗

arXiv2024

25-Fold Resolution Enhancement of X-ray Microscopy Using Multipixel Ghost Imaging

Hard x-ray imaging is indispensable across diverse fields owing to its high penetrability. However, the resolution of traditional x-ray imaging modalities, such as computed tomography (CT) systems, is constrained by factors including beam properties, the absence of optical components, and detection resolution. As a result, typical resolution in commercial imaging systems is limited to a few hundred microns. This study advances high-photon-energy imaging by extending the concept of computational ghost imaging to multipixel ghost imaging with x-rays. We demonstrate a remarkable enhancement in resolution from 500 microns to approximately 20 microns for an image spanning 0.9 by 1 cm^2, comprised of 400,000 pixels and involving only 1000 realizations. Furthermore, we present a high-resolution CT reconstruction using our method, revealing enhanced visibility and resolution. Our achievement is facilitated by an innovative x-ray lithography technique and the computed tiling of images captured by each detector pixel. Importantly, this method can be scaled up for larger images without sacrificing the short measurement time, thereby opening intriguing possibilities for noninvasive high-resolution imaging of small features that are invisible with the present modalities.

physics.med-ph↗

arXiv2023

Confirming X-ray Parametric Down Conversion by Time-Energy Correlation

We present measurements of X-ray Parametric Down Conversion at the Advanced Photon Source synchrotron facility. Using an incoming pump beam at 22 keV, we observe the simultaneous, elastic emission of down-converted photon pairs generated in a diamond crystal. The pairs are detected using high count rate silicon drift detectors with low noise. Production by down-conversion is confirmed by measuring time-energy correlations in the detector signal, where photon pairs within an energy window ranging from 10 to 12 keV are only observed at short time differences. By systematically varying the crystal misalignment and detector positions, we obtain results that are consistent with the constant total of the down-converted signal. Our maximum rate of observed pairs was 130 /hour, corresponding to a conversion efficiency for the down-conversion process of $5.3 \pm 0.5 \times 10^{-13}$.

quant-ph↗

arXiv2023

Monte Carlo Simulations for Ghost Imaging Based on Scattered Photons

X-ray based imaging modalities are widely used in research, industry, and in the medical field. Consequently, there is a strong motivation to improve their performances with respect to resolution, dose, and contrast. Ghost imaging (GI) is an imaging technique in which the images are reconstructed from measurements with a single-pixel detector using correlation between the detected intensities and the intensity structures of the input beam. The method that has been recently extended to X-rays provides intriguing possibilities to overcome several fundamental challenges of X-ray imaging. However, understanding the potential of the method and designing X-ray GI systems pose challenges since in addition to geometric optic effects, radiation-matter interactions must be considered. Such considerations are fundamentally more complex than those at longer wavelengths as relativistic effects such as Compton scattering become significant. In this work we present a new method for designing and implementing GI systems using the particle transport code FLUKA, that rely on Monte Carlo (MC) sampling. This new approach enables comprehensive consideration of the radiation-matter interactions, facilitating successful planning of complex GI systems. As an example of an advanced imaging system, we simulate a high-resolution scattered photons GI technique.

physics.app-ph↗

arXiv2023

High-resolution computed tomography with scattered x-ray radiation and a single pixel detector

X-ray imaging is a prevalent technique for non-invasively visualizing the interior of the human body and opaque instruments. In most commercial x-ray modalities, an image is formed by measuring the x-rays that pass through the object of interest. However, despite the potential of scattered radiation to provide additional information about the object, it is often disregarded due to its inherent tendency to cause blurring. Consequently, conventional imaging modalities do not measure or utilize these valuable data. In contrast, we propose and experimentally demonstrate a high-resolution technique for x-ray computed tomography (CT) that measures scattered radiation by exploiting computational ghost imaging (CGI). We show that our method can provide sub-200 {\mu}m resolution, exceeding the capabilities of most existing x-ray imaging modalities. Our research reveals a promising technique for incorporating scattered radiation data in CT scans to improve image resolution and minimize radiation exposure for patients. The findings of our study suggest that our technique could represent a significant advancement in the fields of medical and industrial imaging, with the potential to enhance the accuracy and safety of diagnostic imaging procedures.

physics.med-ph↗

arXiv2021

Chemical element mapping by x-ray ghost fluorescence

Chemical element mapping is an imaging tool that provides essential information on composite materials and it is crucial for a broad range of fields ranging from fundamental science to numerous applications. Methods that exploit x-ray fluorescence are very advantageous and are widely used, but require focusing of the input beam and raster scanning of the sample. Thus the methods are slow and exhibit limited resolution due to focusing challenges. We demonstrate a new focusing free x-ray fluorescence method based ghost imaging that overcomes those limitations. We combine our procedure with compressed sensing to reduce the measurement time and the exposure to radiation by more than 80%. Since our method does not require focusing, it opens the possibility for improving the resolution and image quality of chemical element maps with tabletop x-ray sources and for extending the applicability of x-ray fluorescence detection to new fields such as medical imaging and homeland security applications

physics.optics↗

arXiv2021

Efficient Interaction of Heralded X-ray Photons with a Beam Splitter

We report the experimental demonstration of efficient interaction of multi kilo electron Volt heralded x-ray photons with a beam splitter. The measured heralded photon rate at the outputs of the beam splitter is about 0.01 counts/s which is comparable to the rate in the absence of the beam splitter. We use this beam splitter together with photon number and photon energy resolving detectors to show directly that single x ray photons cannot split. Our experiment demonstrates the major advantage of x rays for quantum optics: the possibility to observe experimental results with high fidelity and with negligible background.

quant-ph↗

arXiv2020

Measurements of Polarization Dependencies in Parametric Down-Conversion of X-rays into Ultraviolet Radiation

We present measurements of the polarization dependencies of the x-ray signal photons generated by the effect of parametric down-conversion of x rays into ultraviolet radiation. The results exhibit pronounced discrepancies with the classical model for the nonlinearity but qualitatively agree with a newly developed quantum mechanical theory for the nonlinear interaction. Our work shows that the reconstruction of the atomic scale charge distribution of valence electrons in crystals by using nonlinear interaction between x rays and longer wavelength radiation, as was suggested in previous works, requires the knowledge of polarization of the generated x-ray signal beam. The results presented in this work indicate a new methodology for the study of properties of the Wannier functions in crystals.

physics.optics↗

arXiv2020

High-speed x-ray imaging with single-pixel detector

We demonstrate experimentally the ability to use a single pixel detector for two-dimensional high-speed and high-resolution x-ray imaging. We image the rotation of a spinning chopper at 100-kHz at spatial resolution of about 15 microns by using the computational ghost imaging approach. The technique we develop can be used for the imaging of high-speed periodic dynamics or periodically stimulated effects with a large field of view and at low dose.

eess.IV↗

arXiv2019

Sub-Attosecond Metrology via X-Ray Hong-Ou-Mandel Effect

We show that sub-attosecond delays and sub-Angstrom optical path differences can be measured by using Hong-Ou-Mandel interference measurements with x-rays. We propose to use a system comprising a source based on spontaneous parametric down-conversion for the generation of broadband x-ray photon pairs and a multilayer-based interferometer. The correlation time of the photon pairs and the Hong-Ou-Mandel dip are shorter than 1 attosecond, hence the precision of the measurements is expected to be better than 0.1 attosecond. We anticipate that the scheme we describe in this work will lead to the development of various techniques of quantum measurements with ultra-high precision at x-ray wavelengths.

quant-ph↗

arXiv2019

Quantum enhanced X-ray detection

We present the first experimental demonstration of quantum-enhanced detection at x-ray wavelengths. We show that x-ray pairs that are generated by spontaneous down-conversion can be used for the generation of heralded x-ray photons and measure directly the sub-Poissonian statistics of the single photons by using photon number resolving detectors. We utilize the properties of the strong time-energy correlations of the down converted photons to demonstrate the ability to improve the visibility and the signal-to-noise ratio of an image with a small number of photons in an environment with a noise level that is higher than the signal by many orders of magnitude. In our work we demonstrate a new protocol for the measurement of quantum effects with x-rays using advantages such as background free measurements that the x-ray regime offers for experiments aiming at testing fundamental concepts in quantum optics.

quant-ph↗

arXiv2019

Observation of strong nonlinear interactions in parametric down-conversion of x-rays into ultraviolet radiation

Nonlinear interactions between x-rays and long wavelengths can be used as a powerful atomic scale probe for light-matter interactions and for properties of valence electrons. This probe can provide novel microscopic information in solids that existing methods cannot reveal, hence to advance the understanding of many phenomena in condensed matter physics. However, thus far, reported x-ray nonlinear effects were very small and their observations required tremendous efforts. Here we report the observation of unexpected strong nonlinearities in parametric down-conversion (PDC) of x-rays to long wavelengths in gallium arsenide (GaAs) and in lithium niobate (LiNbO3) crystals, with efficiencies that are about 4 orders of magnitude stronger than the efficiencies measured in any material studied before. These strong nonlinearities cannot be explained by any known theory and indicate on possibilities for the development of a new spectroscopy method that is orbital and band selective. In this work we demonstrate the ability to use PDC of x-rays to investigate the spectral response of materials in a very broad range of wavelengths from the infrared regime to the soft x-ray regime.

physics.optics↗

arXiv2019

Multiple Fourier Component Analysis of X-ray Second Harmonic Generation in Diamond

The unprecedented brilliance of X-ray free-electron lasers (XFELs) [1, 2] has enabled first studies of nonlinear interactions in the hard X-ray range. In particular, X-ray-optical mixing [3], X-ray second harmonic generation (XSHG) [4] and nonlinear Compton scattering (NLCS) [5] have been recently observed for the first time using XFELs. The former two experiments as well as X-ray parametric downconversion (XPDC)[6, 7] are well explained by nonlinearities in the impulse approximation[8], where electrons in a solid target are assumed to be quasi free for X-ray interactions far from atomic resonances. However, the energy of the photons generated in NLCS at intensities reaching up to 4 x 1020 W/cm2 exhibit an anomalous red-shift that is in violation with the free-electron model. Here we investigate the underlying physics of X-ray nonlinear interactions at intensities on order of 1016 W/cm2. Specifically, we perform a systematic study of XSHG in diamond. While one phase-matching geometry has been measured in Shwartz et al.[4], we extend these studies to multiple Fourier components and with significantly higher statistics, which allows us to determine the second order nonlinear structure factor. We measure the efficiency, angular dependence, and contributions from different source terms of the process. We find good agreement of our measurements with the quasi-free electron model.

physics.optics↗